Explain why two atoms of the same element may behave identically in chemical reactions yet have different masses.

IB DP Chemistry Standard Level (2023 syllabus) — S1.2 The nuclear atom · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Atoms of the same element have the same number of protons, which defines the element and its chemical behaviour. For neutral atoms the number of electrons is also the same, so the electrons involved in bonding are identical and the atoms react in the same way. However, the atoms can have different numbers of neutrons, making them isotopes. Neutrons add to the mass but carry no charge and do not take part in chemical bonding, so the masses differ while the chemical behaviour remains the same.

Examiner tips

  • Mention atomic number and electron count for identical reactivity
  • Explain isotopes and neutron contribution to mass
  • Use terms ‘isotopes’, ‘neutrons’, ‘chemical behaviour’

Common mistakes

  • Confusing isotopes with ions
  • Saying the atoms have different numbers of protons
  • Ignoring the role of neutrons in mass

Mark scheme (4 marks)

  1. Atoms of the same element have the same number of protons (atomic/proton number), which determines the element's identity and chemical behaviour.
  2. The number of electrons is also the same (for neutral atoms), and since chemical reactions involve electrons, the atoms react identically.
  3. The atoms have different numbers of neutrons, making them isotopes of each other.
  4. Neutrons contribute to mass but carry no charge and do not participate in chemical bonding, so differing neutron numbers cause different masses without affecting chemical behaviour.

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