Which of the following best explains the trend in ionization energy across a period?

It decreases because the distance between the nucleus and the outermost shell increases.

It increases because the number of protons increases but the number of core electrons decreases.

It increases because the outermost electrons are attracted more strongly by protons.

It decreases because the effective nuclear charge decreases while moving left to right.

Answer: It increases because the outermost electrons are attracted more strongly by protons.

Explanation:

Ionization energy is the energy required to remove the most loosely bound electron from a gaseous atom. Metals have low ionization energy as they tend to attain stable configuration by losing the valence electrons.

The size of an atom decreases as we move across the period because the electrons get added to the same shell and the nuclear charge keeps on increasing. Thus the electrons get more tightly held by the nucleus.

As, the size of an element decreases, the valence electrons come near to the nucleus. So, the attraction between the nucleus and the shared pair of electrons increases and thus ionization energy increases.

Hence, the correct statement is It increases because the outermost electrons are attracted more strongly by protons.

A) It decreases because the distance between the nucleus and outermost shell increases