The energy absorbed or released when a neutral atom loses one electron

ionization energy

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. Example: Sodium

Nobles gases have high ionization energy as they do not tend to lose electrons easily due to their high stability.

In a group, the ionization energy decreases as we move down as the size increases and thus the valence shell moves farther from nucleus and thus is easier to remove.

In a period, the ionization energy increases as we move across, the size decreases and thus the valence shell moves near to the nucleus and thus it gets difficult to remove.

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