Chapter Overview 2.1 Chemistry is Critical to Life 2.2 The Nature of Matter: The Atom 2.3 Matter is Transformed Through Chemical Bonding 2.4 Molecules Have a Three-Dimensional Shape The Biological Hierarchy Life can be organized into a number of levels The Atom Matter is composed of many very small units called atoms 100 million carbon atoms, lined end to end, would stretch only about 3 centimeters The Atom Atoms are composed of smaller parts “Subatomic particles” Protons Neutrons Electrons Atomic Structure The Atom Atoms possess positive charges in the form of protons Atoms possess negative charges in the form of electrons The number of protons is typically equal to the number of electrons As a result, the atom is uncharged The Atom Charge Position Protons + nucleus Neutrons 0 nucleus Electrons - outside nucleus Protons and neutrons are packed together to form an atom’s nucleus The much smaller electrons exist outside of the nucleus Chemistry Symbols Each chemical element has its own letter-based symbol H = hydrogen He = helium C = carbon O = oxygen Pt = platinum Etc. The Atom Each element is defined by the number of protons present in its atoms This is an element’s atomic number e.g., All hydrogen atoms possess 1 proton e.g., All helium atoms possess 2 protons e.g., All carbon atoms possess 6 protons e.g., All gold atoms possess 79 protons Atomic Structure (cont.) All atoms of an element have the same number of protons, the atomic number. The atomic number and mass number are often included with the chemical symbol. 12 6 C Mass number Atomic number Chemical symbol The Atom An atom’s nucleus possesses protons and neutrons The number of protons defines the element The number of neutrons can be slightly variable Isotopes are different forms of a single element Possess different numbers of neutrons The Atom Most elements possess multiple isotopes e.g., Hydrogen, carbon, iodine, etc. Some of these isotopes are radioactive e.g., Hydrogen-3 (tritium), carbon-14, etc. Some of these isotopes are used in medicine Uses of Radioactive Isotopes Radioactive isotopes can be used as tracers to follow the movement of that element. Radioactive isotopes can be used to sterilize medical and dental instruments. Radioactive isotopes can also be used to kill cancer cells. The Atom An atom’s electrons are involved in combining the atom with other atoms “Chemical bonds” When atoms come into contact, their outermost electrons can be exchanged or rearranged The atoms become attached through chemical bonds Chemical Bonding Why form chemical bonds? When atoms bond with each other, they can move to a lower, more stable energy state Such reactions will tend to occur “Energy always seeks its lowest state” Chemical Bonding Electrons reside in well-defined “energy levels” or “energy shells” The first shell can hold 2 electrons Each of the next few shells can hold 8 electrons Chemical Bonding Atoms are most stable when their outer electrons shells are completely filled Sometimes this energy shell is normally completely filled e.g., He, Ne, Ar, etc. Other times, this shell becomes completely filled as a result of chemical bonding Chemical Bonding Chemical bonding in water (H2O) Oxygen shares one pair of electrons with a hydrogen atom and another pair of electrons with a second hydrogen atom All electron shells are full The sharing of a pair of electrons is called a covalent bond Chemical Bonding When two or more atoms bond covalently, a molecule is formed e.g., One oxygen atom and two hydrogen atoms form a single water molecule e.g., Sucrose = C12H22O11 Chemical Bonding Molecules are held together by covalent bonds Sharing of a pair of electrons This sharing of electrons may be equal e.g., Two hydrogen atoms form H2 “Nonpolar covalent bond” Chemical Bonding Molecules are held together by covalent bonds Sharing of a pair of electrons This sharing of electrons may be unequal e.g., H-O-H (H2O) The electrons spend most of their time near the oxygen atom (ELECTRONEGATIVE) Oxygen has a partial negative charge Hydrogens have partial positive charge “Polar covalent bond” Chemical Bonding Some atoms share electrons Covalent bonds Some atoms gain or lose electrons e.g., Sodium (Na), Chlorine (Cl), etc. Chemical Bonding A sodium atom has one electron in its outer shell If this electron is lost The outer shell is completely full The atom is more stable The atom has a positive electrical charge Positive “ion” Na+ Chemical Bonding A chlorine atom has seven electrons in its outer shell If it gains an electron The outer shell is completely full The atom is more stable The atom has a negative electrical charge Negative “ion” Cl- Chemical Bonding Sodium ion (Na+) and Chloride ion (Cl-) can interact This interaction is termed an ionic bond The compound NaCl (sodium chloride) is formed when many positive and negative ions interact 1:1 ratio Chemical Bonding Water molecules have polar covalent bonds Hydrogen atoms: partial positive charges Oxygen atoms: partial negative charges A partial positive charge on one water molecule can interact with a partial negative charge on another water molecule “Hydrogen bond” Other molecules with partial charges can also form hydrogen bonds Chemical Bonding Individual hydrogen bonds are relatively weak Large numbers of hydrogen bonds can have a major effect on the shapes of molecules e.g., Hydrogen bonds hold the two strands of a DNA molecule together Shapes of Molecules Molecules depicted on paper appear to be two-dimensional These molecules actually possess a three-dimensional shape Shapes of Molecules The three-dimensional shape of a molecule can be important in their function Particularly when this molecule interacts with another molecule Can you shake my hand while it is making a fist?