Chemistry

Yeah, that is true.. but u can think about that in this way:
Consider the electron configuration of Na, K, Ca and Mg:
Na: 2, 8, 1
K: 2, 8, 8, 1
Ca: 2, 8, 8, 2
Mg: 2, 8, 2
As u go down a group, the number of electron shell increases such that the shielding effect increases (shielding means the replusion between the electrons within the shell). The effective nuclear charge therefore decreases, such that the attraction between the outer most electrons and the nucleus decreased. As a result, the electron is able to lose more easily, forming a positive charge ion. Therefore, it can be explained that potassium is more reactive than sodium.
As u go across the group, (i.e from K to Ca), u can see that from the electron configuration as above, that there is one more electron in the outer most electron shell. THis measn that the number of protons increases as well, the attraction between the protons and the electrons will be stronger, the nuclear effective charge increases (note that the shielding effect stayed the same as the number of electron shell stayed the same), such that the outmost electron could be be lost easily such that the reactivity of Ca is less than K. But due to the samereason as (a), the reactivity of Ca will be hgiher than Mg.

YES .but lose electorn due to its distance between electrons.
for example :
Group 1
you know this group metal their reactivity increase down the group. Li( 2.1 ) the outermost shell electron is very near the neclues,
so it can't easy separate/ reast with any elecment..
K(2.8.8.1)the outermost shell is far the necleus,so easy to react with any elecment/easy to separate.