The element in which outermost shell is completely flled?

The correct option is B

(b) Xenon

Explanation for Correct answer

Option (b) Xenon

1. Xenon has 54 electrons.
2. Its electronic configuration is $2,8,18,18,8$.
3. Thus, it tells us that its outermost shell has 8 electrons and hence is completely filled. Also, it is a noble gas.

Explanation for Incorrect answer

Option (a) Beryllium

1. Beryllium has 4 electrons.
2. Its electronic configuration is $2,2$.
3. Thus, it tells us that its outermost shell has 2 electrons and hence is not completely filled.

Option (c) Aluminium

1. Aluminum has 13 electrons.
2. Its electronic configuration is $2,8,3$.
3. Thus, it tells us that its outermost shell has 3 electrons and hence is not completely filled.

Option (d) Hydrogen

1. Hydrogen has a total of 1 electron.
2. Its electronic configuration is$1$.
3. Thus, it tells us that its outermost shell has 1 electron and hence is not completely filled.

The element in which the outermost shell is completely filled is option (b) Xenon.