What factor(s) may be responsible for the ionization energy within a period? select all that apply.
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.

C) The diameter of the atom decreases somewhat as electrons are added, and D) The attractive force of the nucleus increases.

Explanation:

From left to right in the period the atomic number of the element increases. This increases the number of protons in the nucleus, and thus increases the nuclear forces. Also from left to right in the period gradually more and more electrons are added. Electrons that enter the d orbital have poor shielding effect compared to the s and p block elements. The combination of poor shielding effect and the increase in nuclear charge cause the atomic radii to decrease.