# An element has 35 electrons. In which period of the Periodic Table is this element located?

Jun 24, 2016

The fourth period.

#### Explanation:

The trick here is to realize that if you have access to a periodic table, you can tell the period of the element by looking at its atomic number.

For a neutral atom, the number of electrons that surround the nucleus is equal to the number of protons present in the nucleus.

In this case, $35$ electrons implies $5$ protons present in the nucleus, which in turn implies an atomic number of $35$.

A quick look in the periodic table will reveal that this element is bromine, $\text{Br}$, which is located in period $4$, group 17.

Now, let's assume that you don't have access to a periodic table, but that you're very good at writing electron configurations.

The idea here is that you can find the period in which the element is located by writing out its electron configuration.

The highest energy level on which electrons are located will give you that element's period.

So, the first energy level contains one orbital, the $1 s$ orbital. As you know, each orbital can hold a maximum of $2$ **electrons.

The first two electrons will thus be distributed to the $1 s$ orbital

$\text{X: } 1 {s}^{2}$

The second energy level contains four orbitals

• the 2s-orbital
• three 2p-orbitals

These orbitals will hold

$4 \times {\text{2 e"^(-) = "8 e}}^{-}$

which means that the electron configuration of your element continues like this

$\text{X : } 1 {s}^{2} 2 {s}^{2} 2 {p}^{6}$

At this point, you have distributed $10$ electrons out of the $35$ available.

The third energy level contains nine orbitals

• the 3s-orbital
• three 3p-orbitals
• five 3d-orbitals

These orbitals will hold

$9 \times {\text{2 e"^(-) = "18 e}}^{-}$

which means that the electron configuration continues like this

$\text{X: } 1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2} 3 {p}^{6} 3 {d}^{10}$

You've now used $28$ electrons, so look to add the last $7$ electrons to the fourth energy level.

At this point, you can actually stop and say that because the fourth energy level is the highest that contains electrons, the element will be located in the fourth period of the periodic table.

The complete electron configuration for this element will be

$\text{X: } 1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2} 3 {p}^{6} 3 {d}^{10} \textcolor{red}{4} {s}^{2} \textcolor{red}{4} {p}^{5}$

As you can see, the highest-energy electrons are located in period $\textcolor{red}{4}$.