Atomic Structure — Protons, Neutrons and Electrons

This lesson covers the sub-atomic particles, atomic number, mass number and isotopes as required by AQA GCSE Combined Science Trilogy (8464, Chemistry 4.1.1). You must be able to determine the number of each sub-atomic particle from the periodic table, use atomic notation and calculate relative atomic mass.

---

Sub-Atomic Particles

Atoms are made of three types of sub-atomic particle:

Particle	Relative Charge	Relative Mass	Location
Proton	+1	1	Nucleus
Neutron	0	1	Nucleus
Electron	−1	Very small ($\approx$≈ 1/1836, negligible)	Energy levels (shells) around the nucleus

Key points:

• Protons and neutrons have approximately the same mass (relative mass of 1).
• Electrons have negligible mass compared to protons and neutrons.
• In a neutral atom, the number of protons equals the number of electrons — the charges balance to zero.
• The mass of an atom is concentrated almost entirely in the nucleus.