Question

Calcium has 20 electrons. How many electrons are in the outer shell?

Answer 1

`2`

Explanation:

Electronic configuration of `""_20"Ca"` is

`"1s"^2\ "2s"^2\ "2p"^6\ "3s"^2\ "3p"^6\ underbrace("4s"^2)`
`color(white)(...................)color(blue)"outermost shell"`

Number of electrons in outermost shell (`4^"th"` shell) is `2`

Answer 2

2

Explanation:

Label each shell as n=1, n=2, etc.
Each shell can contain `2n^2` electrons

The first shell holds 2 electrons. We have 18 electrons left.
The second shell holds 8 electrons. 10 electrons left.
The third shell is able to hold 18 electrons.

However, the third shell only holds 8 electrons before the fourth shell starts to fill. The third shell carries on filling up only after 2 electrons are in the 4th shell. So what happens in calcium is that 8 of the remaining 10 electrons are in the 3rd shell, and the other 2 are in the 4th shell.