Atoms And Molecules

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The chemical formula of a compound is a symbolic representation of its composition.

Valency is the combining capacity of an element.

Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound.

Rules for writing the formula of a compound:

1. A formula of the compound is given by writing side by side the symbols of constituent elements.
2. Symbol of the more metallic element is written first in the formula.
3. Number of atoms of each of the constituent elements present in the molecule is indicated by the subscript
4. When either of the ions or both the ions are polyatomic and their valency is more than 1, we enclose the polyatomic ions in brackets. No brackets are necessary if the valency(ies) of polyatomic ion(s) is(are) 1.
5. While writing the formula of the compound if the valency numbers have a highest common factor (H.C.F), divide the valency numbers by H.C.F to get the simple ratio between the combining elements.

The simplest compounds, which are made up of two different elements are called binary compounds.

The formula of a binary compound is written by crisscrossing the valencies of elements present in a molecule of the compound.

Formula of hydrogen sulphide and water.

A chemical compound is always electrically neutral, so the positive and negative valencies or charges of the ions in the compound must add up to zero.