Department of Chemistry, University of Washington; Winter Quarter 2008 1 Chem 142; Instructor: K. P. Daruwala ? All rights reserved (08/2007). Atoms, Molecules, and Ions. CHEMISTRY 142 Winter Quarter 2008 Text: Chemical Principles, (fifth edition), by Steven S. Zumdahl Chapter 2 ? Atoms, Molecules, and Ions Learning Objectives After completing your study of Chapter 2, you should be able to: 1) State and explain the law of conservation of mass, law of definite proportions (or constant composition) and law of multiple proportions. (Section 2.2). 2) Understand the postulates of Dalton's theory and see its relationship to the laws of conservation of mass, definite proportions, and multiple proportions. (Section 2.3). 3) Recognize the interpretation of the laws by Cannizzaro. (Section 2.4). 4) Describe the basic properties of electrons, protons, and neutrons, and explain how these particles are distributed in an atom. Understand and describe the modern theory of atomic structure and know the names of all the scientists who were involved with the development of this theory. (Section 2.5). 5) Learn the modern view of atomic structure. Know what are atomic number, mass number, complete atomic symbol and name of an element. Be able to write the symbols and the names of the isotopes of an element, and deduce the number of protons, neutrons, and electrons that are present in an isotope of any given element. (Section 2.6). 6) Calculate the average atomic mass of an element from the given atomic masses of its isotopes and their relative abundances. Likewise determine the relative or percentage abundances of the isotopes of any given element. (Section 3.1). 7) Know how molecules and ions are formed and be able to write their formulas. Describe the basic difference between a molecular and an ionic compound and be able to categorize a compound as molecular or ionic. (Section 2.7). 8) Know the arrangement of the periodic table, and be able to locate elements and predict their properties from their location within the table. State the periodic law, compare the properties of metals, nonmetals, and metalloids, and recognize the regions within the periodic table where these elements are located. (Section 2.8). 9) By making use of chemical formulas, understand the meaning of subscripts and coefficients. Write the symbols for the cations formed by the metals of Groups I A, II A, and III A, and those of the transition metals, and the anions obtained from the nonmetals of Groups V A, VI A and VII A. (Section 2.9 ? Type I and Type II ionic binary compounds). 10) Write the formulas for ionic compounds, including those involving the polyatomic ions. Know the names, charges and formulas for the polyatomic ions in Table 2.5 on page 38. (Section 2.9 ? Ionic compounds with polyatomic ions). 11) Name simple binary covalent compounds. (Section 2.9 ? Type III covalent binary compounds). 12) Name and write the formulas of the acids. (Section 2.9). Chapter 2 Recommended Study and Practice Problems: 1, 2, 6, 12, 14, 16, 17, 19, 21, 25, 27, 29, 31, 33, 34, 35, 37, 38, 39, 41, 43, 45, 46, 47, 49, 51, 52, 53, 55, 57, 59, 61. Khushroo P. Daruwala 2 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. EARLY CHEMICAL LAWS AND THEORIES The principles of modern chemistry are essentially based upon some of the early laws that were formulated during the late eighteenth and early nineteenth centuries. The Law of Conservation of Mass C + O 2 ? CO 2 According to this law, matter is neither created nor destroyed, but is converted from one form to the other. In other words, this law can also be stated as follows: The Law of Definite Proportions: According to this law, any given compound will display fixed properties. The properties in turn are dependent upon the composition of the compound. It will always melt and boil at the same temperature under normal atmospheric pressure. This law is stated as follows ?a given compound will always contain its constituent elements in certain fixed proportions by mass, irrespective of its source.? e.g. NaCl always contains 1.5 times by weight Cl as Na. H 2 O always contains 8 times by weight O as H. A mixture (of substances) on the other hand will always change from one sample to another. The Law of Multiple Proportions Consider two chemical compounds that are composed of exactly the same elements, but have different proportions by mass of the elements. According to this law, if two different chemical compounds are composed of exactly the same elements but have different proportions by mass of these elements, then the ratio of the masses of the second element that will combine with a fixed amount of the first element will be in the form of small whole numbers. Dalton's Atomic Theory Taking into consideration, the above laws, Dalton proposed his atomic theory to be as follows: 1) All matter is composed of tiny indestructible particles called atoms. 