When magnesium is ignited in air, the magnesium reacts with oxygen and nitrogen.

The reaction between magnesium and nitrogen is represented by the unbalanced equation below.

Mg(s) + N2(g) --> Mg3N2(s)

51 Balance the equation in your answer booklet for the reaction between magnesium and nitrogen, using the smallest whole-number coefficients. [1]

52 In the ground state, which noble gas has atoms with the same electron configuration as a magnesium ion? [1]

53 Explain, in terms of electrons, why an atom of the metal in this reaction forms an ion that has a smaller radius than its atom. [1]

Answer :

51: The only coefficient is in the first one i think and it would be a 3
52: Neon (Ne)
53: Mg has a low nuclear charge so all it's electrons would be lost in the electron shell, making the ion smaller than the atom.


Mg(s) + N₂(g) --> Mg₃N₂(s)

There is one Mg atom on the reactant side and three Mg atoms on the product side. So to balance the number of Mg on both side of the equation a coefficient 3 is placed before Mg on the reactant side. The number of N atoms are two on both side of the reaction. Therefore the balanced chemical equation is,  

2Mg(s) + N₂(g) --> Mg₃N₂(s)


The electron configuration of Mg is 1s²2s²2p⁶3s² or [Ne]3s²

The electron configuration of Mg²⁺ is 1s²2s²2p⁶ or [Ne]

The electron configuration of Ne is 1s²2s²2p⁶

Ne he same electron configuration as a Mg²⁺ ion.


Mg has its valence electron in the 3s shell, but Mg²⁺ has no electron in the 3s shell, the valence electron in Mg²⁺is in the 2p orbital. The size of the 2 shell is smaller than the 3 shell. Hence, Mg²⁺ is smaller than Mg.