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Chemistry · Topic

Atomic structure, bonding and formulae

Ionic formulae come from charge balance; show the charges you balanced.

2 original questions in the bank, worth 4 marks · above the two-per-topic floor

Drill this topic All atomic structure, bonding and formulae questions

The questions, with the examiner reasoning

State1 markBoth boards

A neutral atom is written as ³⁵₁₇Cl. How many protons, neutrons and electrons does it contain?

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Answer: 17 protons, 18 neutrons, 17 electrons

  1. Protons = atomic number = 17
  2. Electrons = protons in a neutral atom = 17
  3. Neutrons = mass number − protons = 35 − 17 = 18
  • A. 17 protons, 18 neutrons, 17 electrons — Correct: the atomic number gives protons (and electrons in a neutral atom); neutrons are 35 − 17.
  • B. 17 protons, 17 neutrons, 18 electrons — The neutron number is the mass number minus the proton number, and a neutral atom has equal protons and electrons.
  • C. 18 protons, 17 neutrons, 18 electrons — The two numbers in the symbol were swapped.
  • D. 35 protons, 17 neutrons, 35 electrons — The mass number was read as a count of protons.

Examiner technique: One rule does all the work: bottom number protons, top minus bottom gives neutrons, and neutral means equal protons and electrons.

Define3 marksBoth boards

Define the term isotope, and explain why two isotopes of the same element have identical chemical properties but different relative atomic masses.

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Answer: Atoms of the same element with the same number of protons but different numbers of neutrons; chemical properties depend on electrons, and both isotopes have the same electron arrangement.

  1. Same number of protons (same atomic number) (1 mark)
  2. Different number of neutrons, so a different mass number (1 mark)
  3. Chemical properties are set by the electrons, and the electron arrangement is the same in both (1 mark)

Examiner technique: A definition question is marked on the words you choose. "Same element, different mass" is not a definition; it is a description, and it usually stops at one mark.