Why does ionization energy increase across a period

 effective nuclear charge increases as electron shielding remains constant. ... The increased distance weakens the nuclear attraction to the outer-most electron, and is easier to remove (requires less energy).

Explanation:

In general, ionization energy increases across a period and decreases down a group. Across a period, effective nuclear charge increases as electron shielding remains constant. ... The increased distance weakens the nuclear attraction to the outer-most electron, and is easier to remove (requires less energy).

electrons

Explanation:

more electrons hold the valence electrons more tightly to the nucleus

valence electrons are the electrons on the outer most ring in the electron cloud

that tool would be a compass