The atomic number of the element which does not usually form compounds because its outermost shell is full is

a. 8

b. 16

c. 18

d. 19

e. 20

Correct answer: 18

The outermost shell is full when that shell reaches 2 (n²) electrons distributed by holding-capacity of each layer, where n is the outermost shell number. The element with an atomic number of 18 has 18 electrons around it. For the first shell, it is full when it has two electrons [2 (1²) = 2 on the first shell]. The second shell is full when it has six electrons [2 (2²) = 2 on the first shell + 6 on the second]. The third shell is full when it has 8 electrons [2 (3²) = 2 on the first shell + 6 on the second + 8 on the third shell]. 18 electrons fill up the outermost shell of the element.