# If an electron has 8 electrons in its ground state, what is the correct electron configuration for the element?

Dec 28, 2015

$1 {s}^{2} 2 {s}^{2} 2 {p}^{4}$

#### Explanation:

When building the electron configuration of an atom from scratch, you need to use

• The Aufbau Principle, which states that electrons will occupy available lower-energy orbitals before moving up to higher-energy orbitals
• Hund's Rule, which states that electrons will first be placed in unoccupied degenerate orbitals, then pair up to fill these degenerate orbitals
• Pauli's Exclusion Principle, which states that electrons that occupy the same orbital must have opposite spins

So, an energy diagram for empty orbitals looks like this

According to the Aufbau Principle, electrons will occupy energy levels in increasing order of energy.

The first energy level, which is lowest in energy, will thus be occupied first. This energy level contains the 1s-orbital.

Since an orbital can hold a maximum of two electrons, the first two electron will be placed in the 1s-orbital. The electron configuration for this atom starts out like this

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

You've used up $2$ of the $8$ electrons available.

Next, the second energy level, which contains a total of four orbitals

• one s-orbital
• three p-orbitals

Since the 2s-orbital is lower in energy than the 2p-orbitals, this orbital will be filled first. The electron configuration for your atom will now be

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

You're down to $4$ electrons.

Now, these three electrons will be placed in the 2p-orbitals. The complete electron configuration for $\text{X}$ will thus be

$\text{X: } 1 {s}^{2} 2 {s}^{2} 2 {p}^{4}$

To get a visual of how these last $4$ electrons will be arranged, you need to keep in mind Hund's Rule - you must add an electron to every empty degenerate orbital before pairing these electrons up.

This means that each 2p-orbital will get one electron. After you distribute $3$ of the $4$ remaining electrons like this, add the last electron to the first 2p-orbital - don't forget to change its spin!

As you know, a neutral atom has equal numbers of protons in its nucleus and electrons surrounding its nucleus. This means that element $\text{X}$ is actually oxygen, for which $Z = 8$.

Notice the placement of the ${8}^{\text{th}}$ electron, which got distributed to the first 2p-orbital having opposite spin.