Given below are some elements of the modem periodic table. Atomic number of the element is given in the parentheses: A(4), B(9), C(14), 0(19), E(20)

(a) Select the element that has one electron in the outermost shell. Also write the electronic configuration of this element.

(b) Which two elements amongst these belong to the same group? Give reason/for your answer.

(c) Which two elements amongst these belong to the same period? Which one of the two has bigger atomic radius?

The electronic configuration of the given elements can be written in tabular form as follows:

(a)The element D is having one electron in its valence shell. The electronic configuration of element D is 2, 8, 8, 1.

(b) A and E belong to the same group;.i.e 2nd group of the periodic table as both have same number of valence electrons (i.e.2) in their outermost shell.

(c) The element which have same number of valence shells belongs to shame period. Therefore, A and B have 2 valence shells and therefore, they both belongs to period 2. D and E consists of 4 shells and therefore, belongs to period 4 of periodic table.

Among element A and element B, element has bigger atomic radius. Among D and E element, D has bigger radius because atomic size decreases on moving from left to right in a period and therefore atomic radius decreases.