# 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

## 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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