Question

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

Answer 1

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.