Answer:
Alkali metals
Explanation:
Alkali metals possesses the lowest first ionization energy in their respective period because the size of alkali metals is largest in their respective period. So the outermost electron experiences less force of attraction from the nucleus and hence can be easily removed.
The value of ionization energy decreases down the group because the size of metal increases due to the addition of new shell along with increase in the magnitude of screening effect.
