Ions

• Compounds composed of metals and non-metals contain charged species. The charged species are known as ions.
• A negatively charged ion is called an “anion” and a positively charged ion is called a ” cation”. Ex-NaCl where Na⁺ and Cl^–
• Ions may consist of a single charged atom or a group of atoms that have a net charge on them.

Polyatomic ion: A group of atoms carrying a charge is known as a polyatomic ion.

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

Characteristics of chemical formulae:

• The valencies or charges on the ion must balance.
• When a compound is formed of metal and non-metal, symbol of metal comes first. Ex- CaO, NaCl, CuO.
• When polyatomic ions are used, the ions are enclosed in brackets before writing the number to show the ratio. Ex- Ca(OH)₂, (NH₄)₂SO₄

Rules for writing chemical formulae

1. We first write symbol of element which form compound.
2. Below the symbol of each element, we should write their valency.
3. Now cross over the valencies of combining atoms.
4. With first atom, we write the valency of second atom ( as a subscript)
5. With second atom, we write the valency of first atom( Subscript).