2) All atoms of the same element are identical in mass and other properties. Atoms of different elements will have different masses and they will display different properties. 3) Chemical compounds are formed, when atoms of different elements combine with each other in the ratio of small whole numbers. 4) Chemical reactions involve the combination, separation, and rearrangement of atoms. Khushroo P. Daruwala 3 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. Question: How does Dalton's theory account for the three laws? a) Law of definite proportions: g of H + g of F = g of HF g of H will always combine with g of F to give g of HF. Since a F atom is 19 times the mass of a H atom, then of the mass of HF is F, while H is . If g of H combines with gof F,then . If g of H combines with gof F,then b) Law of Multiple Proportions: Consider the following two compounds formed from tin (Sn) and chlorine (Cl). Compound A (SnCl 2 ) Compound B (SnCl 4 ) Cl atoms have combined with 1 Sn atom. Cl atoms have combined with 1 Sn atom. c) Law of Conservation of Mass: Atomic Weight 1) Dalton made the hydrogen atom a standard and assigned it a mass of 1. He then designated the masses of other elements relative to that of the hydrogen atom. 2) During the 19 th and 20 th centuries, oxygen was used as a standard for atomic weights. However, problems arose when isotopes of oxygen were discovered. 3) In the 1950's as a standard for atomic weight, scientists adopted the pure isotope of carbon-12. Scientists assigned it a mass of exactly 12 atomic mass units (amu). An amu is thus defined as follows The carbon atom that occurs naturally consists of a mixture of two isotopes, viz. the more abundant 12 C and the less abundant 13 C with a mass of 13.00355 amu. Although the atoms exist as a mixture of isotopes, when the carbon atoms participate in a chemical reaction, it is more convenient to think in terms of a hypothetical "average" atom. Since carbon exists as a mixture of two isotopes, it is understood that the average mass of the carbon atom will lie between 12.00000 amu and 13.00335 amu. Naturally, this average mass will be "weighted" towards the mass of the more abundant carbon-12 isotope. This weighted average of the masses of naturally occurring carbon atoms is 12.011 amu. Thus, the atomic weight or atomic mass of any element is defined as the weighted average of the masses of the naturally occurring isotopes of that element. Khushroo P. Daruwala 4 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. STRUCTURE AND CONSTITUTION OF THE ATOM The Electron: In ancient times, atoms were regarded as the smallest possible components of matter. However, in the 19 th century, experiments with electricity made an indication that the atom itself might be composed of even smaller particles. The experiments that led to the discovery of the electron is chronicled as follows: 1) Benjamin Franklin characterized electrical charges into two types, one positive and the other negative. Being opposite in character, the two charges will attract each other. 2) Humphrey Davy used electricity to decompose chemical compounds. He proposed that attractive forces that are electrical in nature hold elements present in compounds together. 3) Michael Faraday studied the relationship between the amount of electricity used to decompose a chemical compound, and formulated the laws of chemical electrolysis. On the basis of Faraday?s work, George Johnstone Stoney proposed that units of electrical charge are associated with atoms and that these units are called electrons. 4) Experiments with cathode ray tubes. Julius Plücker discovered the cathode ray tube around 1859. When the tube is filled with a gas at low pressure, and a high voltage is applied across the electrodes a stream of cathode rays is discerned from the cathode the negative electrode. If a zinc sulfide (ZnS) coating is applied to the opposite wall of the cathode ray tube, it will begin to glow. Facts about cathode rays. a) The rays travel in a straight line. b) If an object is placed between the cathode and the ZnS screen a shadow of the object is cast. c) When a pinwheel is placed in the path of the cathode rays it will begin to turn and spin. This fact indicates that cathode rays have a mass and are actually composed of particles. d) If an electrical or magnetic field is applied, the cathode rays are bent. This result suggests that the cathode rays are streams