The Structure of the Atom

Section 1 of 6Inside an atom

The nuclear atom

Nucleus

• Nucleus A tiny central region containing protons and neutrons.
• It is positively charged overall and holds almost all of the atom's mass.

Electrons

• Electrons Negatively charged particles that surround the nucleus in shells (energy levels).
• Their mass is so small that it is ignored when finding the mass of an atom.

Empty space

• Empty space Most of the volume of an atom is empty space between the nucleus and the outermost electrons.

The atom as a unit

• The atom as a unit The atom is the smallest part of an element that still has the properties of that element.
  • Protons, neutrons and electrons on their own do not.

Section 2 of 6Relative charges

Particle	Where found	Relative charge
Proton	Nucleus	+1
Neutron	Nucleus	0 (neutral)
Electron	Shells around the nucleus	−1

Section 3 of 6Why atoms have no overall charge

1

Each proton carries a charge of +1 and each electron carries a charge of −1.

2

In a neutral atom the number of protons equals the number of electrons.

3

The positive and negative charges are equal in size, so they cancel exactly.

4

Neutrons have no charge, so the number of neutrons does not affect whether the atom is neutral.

Section 4 of 6Forming ions

Losing electrons

• Losing electrons If an atom loses one electron, there is one more proton than electrons, so it becomes a 1+ ion.
• Losing two electrons gives a 2+ ion, and so on.

Gaining electrons

• Gaining electrons If an atom gains one electron, there is one more electron than protons, so it becomes a 1− ion.

Protons never change

• Protons never change Forming an ion changes only the number of electrons.
  • The number of protons stays the same, so the atoms remain the same element.

Worked example: magnesium

• Worked example: magnesium A magnesium atom has 12 protons and 12 electrons.
  • When it loses 2 electrons it has 12 protons and 10 electrons.
• The overall charge is +12 − 10 = +2, so the ion is Mg²⁺.

Section 5 of 6Key terms

Term	Meaning
Proton	A positively charged particle (relative charge +1) found in the nucleus.
Neutron	A particle with no charge found in the nucleus.
Electron	A negatively charged particle (relative charge −1) found in shells outside the nucleus.
Relative charge	A comparison of the charge on a particle with the charge on a proton, used because real charges are far too small to measure conveniently.
Ion	An atom or group of atoms that has gained or lost electrons, so it has an overall charge.
Nucleus	The central part of the atom, made of protons and neutrons.

Section 6 of 6Exam technique

1

Saying the nucleus contains electrons. The nucleus contains only protons and neutrons.

2

Stating that atoms are neutral because they contain neutrons. It is the equal numbers of protons and electrons that cancel the charge.

3

Forgetting that ions form by gaining or losing electrons, never by gaining or losing protons.

4

Mixing up the charge on an ion. Losing negative electrons leaves the ion positive; gaining them makes it negative.

5

Writing the charge of an electron as +1. Remember: Protons are Positive, Neutrons are Neutral, so electrons must be negative.