Atoms and Molecules - How Atoms Combine?

Valency: It is the combining capacity of an atom of an element. It determines how atoms of one element will combine with atoms of another element. For example, the valency of Carbon is $4$ and Oxygen is $2$.

Chemical Formula: The chemical formula of a compound is a symbolic representation of its composition. The valencies or charges on the ions must balance to form a neutral compound.

Rules for Writing Formulae: 1. The valencies/charges on the ion must balance. 2. When a compound consists of a metal and a non-metal, the symbol of the metal is written first (e.g., $MgO$, $NaCl$). 3. In compounds formed with polyatomic ions, the ion is enclosed in brackets before writing the number (e.g., $(NH_4)_2SO_4$).

Binary Compounds: These are compounds made up of two different elements. To write their formulae, we write the constituent elements and their valencies, then criss-cross the valencies.

Ions: Atoms combine by losing, gaining, or sharing electrons to form ions. Positively charged ions are Cations (e.g., $Na^+$, $Mg^{2+}$) and negatively charged ions are Anions (e.g., $Cl^-$, $O^{2-}$).

Molecular Mass: It is the sum of the atomic masses of all the atoms in a molecule. It is measured in atomic mass units ($u$).