Consider the following elements (atomic numbers are given in parenthesis) Ca(20); K(19); F(9); Be(4)
(a) Select:
(i) The elements having one electron in the outermost shell.
(ii) Two elements of the same group. Write the number of this group.
(b) Write the formula of the compound formed by the union of Ca(20) and the element X(2, 8, 7).
Answer
Atomic number of Ca = 20
Electronic configuration = 2, 8, 8, 2
Atomic number of K = 19
Electronic configuration = 2, 8, 8, 1
Atomic number of Be = 4
Electronic configuration = 2, 2
(a) (i) Potassium has 1 electron in its outermost shell
(ii)Be and Ca have the same number of electrons in the outermost shell (valence shell) ie., 2 . So, they belong to the same group i.e. Group 2.
(b) Calcium has an electronic configuration of 2, 8, 8, 2 which means it has a tendency to d onate 2 electrons in order to attain noble gas configuration.
The element X having electronic configuration 2, 8, 7 has a t endency to gain 1 electron in order to attain noble gas configuration.
This implies that 1 Ca atom will react with 2 X atoms to form CaX 2 and in this way, both of them will become stable.
