Particles in the Atom and Atomic Radius

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Understand that atoms are mostly empty space surrounding a very small, dense nucleus that contains protons and neutrons; electrons are found in shells in the empty space around the nucleus
  2. Identify and describe protons, neutrons and electrons in terms of their relative charges and relative masses
  3. Understand the terms atomic and proton number; mass and nucleon number
  4. Describe the distribution of mass and charge within an atom
  5. Describe the behaviour of beams of protons, neutrons and electrons moving at the same velocity in an electric field
  6. Determine the numbers of protons, neutrons and electrons present in both atoms and ions given atomic or proton number, mass or nucleon number and charge
  7. State and explain qualitatively the variations in atomic radius and ionic radius across a period and down a group

1. The Structure of the Atom

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Atoms Are Mostly Empty Space

An atom is the smallest part of an element that can take part in a chemical change. Atoms are incredibly tiny - a hydrogen atom has a diameter of about 10⁻¹⁰ metres (that's 0.0000000001 metres).

Here's something surprising: atoms are mostly empty space. If you imagine an atom blown up to the size of a football stadium, the nucleus (the centre of the atom) would only be about the size of a pea in the middle of the pitch. The rest of the stadium would be empty space where electrons move around.

The Nucleus

At the very centre of every atom is the nucleus. This is an extremely small, dense region where nearly all of the atom's mass is concentrated.

The nucleus contains two types of particles called nucleons:

  • Protons - positively charged particles
  • Neutrons - particles with no charge (neutral)

The nucleus is positively charged overall because it contains protons.

Electrons and Electron Shells

Outside the nucleus, electrons move around in regions of space. Electrons are negatively charged particles that are much, much smaller and lighter than protons or neutrons.

We can think of electrons moving in electron shells (also called energy levels) around the nucleus. Each shell is at a certain distance from the nucleus and has its own energy level. The shells are numbered n = 1, n = 2, n = 3, and so on, with n = 1 being closest to the nucleus.

In a neutral atom (an atom with no overall charge), the number of electrons equals the number of protons. This means the positive charges and negative charges balance out.

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