GCSE Chemistry: Atomic Structure and the Periodic Table Guide

Structure of the atom

An atom has a nucleus containing protons (positive charge) and neutrons (no charge), surrounded by electrons (negative charge) in shells. The number of protons equals the number of electrons in a neutral atom.

Atomic number and mass number

The atomic number is the number of protons in an atom, which defines the element. The mass number is the total number of protons and neutrons. Isotopes are atoms of the same element with different numbers of neutrons.

Electron configuration

Electrons fill shells in a set order, with the first shell holding up to 2 electrons, and the second and third shells holding up to 8 each. An element's electron configuration (e.g. 2,8,1 for sodium) determines its chemical properties.

The periodic table

Elements are arranged in order of atomic number. Groups (columns) contain elements with the same number of outer electrons and similar properties. Periods (rows) show a pattern of properties as atomic number increases across each row.

Common mistakes

• Mixing up atomic number (protons) and mass number (protons + neutrons).
• Forgetting that isotopes have the same chemical properties but different masses.
• Confusing groups (columns, similar properties) with periods (rows).