In a chemical reaction, matter cannot be created or destroyed. mass of products = mass of reactants
The Law of definite proportions or constant composition (Joseph Proust)
All samples of a given pure compound, regardless of their source or how they were made always contain same elements in the same parts (fraction) of mass.
The Law of Law of multiple proportions (John Dalton)
when two elements A and B, react to form more than one compound, the different masses of B that combine with a fixed mass (1g) of A can be expressed as a ratio of small whole numbers.
-smallest particle of an element. -splits into subatomic particles. such as: electrons (-ve), protons (+ve) and neutrons (neutral) -concept of splitting atom started with discovery of: cathode rays, x-rays, the electrons.
The electron (J.J Thompson)
Discovered the electron while working on his cathode ray experiment. proposed that cathode rays consist of tiny negatively charged particles, which we now call electrons. measured the charge: mass ratio of the electron. mass/charge = -1.76x108 coulombs.
The electron (Robert Millikan)
used the oil drop experiment to calculate the charge on a single electron.\ charge on any drop was always a whole number multiple of -1.6022x10-19 coulombs. further calculated the mass of an electron to be 9.10x10-28 g.
The atomic model
J.J Thompson proposed the "plum pudding" or "chocolate chip cookie" model of the atom. atoms consists of diffused cloud of positive charges and electrons are embedded randomly in it.
Henri becquerel and the curies ( Marie and Pierre) discovered and studied radioactivity. 3 types of rays produced: alpha (+ve), beta (-ve) and gamma (no charge).
Ernest Rutherford (1871-1937)
used alpha particles to study the inner structure of gold atom. beam of dense, positively charged alpha particles emitting from radium was aimed at thin gold foil. demonstrated the existence of a positively charged nucleus, a dense central core within the atom.
Rutherford's nuclear atom
majority of the particles penetrated the foil undeflected. a few particles were deflected. occasionally a very few particles were deflected at very large angles. thus he proposed nuclear atom of the atom. an atom consists of a tiny massive center of positive charge (the nucleus) surriunded by electrons, which are moving around the nucleus.
Atomic no. (Z)
no. of protons in the nucleus of an atom. determines the chemical identity of an atom.
Mass no. (A)
= no. of protons + no. of neutrons= atomic no. + no. of neutrons.
weighted average of the masses of the isotopes of the element. masses of atomic particles expressed in atomic mass unit (u). mass of isotope 1 in amu) x (% abundance isotope1/100)
Mendeleev"s periodic table
Dmitri Mendeleev. listed the elements in order of their atomic masses and found that elements with similar chemical properties fell in the same column. proposed table base on observation supported by scanty understanding.
atoms with same atomic no. but different mass nos.
The periodic law
Fundamental basis of the periodic table is atomic no. rather that atomic mass.
Modern periodic table.
known elements are arranged in order of increasing atomic no. (z). each element is represented by its symbol and atomic mass.
Periodic table contains
8 principle columns called groups (also called families) and 7 main horizontal rows called periods. elements can be classified as metals, non-metals, metalloids or semi-metals. main group elements- A transition elements or metals- B
-mostly solids -good electrical conductors. -good heat conductors. -lustrous (shiny) -malliable and ductile -lose electrons during chemical change.
-many are gases -poor conductors or non-conductors -poor heat conductors -dull in appearance -brittle -gain electrons during chemical change.
diatomic polyatomic polyatomic monoatomic
Groups of atoms in which one or more pairs of electrons are shared between bonded atoms. diatomic: H2 , N2 , F2, HCl and CO polyatomic: H2 O, NH3, CH4,
An atom or groups of atoms with a net positive or negative charge. monatomic: Na+, S2- polyatomic: OH-, NH4
an ion with positive charge. formed when an atom looses one or more electrons.
an ion with negative charge. formed when an atom gains one or more electrons.
A mole: a chemist's dozen
Latin word meaning "a huge pile". amount of any substance that contains the same no. of entities (atoms, molecules or ions) as there are in exactly 12g of carbon-12 isotopes. 1 mole of atoms=6.022x1023 atoms=avogadro's no. (NA)
chemical bonds (ionic bond)
transfer of one or more elements between a metal and a non-metal.
chemical bonds (covalent bond)
sharing of a pair or more of electrons between non-metals.