of fast-moving negatively charged particles. These negatively charged particles are the electrons. e) The same beam of electrons is generated when different metals are used for the cathode or when the cathode ray tube is filled with different gases. f) When two oppositely charged plates are placed on either side of the cathode rays, i.e. when an electrical field is applied, the rays are deflected from their usual straight-line path towards the positively charged plate. The degree of deflection of the cathode rays is dependent upon: i) the magnitude of the charge (q) of the particle: Khushroo P. Daruwala 5 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. ii) the mass (m) of the particle: The combination of the above two factors determines the extent to which the electrons are deflected from a straight-line path in an electrical field. Thus, the degree of deflection of the electron = g) If a magnetic field is applied to a beam of cathode rays, the degree of deflection is perpendicular or orthogonal to the applied electrical field. This information points out that the rays or electrons are capable of traveling in both directions of the same plane. h) J. J. Thomson (1897) determined the value of the charge to mass ratio (q / m) by studying the deflection of cathode rays in the presence of both electric and magnetic fields. Thomson measured the radius of curvature of the deflection caused by a magnetic field of known field strength. He then determined the strength of the electric field required to balance the magnetic field so that there was no net deflection. From the measurements that Thomson made, he was able to calculate the value of the charge to mass (q / m) ratio of an electron. Thomson observed that the same q / m ratio was obtained when The constant value for the q / m ratio that Thomson obtained was 5) Determining the charge on an electron. The charge on an electron was determined by means of an experiment called the Millikan?s oil drop experiment. When the horizontal plates of the chamber are electrically charged, the rate of fall of the drop is altered, since the negatively charged drop is now attracted towards the positively charged upper plate. The amount of charge on the plates is adjusted in such a way that the oil drops stop falling and remain in a state of suspension. By making the appropriate adjustment of the electrical charge on the plate, the charge on the drop is calculated from the mass of the drop and the charges on the plate. Millikan observed that different droplets had different charges. However, he noticed that each charge was a whole number multiple of a smaller charge, which was calculated to be Khushroo P. Daruwala 6 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. With the help of Millikan?s data, Thomson was able to calculate the mass of an electron as follows: The Proton: When atoms of molecules of a gas present in an electrical discharge tube, the cathode rays will remove the electrons from the atoms or molecules of the gas present in the tube. When electrons are removed from a neutral atom or molecule, positive ions or particles result and are formed. If holes were punctured into the cathode of an electric discharge tube, then the positive ions so produced would be able to pass through these holes. These streams of positive particles are known as positive or canal rays. The values of the q / m ratio were determined by using the same method as the one employed in the study of the cathode rays. However, when different gases are employed in the discharge tube, a different q / m ratio is obtained for the positively charged particles. This result suggests that a different positive ion is being generated in the gas discharge tube. When hydrogen gas was used, the positively charged ion that resulted had the smallest mass. Therefore, the largest value for the q / m ratio was obtained for the hydrogen gas, which was found to be The positively charged particles in the canal rays are known as protons, and they possess a unit positive charge + e that is equal but opposite in sign to that of an electron. The mass of a proton can then be calculated as follows: The ratio of the mass of the proton to that of the electron, is mass of the proton mass of the electron = Khushroo P. Daruwala 7 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. The Neutron: All atoms are neutral species since they contain an equal number of protons and electrons. Yet the masses of all atoms were found to be much greater than what would be predicted on the basis of the presence of just the protons and the electrons. Therefore, in addition to electrons and protons, there has to be some other