CHAPTER 3 ATOMS AND MOLECULES
LAWS OF CHEMICAL COMBINATION
There are two laws of chemical combination:
DALTON’S ATOMIC THEORY
The postulates of this theory are:
An atom is the building block of all matter. The size of an atom is very, very small. In fact, the radius of an atom is around 1 nm i.e. 10-9 m.
Atomic Mass: One atomic mass unit(amu) is equal to 1/12th the mass of one atom of carbon 12.
A molecule is a group of two or more atoms held together by attractive forces. For e.g. CO2 is a molecule of carbon dioxide.
The charged particles in substances are called ions. A positively charged ion is called a cation and a negatively charged ion is called anion. For e.g. HCl contains H+ ions and Cl- ions.
The chemical formula of a compound is determined by the valency of each element.
For e.g. The valency of H is 1 and that of S is 2. The valencies are exchanged and the formula becomes H2S.
The molecular mass is the sum of the atomic masses of all the atoms in the molecule.
FORMULA UNIT MASS
The formula unit mass is the sum of the atomic masses of all atoms in a formula unit of the compound.
One mole of a substance is that quantity which has mass equal to its atomic or molecular mass in grams.
The number of particles present in 1 mole of any substance is constant i.e. 6.022 x 1023. This value is known as Avogadro’s Constant (N0).
The mass of 1 mole of a substance is called molar mass.