# Question 89d4c

Oct 21, 2017

$\left[\text{Ne}\right]$

#### Explanation:

Start by looking for calcium, $\text{Ca}$, in the Periodic Table. You will find that this element is located in period $4$, group $2$ and that it has an atomic number equal to $20$.

This tells you that an atom of calcium contains $20$ protons inside the nucleus and $20$ electrons surrounding the nucleus.

You can thus say that the electron configuration of an atom of calcium must account for a total of $20$ electrons.

As you know, you can use the Aufbau Principle to write the full electron configuration for an atom of calcium.

You will end up with

$\text{Ca: } 1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2} 3 {p}^{6} 4 {s}^{2}$

Now, when calcium loses two electrons, it becomes a cation with an overall $2 +$ charge. These two electrons will be removed from the fourth energy level, the valence shell of an atom of calcium. More specifically, these two electrons will come from the $42$ orbital.

$\text{Ca: } 1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2} 3 {p}^{6} \textcolor{red}{\cancel{\textcolor{b l a c k}{4 {s}^{2}}}}$

This implies that the full electron configuration of a calcium action will look like this

${\text{Ca}}^{2 +} : 1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2} 3 {p}^{6}$

Finally, to write the abbreviated electron configuration of a calcium cation, you need to look in the Periodic Table for the noble gas that comes immediately before calcium.

This noble gas is neon, $\text{Ne}$. The full electron configuration of an atom of neon looks like this

$\text{Ne: } 1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2} 3 {p}^{6}$

Notice that this is the same electron configuration as the one you have for the calcium cation. To show this, you can write

"Ca"^(2+): ["Ne"] -> the nogle gas shorthand of a calcium cation

Here $\left[\text{Ne}\right]$ simply means the full electron configuration of neon.

Notice that you can write the noble gas shorthand of an atom of calcium like this

"Ca: " ["Ne"] 4s^2#

Once again, the $\left[\text{Ne}\right]$ simply means the electron configuration of neon, the noble gas that comes immediately before calcium in the Periodic Table.