uncharged particle present in an atom. James Chadwick discovered the presence of a third particle called the neutron in 1932 through the use of nuclear reaction techniques. Chadwick was able to determine the mass of the neutron as 1.6750 × 10 ?24 g. The ratio of the mass of the neutron to that of the electron, is mass of the neutron mass of the electron = Particle Mass in grams Charge Electron 9.109535 × 10 ?28 g Proton 1.672649 × 10 ?24 g Neutron 1.674954 × 10 ?24 g The Nucleus: Thomson postulated that the atom is a uniform sphere of positively charged matter within which thousands of electrons circulate in coplanar rings (the plum pudding model). Thomson and his colleagues tested this model by directing a beam of electrons at a very thin metal foil. According to Thomson?s hypothesis when a beam of electrons encounters the electrons present in the atoms of the metal foil, the negative charges of the electrons of the beam will repel the negative charges of the electrons in the metal foil. However, upon doing so, Thomson observed only a small deflection of the beam. Based on this result of a rather small deflection, Thomson and his colleagues were forced to lower their estimate of the number of electrons present in an atom. There are atoms of certain elements, which are found to exist in unstable combinations of subatomic particles. These atoms spontaneously emit rays, and by means of this process get converted into atoms of a completely different chemical identity. This phenomenon discovered by Henri Becquerel is known as radioactivity. In general, all radioactive substances are capable of emitting three types of radiation. Ray Composition Charge of Component alpha (?) particles containing 2 protons and 2 neutrons speed = 16,000 km / s beta (?) electrons speed = 130,000 km / s gamma (?) short wavelength electromagnetic radiation sped simlar toX-rays Rutherford elaborated upon the model of Thomson by directing a beam of ? particles against a very thin (0.0004 cm thick) foil of gold or silver or copper. Rutherford observed that when the beam was directed at a gold foil, most ? particles went through the foil, while some were deflected from their straight-line path, and a few even recoiled back towards their source. Based upon these results, Rutherford proposed that an atom of an element consists of two parts. 1) The nucleus which exists in the center of the atom and which is responsible for the mass and positive charge of the atom. He calculated the radius of the nucleus to be 10 ?12 cm, indicating that the nucleus was found to be 100,000 times smaller than the atom. 2) The electrons occupy the rest of the volume or space that is considered to be extra nuclear. Since an atom is an electrically neutral species, the number of its protons = Khushroo P. Daruwala 8 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. IMPORTANT TERMS AND CONCEPTS REGARDING ATOMIC STRUCTURE Atomic Number (Z): Represents the number of protons (only) that are present in the nucleus of an atom. Mass Number (A): Corresponds to the number of protons and neutrons that are present in the nucleus of an atom. Therefore, the number of neutrons (N) that are present in an electrically neutral element will be = The complete atomic symbol of an element can be written as follows: For example: Li 7 3 Cl 37 17 More often the atomic number (Z) will not be indicated in the element symbol, since it can be easily procured from the periodic table. e.g. 7 Li and 37 Cl. Writing the element name from the complete element symbol. The format by which the element name is written from its given atomic symbol is For example: Li 7 3 = Cl 37 = ILLUSTRATIVE EXAMPLES: 1) How many protons, neutrons, and electrons are present in each of the following? Element Protons Neutrons Electrons 63 Cu aluminum?27 2) What are the complete atomic symbol and the name of the element that contains 19 protons and 22 neutrons? Isotopes: Atoms of the same element, which differ from each other in the number of their neutrons, are referred to as isotopes. For example, Since the chemistry of the elements arises from its electrons, isotopes of an element display identical chemical properties. Isotopes however differ from each other in their natural or relative abundances. The atomic weight of a particular element is thus obtained to be its average atomic weight or mass since it is calculated as an average from the atomic masses of its isotopes. In general, the average atomic mass of an element can be calculated by means of the following equation. Average atomic mass = % abundance of isotope 1 × (mass of isotope 1) + 100 % abundance of isotope 2 × (mass of isotope 2) + ????.. 