Atomic Theory

Diatomic molecules of elements:

These molecules are made up of two like atoms. The following seven elements are all made up of diatomic molecules: H2, O2, N2, F2, Cl2, Br2, I2.

Diatomic molecules of compounds:

These molecules are made up two unlike atoms. For example, HCl, NaCl, CaO, NO, etc.

Triatomic molecules: Atomicity = 3

These molecules are made up of three atoms only. Triatomic molecules could belong to both elements and compounds.

Triatomic molecules of elements:

These molecules are made up of three like atoms. For example, O3 – ozone.

Triatomic molecules of compounds:

These molecules are made up of at least two different types of atoms. For example, H2O,  MgCl2, NO2, Na2O etc….

Tetraatomic molecules: Atomicity = 4

These molecules are made up of four atoms only. Tetraatomic molecules could belong to either elements or compounds.

Tetraatomic molecules of elements:

These molecules are made up of four like atoms. For example, P4 – white or yellow phosphorus.

Tetraatomic molecules of compounds:

These molecules are made up of at least two types of atoms. For example, AlCl3. PH3, C2H2, SiO3 etc….

FORMULAE OF COMPOUNDS

     A formula of a compound gives:

a.    The elements which make up the compound. The symbols of these elements are found in the formula.

b.    The number of atoms of the elements present. This is indicated in the formula by a number placed to the right and below the element concerned.

Examples: CuO     = 1 atom of copper and   atom of oxygen

H2SO4 = 2 atoms of hydrogen, 1 atom of sulfur and 4 atoms of oxygen

Ca (NO3)2 = 1 atom of calcium, 2 atoms of nitrogen and 6 atoms of   oxygen

     (NH4)2SO4= 2 atoms of nitrogen, 8 atoms of hydrogen, 1 atom of sulfur

      and 4 atoms of oxygen

Empirical Formulae

The empirical formula of a compound is the simplest formula for that compound.