Select the factor(s) that may be responsible for the ionization energy within a period. the valence electrons are further from the nucleus when moving from left to right. the valence electrons remain at the same distance from the nucleus when moving left to right. the diameter of the atom decreases somewhat as electrons are added. the attractive force of the nucleus increases.

The attractive force of the nucleus increases.

Explanation:

Ionization energy is the energy required to remove the most loosely bound electron from an isolated gaseous atom.

In a period, the ionization energy increases as we move across a period as the size decreases. The electrons get added to the same shell and the nuclear charge increases.The valence shell moves nearer to the nucleus.

Thus the valence electron gets tightly held by the nucleus due to attractive forces of the nucleus and thus more energy is required to remove the valence electron.

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