Which element has
(a) two shells, both of which are completely filled with electrons?
(b) the electronic configuration 2, 8, 2?
(c) a total of three shells, with four electrons in its valence shell?
(d) a total of two shells, with three electrons in its valence shell?
(e) twice as many electrons in its second shell as in its first shell?

Explanation

(a) Since noble gases (Group 18) have all their shells completely filled with electrons therefore the given element must be a noble gas. Since it has two completed shell (2,8) it belongs to 2nd period. Thus, the elements is neon (Ne).
(b) Since each period begins with the filling of a new shell, therefore the element having electronic configuration 2, 8, 2 belongs to 3rd period. Further , since the element has two electrons in the outermost shell, it belongs to group 2. In other words the given element belongs to group 2 of the 3rd period. thus the element is magnesium (Mg).
(c) Since the given element has total three energy shells with a total of 4 electrons in the valence shell therefore , the electronic configuration of the element is 2, 8, 4 , . In other words the element belongs to group 14 of the 3rd period. Thus the element is silicon (Si).
(d) Since the given element has a total of two energy shells with three electrons in the valence shells therefore the electronic configuration of the elements 2, 3. In other words the element belongs to group 13 of the 2nd period. thus the element is boron (B).
(e) The given element has two shells. But in the first shells the maximum of electrons is 2. Since the number of the electrons in the 2nd shell is twice the electrons in the Ist shell , therefore the number of electrons in the 2nd shell is 4 and the complete electronic configuration of the element is 2, 4. In other words the given element belongs to group 14 of the 2nd period. Thus the element is carbon (C).