Particles in the atom and atomic radius

AS Level Chemistry · Atomic structure. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

An atom is mostly empty space. At its centre is a tiny, dense nucleus that holds the protons and neutrons, so it carries all the positive charge and almost all the mass. The electrons move in shells in the space around it.

The proton number tells you which element the atom is. The nucleon number is protons plus neutrons. In a neutral atom, electrons equal protons. An ion has gained or lost electrons only.

The size of an atom or ion depends on how strongly the nucleus pulls on the outer electrons: more protons pull them in, more shells push them out.

Rules to remember

Common mistake

Students change the number of protons when they work out an ion. Only the electrons change: protons and neutrons stay exactly as in the atom.

Worked example

How many protons, neutrons and electrons are in one 2713Al3+ ion?

  1. Protons = proton number = 13.
  2. Neutrons = nucleon number − proton number = 27 − 13 = 14.
  3. Electrons = 13 − 3 = 10, because a 3+ ion has lost three electrons.

Answer: 13 protons, 14 neutrons, 10 electrons.

Practice questions

Try each one, then open the answer.

1. How many protons, neutrons and electrons are present in one 5626Fe3+ ion?

  1. A
    23 protons, 30 neutrons, 26 electrons
  2. B
    26 protons, 56 neutrons, 23 electrons
  3. C
    26 protons, 30 neutrons, 29 electrons
  4. D
    26 protons, 30 neutrons, 23 electrons
Show answer

Answer: D. Proton number 26 gives 26 protons; neutrons = 56 − 26 = 30. A 3+ ion has lost three electrons: 26 − 3 = 23. The common slip is adding electrons for a positive charge.

2. The ions Na+, Mg2+, O2− and F− all have ten electrons. Which has the largest ionic radius?

  1. A
    Na+
  2. B
    Mg2+
  3. C
    F−
  4. D
    O2−
Show answer

Answer: D. With the same number of electrons, the ion with the smallest nuclear charge (O, 8 protons) attracts them least strongly, so it is the largest. Mg2+ (12 protons) is the smallest.

3. Why does atomic radius decrease across Period 3 from sodium to chlorine?

  1. A
    nuclear charge increases while electrons are added to the same shell, so shielding stays about the same and the outer electrons are pulled closer
  2. B
    the number of occupied electron shells decreases
  3. C
    the outer electrons repel each other more strongly as more are added
  4. D
    shielding by inner electrons increases greatly across the period
Show answer

Answer: A. Across the period protons are added to the nucleus but the outer electrons all go into the third shell, with the same inner-shell shielding. The greater effective nuclear charge pulls the outer shell in.

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