10 ???.. % abundance of isotope n × (mass of isotope n) 10 Khushroo P. Daruwala 9 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. ILLUSTRATIVE EXAMPLES: 1) An element consists of 99.759 % of isotope 1 of atomic mass 15.9949 atomic mass units (amu), 0.037000% of isotope 2 of atomic mass 16.9993 amu, and 0.20400 % of isotope 3 of atomic mass 17.9992 amu. Determine the average atomic mass of this element and identify it. 2) Vanadium occurs in nature as a mixture of two isotopes: 50 V, which has an atomic mass of 49.9472 amu, and vanadium?51, which has an atomic mass of 50.9440 amu. The average atomic mass or weight of vanadium is 50.9415 amu. What is the percent abundance of each of the two isotopes? 50 V = 51 V = Khushroo P. Daruwala 10 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. INTRODUCTION TO THE PERIODIC TABLE The periodic table is a device, which has been used to correlate the properties of the elements. The elements arranged or listed in the periodic table are in an order of increasing atomic number. When elements are studied in order of their increasing atomic number, similarities in properties recur periodically. This is the basis of the Periodic Law, and the periodic table is designed upon this law. The arrangement of the elements in the periodic table is such that similar elements are grouped together, and that the properties of all the elements can be predicted from their positions in the table. The arrangement of the periodic table is as follows: 1) Elements with similar physical and chemical properties are arranged in vertical columns called groups. 2) In general, the elements designated as ?A? groups are classified in the US as main or representative groups, while those elements located in the ?B? groups are known as the transition elements. 3) The horizontal rows of the periodic table are classified as periods. There are 2 elements in the first period, and 8, 8, 18, 18, and 32 elements in the 2 nd , 3 rd , 4 th , 5 th and 6 th periods respectively. At the latest count, the 7 th period also has about 32 elements. 4) The periodic table is divided into several regions according to the properties of the elements. c) Elements along the staircase line are the a) Elements to the left of the staircase line are the metals. Metals are solids at room temperature, can conduct heat and electricity, and are ductile and malleable, and can form alloys. b) Elements to the right of the staircase line are the nonmetals. Nonmetals are either solids, liquids, or gases. They are however, of heat and electricity. "staircase line" 5) Some of the groups in the periodic table are often designated by special names as follows: a) Elements of Group VIII A or Group 0, are colorless gases of low reactivity and are known as the noble gases. Because of their extremely low reactivity they are often referred to as the inert gases. b) Each element located in Group I A follows the noble gas element in the period before it. These highly reactive soft metals are known as the alkali metals. c) The elements of Group II A, which follow those of Group I A are also metallic by nature. These metals give alkaline solutions when reacted with water and therefore are known as the alkaline earth metals. d) The group of nonmetallic elements located in Group VII A that precede the noble gases in terms of their atomic numbers. These elements are very reactive and are collectively known as the halogens. e) There are two rows of 14 elements each that are placed at the bottom of the periodic table. These elements are collectively known as the inner transition elements. i) Elements with atomic numbers 58 to 71 that appear after Lanthanum (atomic number = 57) in the sixth period are known as the lanthanides. Khushroo P. Daruwala 1 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. ii) Elements with atomic numbers 90 to 103 that appear after Actinium (atomic number = 89) in the seventh period are known as the actinides. (It is important to note, that the location of these two rows at the bottom of the periodic table is because of convenience and a lack of space on a single sheet). Periodic Trends: 1) The arrangement of the elements in the periodic table has been such that the elements within a group have similar (and not identical) properties. The properties of the first member of each group often differ from those of the lower members of the same group. However, members of a given group show a tendency to form similar compounds with other elements. For example a) Sodium (Na) and potassium (K) from Group I A form NaCl and KCl respectively with chlorine (Cl). b) Chlorine (Cl) and bromine (Br) from Group VII A respectively form CaCl 2 and CaBr 2 with calcium, while these same elements react with carbon to respectively form CCl 4 and CBr 4 . 