# What is the ground state shorthand notation for arsenic (as)?

May 13, 2018

Rather than just give an answer, let us build it. Literally. See the explanation.

#### Explanation:

Shorthand notation means you do Not specifically write all the electrons. You just write the valence electrons after a single symbol for the noble as core lying beneath them. It's done using what is known as the Aufbau Prinzip*, German for "build-up principle". (Yes, taking foreign language courses in school can come in handy.)

Let's do this for arsenic, and you should have a Periodic Table handy.

1) Identify the noble gas core as the heaviest noble gas preceding your element. For arsenic this is argon so you put in

$\text{[Ar]}$.

2) Now you basically count blocks of elements in the Periodic Table from the noble gas up to your element, here from argon up to arsenic. First, there are two elements (potassium, calcium) where the 4s subshell is being filled, in the two "main groups" on the left. These are where s electrons come in, and in the fourth period they are 4s electrons. So you add them to the argon core getting

$\text{[Ar]} 4 {s}^{2}$.

3) Now there are ten elements in the middle, the transition metals, where you are filling up a d subshell. The shell number in this d subshell is one less than the period, so the fourth period is 3d. You go through all ten elements here so you now have

$\text{[Ar]} 4 {s}^{2} 3 {d}^{10}$.

Note for future reference: If you have to work with the f subshells in the elements on the bottom (lanthanides, actinides), the shell number for those is two less than the period, but arsenic comes too early in the Periodic Table for that.

4) To get to arsenic, you now need just three more elements in the right hand "main groups" where p subshells are filled. In the fourth period this is 4p, you do not have to decrease the period number as in d and f subshells. So you arrive at this result for arsenic:

$\text{[Ar]} 4 {s}^{2} 3 {d}^{10} 4 {p}^{3}$.