Question

Using the formula $2n^2$, identify the elements that have incomplete shells.

• A. Helium (Z = 2)
• B. Zinc (Z = 30)
• C. Argon (Z = 18)
• D. Neon (Z = 10)

Answer 1

Answer: (B), (C)

The correct options are
B Zinc (Z = 30)
C Argon (Z = 18)

The formula to find the maximum number of electrons that can be accommodated in a shell = $2n^2$, where ‘n’ is the number of the given shells from the nucleus.
For example, the maximum number of electrons that can be filled in the:
First shell (K - Shell) = $2(1{)}^2$ = 2 electrons.
Second shell (L - Shell) = $2(2{)}^2$ = 8 electrons.
Third shell (M - Shell) = $2(3{)}^2$ = 18 electrons.

So, Helium has its shell completely filled with 2 electrons. Neon has its shells completely filled as it has 10 electrons in 2 shells K and L, which can accommodate 2 and 8 electrons respectively. Argon has 18 electrons in 3 shells and is a Noble gas. But the maximum number of electrons that can be accommodated in 3 shells is 2 + 8 + 18 = 28.
So Argon and Zinc don't have completely filled shells.