From one element to the next, an extra shell of electrons is added. This increases the electron ‘bulk’ and the outer electrons are increasingly less strongly held .
The radii of the adjacent Group 2 atom is smaller than Group 1 atom on the same period, because the nuclear charge has increased by one unit (L to R ), but is attracting electrons in the same shell.
Similarly the radii of Group 2 M2+ ion is smaller than the adjacent Group 1 M+ ion on the same period, because the nuclear charge has increased by one unit (L to R ), but is attracting the same number of electrons in the same shells.
The alkali metals are all highly reactive, losing their one outer electron to form a 1+ ion with non-metals. They give up 1 electron easily as losing 1 is easier than gaining 7 to complete the octet.
They all have the common properties of metals, being silvery-grey in colour, and good conductors of heat and electricity.