2) Each period with the exception of the first one, begins with a highly reactive metal, viz. the alkali metals. Going across the period from left to right, the properties change from element to element. The metallic properties gradually fade and are replaced by nonmetallic ones. With the exception of the first period (which has only two elements hydrogen (H) and helium (He), each period ends with a highly reactive nonmetal the halogen, followed by an unreactive noble gas. IONS AND THE FORMATION OF IONIC COMPOUNDS An ion is a particle that is made of an atom or a group of atoms and which bears an electrical charge. Ions can be classified as follows: IONS Cations Anions Positively charged ions, which have lost electrons. Negatively charged ions, which have gained electrons. Monoatomic Polyatomic Monoatomic Polyatomic Cations formed from a single atom. e.g. Cu 2+ , Al 3+ , Ni 2+ Cations that contain more than one atom. e.g. NH 4 + Anions formed from a single atom. e.g. Cl - , O 2- Anions that contain more than one atom. e.g. OH - , SO 4 2- Cations: Since metallic elements have a tendency to lose electrons, they will form positive ions. The magnitude of the charge on a monoatomic cation corresponds to the number of electrons lost by the metal. The loss of one electron results in an ion of a 1 + charge, while the loss of two electrons will yield a cation with 2 + charge. The following are examples of some monoatomic cations. Element Z Charge Symbol of Ion Potassium (K) 19 1 + K + Magnesium (Mg) 12 2 + Mg 2+ Gallium (Ga) 31 3 + Ga 3+ Khushroo P. Daruwala 12 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. Question: How can I determine whether a particular metal will lose one, two, or more electrons? The number of electrons that are lost by a metal depends upon the location of the metal in the periodic table. a) Metals of the main or representative groups (i.e. those located in Groups I A, II A, III A, IV A, and V A) lose electrons equal to the group number of the metal. The above three metals are located in Groups I A, II A, and III A respectively. Therefore, they will respectively lose 1, 2, and 3 electrons. The corresponding cations that result from such a loss will accordingly have charges of 1 +, 2 +, and 3 + respectively. ILLUSTRATIVE EXAMPLE: Complete the following table with the information requested: Element Group No. No. of e ? lost Charge Symbol of Ion strontium (Sr) cesium (Cs) indium (In) b) For transition metals there is no predictable pattern for the metal to lose any set number of electrons and form a cation. The loss of electrons is at random and not according to group numbers. Element Group No. No. of e ? lost Charge Symbol of Ion titanium (Ti) IV B chromium (Cr) VI B chromium (Cr) VI B copper (Cu) I B copper (Cu) I B As can be seen in the above, one particular element can form more than one cation depending upon the number of electrons it will tend to lose. A polyatomic cation is formed from several atoms. It is important to note that the net charge on the polyatomic cation is the net charge of the aggregate. For example, ammonium ion = mercurous ion = Khushroo P. Daruwala 13 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. Anions: Monoatomic anions are generally formed when atoms of nonmetals gain electrons. Just like in the case of cations, the gain of one electron by a nonmetal results in an ion of charge 1 ?, while a gain of two electrons will yield an anion with a 2 ? charge. Element Z Charge Symbol of Ion oxygen (O) 8 2 ? O 2? fluorine (F) 9 1 ? F ? phosphorus (P) 15 3 ? P 3? Question: How can I determine if a nonmetal will gain any particular number of electrons? Unlike the cations for the main group elements when the number of electrons lost corresponds to the group number of the metal, the number of electrons gained by a nonmetal is not according to the group in which the nonmetal is located. However, the following two formulas are important towards determining the number of electrons gained by a nonmetal and the charge it acquires. number of electrons gained by a nonmetal = charge acquired by a nonmetal = ILLUSTRATIVE EXAMPLE: Complete the following table with the information requested: Element Group No. No. of e ? gained Charge Symbol of Ion chlorine (Cl) VII A carbon (C) IV A selenium (Se) VI A nitrogen (N) V A Polyatomic anions are negatively charged particles that contain more than one atom. Once again, as in the case of polyatomic cations, the net charge on the polyatomic anions is the charge of the aggregate. Some examples are: nitrate ion = sulfite ion = phosphate ion = According to G. N. Lewis, the electrons of an atom are located or arranged in shells about the nucleus. The electrons that are located in the outermost shell (i.e. the one that is located the furthermost from the nucleus) are called the valence electrons. (The outermost shell of an atom is also known as the valence shell). The valence electrons are the ones that are involved in bond formation. The number of valence electrons for the main or representative group elements (only) will always equal the group number in which that element is located. ILLUSTRATIVE EXAMPLE: Give the number of valence electrons for each one of the following main or representative group elements. calcium (Ca) = ; arsenic (As) = ; krypton (Kr) = . Khushroo P. Daruwala 14 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. It is important to note that: a) The electrons lost by a metallic element to form a cation are its valence electrons. b) When a nonmetallic element gains electrons, the entering or newly acquired electrons will always be lodged in the valence or outermost shell. Rule for the number of electrons lost by the main or representative group metal or gained by the nonmetals. ?The total number of electrons present in a cation formed from the loss of electrons by a main or representative group element and the number of electrons present in an anion formed from the gain of electrons by a nonmetal will always be equal to the number of electrons present in a noble gas.? a) For cations: Atoms of the main or representative group metals will lose electrons to acquire the same number of electrons that are present in the nearest noble gas of lower atomic number. In other words, the resultant cation (of the main or representative elements) will be said to be isoelectronic to (i.e. have the same number of electrons as) the noble gas that is located in the period above it. Element Z No. of e ? lost Symbol of Ion No. of e ? in cation Nearest noble gas of lower Z No. of e ? in the noble gas (i.e. its Z) potassium (K) 19 1 K + 18 argon (Ar) 18 magnesium (Mg) 12 2 Mg 2+ 10 neon (Ne) 10 aluminum (Al) 13 francium (Fr) 87 HOWEVER, from the fourth period onwards, for main or representative group elements located in groups to the right of the transition element groups (i.e in Groups III A to Group VIII A or 0), the number of electrons present in their cations will NOT be isoelectronic to the noble gas that is located in the period above it. Element Z No. of e ? lost Symbol of Ion No. of e ? in cation Nearest noble gas of lower Z No. of e ? in the noble gas (i.e. its Z) gallium (Ga) 31 3 Ga 3+ 28 argon (Ar) 18 lead (Pb) 82 b) For anions: Atoms of nonmetals will gain sufficient electrons to achieve the same number of electrons that are present in the nearest noble gas of higher atomic number. In other words, the resultant anion will be isoelectronic to the noble gas that follows it in the same period. Element Z No. of e ? gained Symbol of Ion No. of e ? in anion Nearest noble gas of higher Z No. of e ? in the noble gas (i.e. its Z) carbon (C) 6 4 C 4? 10 neon (Ne) 10 phosphorus (P) 15 selenium (Se) 34 2 Se 2? 36 krypton (Kr) 36 iodine (I) 53 Khushroo P. Daruwala 15 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. IONIC COMPOUNDS: Compounds that result from the electrostatic attraction of ions of opposite charge are called ionic compounds. An ionic bond is created when cations formed from the loss of electrons are attracted by anions formed by the gain of electrons. An ionic compound therefore consists of a fixed geometric arrangement of cations and anions so as to form a crystalline solid. The formula of an ionic compound thus indicates the simplest ratio of each type of ions required to produce a crystal. Since the crystal of an ionic compound is considered to be electrically neutral, the total charge of all its positive ions will be equal to the total charge on all its negative ions. The simplest combination of cations and anions showing electrical neutrality in an ionic compound is known as the formula unit. (It is important to note that the formula unit of an ionic compound is actually a hypothetical unit since no discrete formula unit exists within its crystal lattice). Naming Ionic Compounds: A) Binary Compounds (Type I: Ionic, where metals form only one type of cation). The steps for writing the formula and the name of the binary compounds of this type are as follows: a) When writing the formula of an ionic compound, the symbol of the cation is written before that of the anion. b) There are always two words in the name of a simple ionic compound. The name of the cation is always written before that of the anion. A