# Question 78c17

Sep 26, 2015

That's just a notation used to express argon's electron configuration.

#### Explanation:

The are two ways of writing electron configurations, the long way, which is what you have when you write complete electron configurations, and the short way, which is what you have when you use the noble gas shorthand notation.

In your example, the complete electron configuration of copper is

$\text{Cu} : 1 {s}^{2} 2 {s}^{2} 2 {p}^{6} 3 {s}^{2} 3 {p}^{6} 4 {s}^{1} 3 {d}^{10}$

The noble gas shorthand notation allows you to skip a part of this electron configuration. More specifically, you can replace a part of this configuration with the electron configuration of the noble gas that comes before copper in the periodic table.

In copper's case, the noble gas that comes before it in the periodic table is argon. Argon's complete electron configuration is

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

Notice that copper's electron configuration builds on the electron configuration of argon.

"Cu": underbrace(1s^2 2s^2 2p^6 3s^2 3p^6)_(color(blue)("argon")) 4s^1 3d^10

This means that you can write

"Cu": ["Ar"] 4s^1 3d^10

So, in essence, the $\left[\text{Ar}\right]$ simply means the electron configuration of argon.

You can use the same technique to write argon's noble gas shorthand configuration. The noble gas that comes before argon in the periodic table is neon, $\text{Ne}$.

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

Notice that argon's electron configuration builds on the electron configuration on neon

"Ar": underbrace(1s^2 2s^2 2p^6)_(color(green)("neon")) 3s^2 3p^6

This means that argon's noble gas shorthand configuration is

"Ar": ["Ne"] 3s^2 3p^6#