# Question #f70af

Oct 27, 2017

Two electrons.

#### Explanation:

The trick here is to realize that the $s$ subshell, which is denoted by $l = 0$, contains one orbital.

As you know, the principal quantum number, $n$, tells you the energy shell in which an electron is located. The angular momentum quantum number, $l$, tells you the energy subshell in which an electron is located.

In your case, you know that you have

$n = 4 \to$ the fourth energy shell

and

$l = 0 \to$ the $s$ subshell

The number of orbitals present in each energy subshell is given by the number of values that the magnetic quantum number, ${m}_{l}$, can take for a given energy subshell.

${m}_{l} = \left\{- l , - \left(l - 1\right) , \ldots , - 1 , 0 , 1 , \ldots \left(l - 1\right) , l\right\}$

You can say that the $s$ subshell contains a single orbital because in this case, the magnetic quantum number can take a single value.

$l = 0 \implies {m}_{l} = 0$

Now, you know that each orbital can hold a maximum of $2$ electrons of opposite spins, as shown by Pauli's Exclusion Principle.

Since the $4 s$ subshell, i.e. the $s$ subshell located in the fourth energy shell, contains a single orbital, the $4 s$ orbital, you can say that a maximum of $2$ electrons can share

$n = 4 , l = 0$