Atomic structure and the Periodic Table

O Level / IGCSE Chemistry · Atoms, elements and compounds. 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 central nucleus containing protons and neutrons, with electrons moving around it in shells. Almost all the mass of the atom is in the nucleus. An atom is neutral because it has equal numbers of protons and electrons.

The proton number tells you which element the atom is. The nucleon number is the total number of protons and neutrons.

Electrons fill the shells from the inside outwards: up to 2 in the first shell, then 8, then 8. The arrangement, such as 2,8,3 for aluminium, shows the position in the Periodic Table. An ion has the same nucleus as its atom, but electrons have been lost or gained.

Rules to remember

Common mistake

For the configuration of an ion, students often use the proton number unchanged. First add electrons for a negative ion or remove electrons for a positive ion, then fill the shells.

Worked example

An atom of potassium has proton number 19 and nucleon number 39. How many neutrons does it have, where is potassium in the Periodic Table, and what is the electronic configuration of the ion K+?

  1. Neutrons = nucleon number − proton number = 39 − 19 = 20.
  2. The atom has 19 electrons: 2,8,8,1. One outer electron means Group I; four occupied shells means Period 4.
  3. K+ has lost one electron, so it has 19 − 1 = 18 electrons: 2,8,8.

Answer: 20 neutrons; Group I, Period 4; K+ is 2,8,8.

Practice questions

Try each one, then open the answer.

1. An element is in Group II and Period 4 of the Periodic Table. What is the electronic configuration of its atom?

  1. A
    2,8,8,2
  2. B
    2,8,2
  3. C
    2,8,4
  4. D
    2,2,8,8
Show answer

Answer: A. Period 4 means four occupied shells; Group II means two outer electrons. 2,8,8,2 is calcium.

2. An element is in Group V and Period 3 of the Periodic Table. What is its proton number?

  1. A
    5
  2. B
    13
  3. C
    15
  4. D
    18
Show answer

Answer: C. Period 3 means three occupied shells; Group V means five outer electrons. The configuration is 2,8,5, so there are 15 electrons and 15 protons (phosphorus).

3. Which row gives the relative charge and relative mass of a neutron?

  1. A
    charge +1, mass 1
  2. B
    charge 0, mass 1/1840
  3. C
    charge −1, mass 1/1840
  4. D
    charge 0, mass 1
Show answer

Answer: D. Neutron: no charge, mass 1. Proton: +1, mass 1. Electron: −1, mass about 1/1840.

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