suffix ?ide? is attached to the anion. Cation Anion Formula of the binary ionic compound Name of the binary ionic compound K + Br ? KBr potassium bromide Ca 2+ Cl ? CaCl 2 calcium chloride Al 3+ F ? Na + S 2? Ba 2+ Se 2? Al 3+ O 2? Li + N 3? Mg 2+ N 3? Al 3+ P 3? B) Binary Compounds (Type II: Ionic, where metals form more than one type of a cation). The steps for writing the formula and the name of the binary compounds of this type are the same as shown above. However, a Roman numeral is included in parenthesis right after the name of the metal cation to indicate its charge in the compound. As in the category A) above, there are two words to the name and the ending or suffix for the anion is still ?ide?. The transition metals and those metals of Groups III A, IV A and V A fall in this category. If old Latin names are employed for naming the metal cation, then the suffix for the metal cation with the lower charge is ?ous?, while the suffix or ending for the metal cation with the higher charge is ?ic?. Khushroo P. Daruwala 16 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. Cation Anion Formula of the binary ionic compound Name of the binary ionic compound Cu + Br ? CuBr copper (I) bromide (or cuprous bromide) Cu 2+ I ? CuI 2 copper (II) iodide (or cupric iodide) Cr 2+ Cl ? Cr 3+ O 2? Fe 2+ S 2? Fe 3+ O 2? Sn 2+ Cl ? SnCl 2 tin (II) chloride (or stannous chloride) Sn 4+ O 2? Hg 2 2+ Br ? C) Ionic Compounds with polyatomic ions: Some important points to make note of are i) Ionic compounds with polyatomic ions are not considered to be binary compounds. ii) Most of the polyatomic anions are ?oxyanions? since they contain an atom of a given element (either a metal or a nonmetal) and different numbers of oxygen atoms. iii) The suffix for the oxyanions with a fewer number of oxygen atoms is ?ite?, while those that have a greater number of oxygen atoms is ?ate?. iv) For oxyanions that contain a halogen atom, four possible polyatomic ions are possible. They are hypobromite = BrO ? bromite = BrO 2 ? bromate = BrO 3 ? perbromate = BrO 4 ? Cation Anion Formula of the binary ionic compound Name of the binary ionic compound K + NO 2 ? KNO 2 potassium nitrite Sr 2+ NO 3 ? Sr(NO 3 ) 2 strontium nitrate Fe 2+ SO 3 2? FeSO 3 iron (II) sulphite (or ferrous sulphite) Cr 3+ SO 4 2? Cs + PO 3 2? Ba 2+ PO 4 3? Cu 2+ ClO ? NH 4 + SO 4 2? NH 4 + PO 4 3? Khushroo P. Daruwala 17 ? All rights reserved (08/2007). Atoms, Molecules, and Ions. D) Binary Compounds (Type III: Covalent). For these compounds no distinct ions are present in them since they are essentially made of nonmetallic elements. The two nonmetals in these compounds are bonded together by means of covalent bonds where a sharing of electrons between them is involved. Thus, these compounds are also called binary molecular compounds. The rules for the naming of these compounds is as follows: i) The element that is written first in the given formula is named first. ii) If the number of the first element is more than one, then prefixes are used to indicate its number. However, if only one atom of the first element is present in the given formula no prefix is used. In other words, the prefix ?mono? is not used. iii) The second element in the given formula of a binary covalent compound is named as anion with the ending or suffix as ?ide?. iv) Prefixes are used to indicate the number of the second element irrespective if it is one or more than one. Formula Compound Name Formula Compound Name CO carbon monoxide CO 2 carbon dioxide N 2 O NO 2 nitrogen dioxide IF 5 IBr PCl 3 S 2 Cl 2 N 2 O 4 P 2 O 5 E) Acids: Acids are compounds that contain one or more H + ions. In acids, the H + ion will be bonded to an anion of a nonmetal or an oxyanion. There are two words in the name of an acid, of which the second word is always ?acid?. The rules for the naming of these compounds is as follows: i) If the anion of the acid is not an oxyanion but the anion of a nonmetal, then the prefix ?hydro? and the suffix ?ic? are used in the name ii) The naming of an acid that contains an oxyanion will depend upon the number of oxygen atoms in the oxyanion. a) The suffix for the oxyanion with the fewer number of oxygen atoms is ?ite?. The acids derived from these oxyanions will have the suffix ?ous? in their names. b) The suffix for the oxyanion with the greater number of oxygen atoms is ?ate?. The acids derived from these oxyanions will have the suffix ?ic? in their names. Formula Compound Name Formula Compound Name HBr hydrobromic acid H 2 Se HNO 2 nitrous acid HNO 3 nitric acid HBrO HBrO 2 HBrO 3 HBrO 4 visitor Microsoft Word - ch2lo.doc
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