A lithium atom has an atomic number of 3 and a mass number of 7. Describe the structure of a lithium atom in terms of its subatomic particles, including their charges, relative masses, and locations within the atom.

OCR GCSE Chemistry A: Gateway Science (J248) — C1.2 Atomic structure · Describe · 4 marks · View as Markdown

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

Lithium is a soft metal used in rechargeable batteries. Understanding its atomic structure helps chemists predict how it will react.

Model answer (4 marks)

A lithium atom has 3 protons in its nucleus, each with a +1 charge and a relative mass of 1. It also has 4 neutrons, which are neutral and have a relative mass of 1. The nucleus therefore contains 7 nucleons and accounts for almost all of the atom’s mass. Three electrons orbit the nucleus in shells or energy levels; each electron carries a –1 charge and has a negligible mass compared with the protons and neutrons. Because the nucleus is extremely small compared with the overall size of the atom, most of the atom is empty space.

Examiner tips

  • State the number and type of subatomic particles and their charges. Mention the relative masses of protons, neutrons and electrons. Explain that the nucleus contains almost all the mass and is much smaller than the atom.
  • Use the exact wording from the mark scheme (e.g. "nucleus", "protons", "neutrons", "electrons", "relative mass", "charge").

Common mistakes

  • Confusing the number of neutrons (writing 3 instead of 4). Forgetting to note that electrons have negligible mass. Using vague terms like "most of the mass" without specifying the nucleus.”

Mark scheme (4 marks)

  1. The nucleus contains 3 protons (positively charged, relative mass 1) AND 4 neutrons (neutral, relative mass 1)
  2. 3 electrons orbit the nucleus / are found in shells / energy levels around the nucleus
  3. Electrons are negatively charged with negligible / very small relative mass
  4. Most of the mass is in the nucleus / the nuclear radius is much smaller than the atomic radius / the atom is mostly empty space

Key terms in this question

atomic number · mass number · subatomic particles

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