The Atom

The Atom

Everything in the universe is made of atoms. They’re incredibly small—so small that millions could fit on the head of a pin. Atoms are the building blocks of all matter.

What is an atom?

An atom is the smallest part of an element that can still be recognised as that element. Every element on the periodic table is made of atoms, and all atoms of a particular element have the same number of protons.

Atoms are neutral overall, meaning they contain the same number of positive protons and negative electrons.

The structure of the atom

Atoms have a tiny, dense nucleus at their centre. This nucleus contains:

Particle

Charge

Relative Mass

Location

Proton

+1

1

Nucleus

Neutron

0

1

Nucleus

Electron

-1

Very small

Electron shells

The electrons orbit the nucleus in energy levels or shells.

💡 In a neutral atom, the number of electrons = protons.

Electron shells

Electrons are arranged in shells around the nucleus. These shells fill in a specific order:

  • 1st shell: holds up to 2 electrons
  • 2nd shell: holds up to 8 electrons
  • 3rd shell: also holds up to 8 (at GCSE level)

You’ll need to be able to draw the electronic structure of the first 20 elements.

This model shows the basic structure of an atom: a central nucleus made of protons and neutrons, with electrons orbiting in energy shells around it.
This model shows the basic structure of an atom: a central nucleus made of protons and neutrons, with electrons orbiting in energy shells around it.

Atomic number and mass number

Each atom has two key numbers:

  • Atomic number – the number of protons
  • Mass number – the total number of protons + neutrons

Since atoms are neutral, the atomic number also tells you the number of electrons.

You can calculate the number of neutrons by:

Mass number – Atomic number = Number of neutrons

Example:

Element

Atomic Number

Mass Number

Protons

Neutrons

Electrons

Chlorine

17

35

17

18

17

What are isotopes?

Isotopes are atoms of the same element with:

  • The same number of protons
  • A different number of neutrons

They have the same chemical properties but different masses.

Example:

Isotope

Protons

Neutrons

Electrons

Carbon-12

6

6

6

Carbon-14

6

8

6

Questions 

  1. What are the three subatomic particles found in an atom?
  2. What is the charge and mass of a neutron?
  3. What does the atomic number tell you?
  4. How do you calculate the number of neutrons in an atom?
  5. What is an isotope?

Summary 

  • Atoms are the smallest particles of elements

  • They consist of a nucleus (protons and neutrons) and electrons in shells.

  • The atomic number tells you the number of protons (and electrons in a neutral atom).

  • The mass number is the total of protons and neutrons.

  • Isotopes have the same number of protons but different numbers of neutrons.