## What is an Electron Configuration?

Electron configuration is the distribution of electrons of an atom (or molecule) in atomic or molecular orbitals.

### What is an orbital? (Simple definition)

An electron can be found in any place around the nucleus. An orbital is the most likely location of an electron around an atom.

If you want to actually see what an orbital looks like:

Orbitron

### Configuration example (Notation)

The electron configuration of the neon atom is 1s² 2s² 2p⁶.

### Orbital example

1s² is a specific orbital. In this example:

• "1" is the energy level.
• "s" is the orbital type.
• "2" is the number of electrons in it.

Note: "2s²" and "2p⁶" are also orbitals.

## Shells and subshells

Electron configurations are divided by shells and subshells.

### What is a electron shell? (Simple definition)

An electron shell is a piece of the outside of an atom. It is a group of orbitals with the same value of the quantum number.

They are given numbers or letters from "K" to "Q".

In the neon example:

• 1s² (1 is the quantum number and shell)
• 2s² (2 is the quantum number and shell)

### What is a electron subshell? (Simple definition)

A subshell is a subdivision of electron shells separated by electron orbitals. Subshells are labelled s, p, d, and f.

In the neon example:

• 1s² (s is the subshell)
• 2p⁶ (p is the subshell)

## Why is the electron configuration important?

You certainly never heard of a proton or neutron configuration, right?

That's because they are easy to find, we know where they are. You can't say the same about electrons.

In fact when we say an electron is an orbital, it is because it has a high probability of being there. Not because we are sure about it. That's a definition for "orbital".

So the main reasons why we study electron configuration are:

1. Electrons are hard to find.
2. Electrons are the reason why atoms and molecules interact with each other.
3. It helps us predict the properties of an element.
4. It helps us determine the valency of an element.

### Other applications

• LCAO (Linear combination of atomic orbitals)
• DFT (Density functional theory)
• ES (Emission spectrum)

## Writing electron configurations

First we need to understand how electrons choose where they will be. Also known as "General rules".

Next I will explain the traditional way to write an electron configuration and then explain a cool hack you can use.

### Rule 1: Distribution by energy levels

Out intuition may lead us to believe that electrons will fill orbitals that are closer to the nucleus first.

But that's not exactly true. They fill the lower energy orbitals first. Most of these are closer to the nucleus, but not always.

### Rule 2: Distribution by distance

When they can choose between same energy orbitals, they will prefer to be as far as possible.

### Rule 3: Distribution by electron spin

When the previous rules are satisfied, electrons will pair up by spin. But they can't have the same spin.

For example, one electron will spin up and the other one will spin down:

We use a memory aid to comply with rule 1 (Above):

 Credits: Byjus

Just follow the line from the top to the bottom. Fill the orbital and move to the next one.

You need to respect the maximum number of electrons in each subshell:

• s: 2
• p: 6
• d: 10
• f: 14

Example of noble gas configuration:

• He: 1s2
• Ne: 1s2 2s2 2p6
• Ar: 1s2 2s2 2p6 3s2 3p6
• Kr: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6
• Xe: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6
• Rn: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p6 4f14 5d10 6s2 6p6

The problem with method is:

1. You have to remember this memory aid.
2. You have to control the number of electrons you’ve used so far.
3. You have to remember how many electrons fit into each subshell (s, p, d, f).
4. It takes a lot of time.

### The block method (The hack)

I'll explain here the hack I learned from cambridge coaching:

Step 1: Label your period table in blocks.

Step 2: Identify the element of interest on the periodic table and circle it.

Step 3: Locate hydrogen as your starting point.

Step 4: Glide across each row, left to right and top to bottom, writing out the electron configuration until you get to your element.

Ge: 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p2

Step 5: Check your work by adding all the superscripts and seeing if it adds up to the total number of electrons in your element of interest. this is optional.

2+2+6+2+6+2+10+2 = 32

What makes this a better method:

• You don’t have to remember how many electrons fit into each subshell (s, p, d, f).
• You don’t have to remember that memory aid.
• You don’t have to keep track of the electrons you’ve used so far.
• It takes much less time.

## Abbreviated Electron Configuration

As you can see above, the standard distribution often results in a big electron configuration.

In these cases we can use an abbreviated configuration (Condensed electron configuration). We can call this an official hack.

Why? Well, you will notice there is always a complete set of subshells in every heavy atom. This is also the same configuration of the previous noble gas in the periodic table.

So what we do is put the last noble gas in square brackets.

### Example

Sodium's electron configuration is 1s² 2s² 2p⁶ 3s¹. How do we write it in the abbreviated form?

Step 1: We pick the last noble gas. In this case it is Neon element.

Neon configuration is 1s² 2s² 2p⁶, so we replace it for "[Xe]":

[Ne]3s¹.

Neon could be abbreviated as [He] 2s² 2p⁶.

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