Question

Four atoms are labelled from A to D:

Atoms	Mass No.	Atomic no.
A	19	9
B	7	3
C	16	8
D	14	7

Which element has 3 shells?

Answer 1

• The protons and neutrons are present inside the nucleus which is surrounded by electrons that revolve around the nucleus in certain orbits.
• The shells are named as K, L, M, N… where the shell number is n=1,2,3,4,..respectively.
• The electrons are first filled in the inner shells.
• The maximum number of the electron in a shell is given by the formula- $2n^2$, where n is the number of shells.

Hence, the number of the electron in the K shell is 2, the number of electrons in M shell is 8, the number of electrons in the L shell is 18 and so on.

The element which has 3 shells must have electrons of more than 10. Hence, none of the atoms given in the list has 3 shells in their structure.