Atomic Structure and Electronic Configuration

Every atom is made up of three subatomic particles: protons (positive charge), neutrons (no charge), and electrons (negative charge). The protons and neutrons hang out in the nucleus at the centre, while electrons zoom around in shells.

When atoms lose or gain electrons, they become ions with an overall charge. This happens because the number of positive protons no longer matches the number of negative electrons.

Electronic structure tells you how electrons are arranged in shells around the nucleus. For example, calcium has 20 electrons arranged as 2,8,8,2. Since it has 2 electrons in its outer shell, calcium sits in group 2 of the periodic table.

Quick Tip: The number of electrons in the outer shell determines which group an element belongs to on the periodic table!

Mass Numbers and Isotopes

The mass number (A) equals the total number of protons plus neutrons, whilst the atomic number (Z) just counts the protons. Think of isotopes as different versions of the same element - they have identical numbers of protons but different numbers of neutrons.

Relative atomic mass is the average mass of an element's isotopes, calculated using their abundance percentages. For chlorine: (35% × 35) + (65% × 37) ÷ 100 = 35.5.

Relative formula mass (Mr) adds up all the atomic masses in a compound. For example, CO₂ = (1 × 12) + (2 × 16) = 44.

Remember: OIL RIG - Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). This will save you in chemistry exams!