Explain why atoms have no overall electrical charge, and describe how the number of protons in an atom determines its identity as a specific element.

Edexcel A-Level Chemistry (9CH0) — 1.1 Atomic structure · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

All matter is made up of atoms. Each atom contains a nucleus surrounded by electrons arranged in shells. The nucleus contains protons and neutrons. The relative charges and masses of these subatomic particles are well established.

Model answer (5 marks)

Protons have a relative charge of +1 and electrons have a relative charge of –1. In a neutral atom the number of protons equals the number of electrons, so the positive and negative charges cancel and the atom has no overall electrical charge.

The number of protons in an atom is called the atomic number. Each element has a unique atomic number, so the proton number identifies which element the atom is.

Examiner tips

  • Use the terms ‘protons’, ‘electrons’, ‘atomic number’ and ‘neutral atom’
  • Show the charge balance explicitly: +1 × protons = –1 × electrons

Common mistakes

  • Confusing neutrons with electrons as charge carriers
  • Saying the atom has no charge because it has no neutrons
  • Not linking the atomic number to the identity of the element

Mark scheme (5 marks)

  1. Protons have a relative charge of +1 (positive)
  2. Electrons have a relative charge of −1 (negative)
  3. The number of protons equals the number of electrons, so the charges cancel / balance out, giving no overall charge
  4. The number of protons in an atom is called the atomic number (proton number)
  5. Each element has a unique number of protons / atomic number, so the number of protons identifies which element the atom is

Key terms in this question

proton

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