The nuclear model of the atom

O Level / IGCSE Physics · Nuclear physics. 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 has a tiny, positively charged nucleus with negatively charged electrons in orbit around it. The nucleus contains protons (positive) and neutrons (no charge), and it holds nearly all the mass of the atom.

The evidence comes from firing alpha particles at thin gold foil. Most went straight through, so the atom is mostly empty space. A few were deflected, and a very small number came back, so there must be a small, massive, positive centre repelling them.

The proton number fixes which element it is. Isotopes are atoms of the same element with different numbers of neutrons. A neutral atom has equal numbers of protons and electrons, and becomes an ion when it loses or gains electrons.

Rules to remember

Common mistake

Giving the nucleon number as the number of neutrons. The nucleon number counts protons and neutrons together, so subtract the proton number to get the neutrons.

Worked example

An ion of potassium, 3919K, has a charge of 1+. How many protons, neutrons and electrons does the ion have?

  1. Protons = proton number = 19.
  2. Neutrons = A − Z = 39 − 19 = 20.
  3. A neutral atom would have 19 electrons. A charge of 1+ means one electron has been lost: 19 − 1 = 18.

Answer: 19 protons, 20 neutrons, 18 electrons

Practice questions

Try each one, then open the answer.

1. Which statement about the nucleus of an atom is correct?

  1. A
    it contains protons and electrons
  2. B
    it is negatively charged
  3. C
    it contains almost all of the atom's mass in a very small volume
  4. D
    it is about the same size as the atom
Show answer

Answer: C. The nucleus of protons and neutrons is tiny compared with the atom but holds nearly all its mass; it is positively charged.

2. The nuclide 2311Na has how many protons, neutrons and electrons in a neutral atom?

  1. A
    11 protons, 12 neutrons, 11 electrons
  2. B
    11 protons, 23 neutrons, 11 electrons
  3. C
    12 protons, 11 neutrons, 12 electrons
  4. D
    23 protons, 11 neutrons, 23 electrons
Show answer

Answer: A. Proton number Z = 11; nucleon number A = 23, so neutrons = 23 − 11 = 12. A neutral atom has 11 electrons.

3. Which statement about the particles in the nucleus of an atom is correct?

  1. A
    protons are positive and neutrons are negative
  2. B
    protons and neutrons are both positive
  3. C
    protons are uncharged and neutrons are positive
  4. D
    protons are positive and neutrons are uncharged
Show answer

Answer: D. The nucleus is made of protons (positive) and neutrons (no charge), so it is positive overall. The negative electrons are outside the nucleus.

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