Which has the larger first ionization energy—magnesium or phosphorus? a

Phosphorus has the greater first ionization energy.

b

Magnesium and phosphorus have equal first ionization energies.

c

Magnesium, but not phosphorus, has a measurable first ionization energy.

d

Magnesium has the greater first ionization energy.

The correct option is a

Explanation:

First ionization energy is the minimum amount of energy required to remove the most loosely bound electron (valence electron) from the outermost shell of a gaseous or neutral atom. Since it takes more amount of energy to remove more electrons, it (ionization energy) generally increases across the period and decreases down a group because the closer the outermost shell is to the nucleus, the more difficult/energy required to remove electrons in that shell.

Since magnesium and phosphorus are on the same period (period 3), it can be deduced from the explanation above that phosphorus has a greater first ionization energy than magnesium because phosphorus (5 valence electrons) has more electrons in it's outermost shell than magnesium (2 valence electrons).

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