Electronic configuration of elements with atomic numbers:
20 (Ca): [Ar] 4s2
12(Mg): [Ne] 3s2
19(K): [Ar] 4s1
21(Sc): [Ar] 3d1 4s2
38(Sr): [Kr] 5s2
The outer electronic configuration of Mg. Ca and
Sr shows that these elements belong to same group. Thereforem Mg and Sr will
have physical and Chemical properties resembling calcium.
Although the last electron in Sr enters 5s
orbital, but on moving from Rb to Sr, the nuclear charge ncreases the thus
atomic size of Sr gets smaller than K. Therefore, K has the largest atomic size
and Mg has smallest atomic size since its last electron enters 3s orbital.
K has only one electron in its outermost sub
shell (4s) . By loosing this electron, will gain stable configuration of noble
gas Ar. Therefore, K has the maximum tendency to loose electron.