Atomic Structure Revision Questions and Answers (O level Chemistry)

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Question 1

The table shows some information about six particles.

  1. Complete the table.
  2. ParticleProton (atomic) numberNumber of neutronsNumber of electrons
    1. What is meant by the term isotopes?
    2. Identify two atoms which are isotopes of the same element.

Solution 1

  1. Explanation on how to fill the table
    • First row – Number of electrons = number of protons in a neutral atom. Therefore if 35Cl has a proton number of 17, it means that it has 17 protons and 17 electrons.
    • Second row – The proton number is 17, which means the element is Chlorine. However it has 20 Neutrons which means the nucleon number (protons + electrons) is 37. Therefore the element is 37Cl.
    • Third row – 39K+ is a positively charged Potassium ion. A positively charged ion has a deficiency of electrons when compared to the neutral atom, but however the number of protons is the same. The number of neutrons of an ion (or atom) = Nucleon number – Proton number = 39 – 19 = 20.
    • Fourth row – A 79Br ion has an excess of 1 electron, hence its -1 charge, which means the number of protons = number of electrons – 1 = 36 – 1 = 35.
    • Fifth row – Number of neutrons = nucleon number – proton number = 79 – 35 = 46.
    • Sixth row – On the periodic table, element with proton number 37 is Rb. In the table above, there is a deficiency of 1 electron, which makes it an ion of charge +1.
    ParticleProton (atomic) numberNumber of neutronsNumber of electrons
    1. Either of the following:
      • Atoms with same number of protons but different number of neutrons.
      • Atoms with same atomic number but different nucleon number.
      • Atoms of the same element with different number of neutrons.
    2. 35Cl and 37Cl

Sydney Chako

Mathematics, Chemistry and Physics teacher at Sytech Learning Academy. From Junior Secondary School to Tertiary Level Engineering Mathematics and Engineering Science.


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