The correct answer is C because:

The ionic radius is dependent on the:
- Number of protons / nuclear charge
- Number of electrons
- Distance of the outer electrons from the nucleus
Focussing only on the number of protons, you would expect:
- Magnesium to have the largest atomic radius because it has the most protons
- Oxygen to have the smallest atomic radius because it has the least protons
However, all four options are ions
- The ionic radii of the sodium and magnesium ions will decrease as they are losing electrons
  - The sodium ion now has 11 protons acting on 10 electrons
  - The magnesium ion now has 12 protons acting on 10 electrons
- The ionic radii of the oxide and fluoride ions will increase as they are gaining electrons
  - The oxide ion now has 8 protons acting on 10 electrons
  - The fluoride ion now has 9 protons acting on 10 electrons
- These factors lead to the following ionic radii:
  - O^2– = 126 pm
  - F² = 119 pm
  - Na⁺ = 116 pm
  - Mg²⁺ = 86 pm

A is incorrect as	Na⁺ has more protons than oxygen and nitrogen but a lower charge than magnesium
B is incorrect as	Mg²⁺ is the smallest as it has the most protons AND a higher charge than sodium
D is incorrect as	F⁻ has one more proton than oxygen AND one less electron added to the atom

Ionic & Metallic Bonding & Structure (Edexcel International AS Chemistry)

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