# Electron Configuration

> How electrons fill shells, Bohr diagrams for the first 20 elements, valence electrons and subshells.

From HSC Chemistry on WhizzNotes, free study notes for NSW students: https://whizznotes.com/chemistry/electron-configuration/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The cinema seats
Picture a small cinema with rows of seats. The front row holds only 2 people. The next row holds 8. The row after that holds 8 for now (it can take more later on).

People always fill the front row first, because it is closest to the screen. Only when it is full does anyone sit in row two.

Electrons behave the same way. They fill the shell closest to the nucleus first, because that is the lowest energy. How they are seated decides almost everything about how an atom reacts.

**In short:** Electron configuration is how an atom's electrons are arranged in shells: for the first 20 elements they fill 2, then 8, then 8, so sodium is 2, 8, 1. The outer (valence) electrons decide how an atom reacts and which group of the periodic table it is in.

## Shells and how they fill

Electrons sit in **shells** (energy levels) around the nucleus. Shell 1 is closest and lowest in energy.

For the first 20 elements, use this rule:

| Shell | Holds up to |
|---|---|
| 1st | 2 electrons |
| 2nd | 8 electrons |
| 3rd | 8 electrons (for the first 20 elements) |
| 4th | the rest |

The **electron configuration** lists how many electrons are in each shell, separated by commas.

### Worked example: Writing configurations
- Carbon, 6 electrons: **2, 4**
- Sodium, 11 electrons: **2, 8, 1**
- Chlorine, 17 electrons: **2, 8, 7**
- Calcium, 20 electrons: **2, 8, 8, 2**

## The first 20 elements

| Element | Z | Configuration | | Element | Z | Configuration |
|---|---|---|---|---|---|---|
| H | 1 | 1 | | Na | 11 | 2, 8, 1 |
| He | 2 | 2 | | Mg | 12 | 2, 8, 2 |
| Li | 3 | 2, 1 | | Al | 13 | 2, 8, 3 |
| Be | 4 | 2, 2 | | Si | 14 | 2, 8, 4 |
| B | 5 | 2, 3 | | P | 15 | 2, 8, 5 |
| C | 6 | 2, 4 | | S | 16 | 2, 8, 6 |
| N | 7 | 2, 5 | | Cl | 17 | 2, 8, 7 |
| O | 8 | 2, 6 | | Ar | 18 | 2, 8, 8 |
| F | 9 | 2, 7 | | K | 19 | 2, 8, 8, 1 |
| Ne | 10 | 2, 8 | | Ca | 20 | 2, 8, 8, 2 |

## Bohr diagrams

A **Bohr diagram** shows the nucleus in the centre and the electrons as dots or crosses on circles.

*Diagram: Bohr diagrams of oxygen with 2 and 6 electrons and sodium with 2, 8 and 1 electrons*
*Oxygen has 6 outer electrons. Sodium has 1 outer electron (shown in red).*

## Valence electrons

The electrons in the **outer shell** are called **valence electrons**. They are the ones that take part in reactions.

- For groups 1 and 2, the number of valence electrons equals the group number.
- For groups 13 to 18, it is the group number minus 10 (so group 17 has 7).
- The number of **shells** used equals the **period** (row) number.

Sodium is 2, 8, **1**: 3 shells, so period 3. 1 valence electron, so group 1.

## Why atoms react: the full outer shell

Atoms with a **full outer shell** are very stable. The **noble gases** (helium 2, neon 2, 8, argon 2, 8, 8) already have one, which is why they hardly react at all.

Other atoms gain, lose or share electrons to get a full outer shell, like a noble gas.

- Sodium 2, 8, 1 **loses** 1 electron: $\mathrm{Na^+}$ is 2, 8, like neon.
- Chlorine 2, 8, 7 **gains** 1 electron: $\mathrm{Cl^-}$ is 2, 8, 8, like argon.

### Exam tip: Ions of the first 20 elements
Metals with 1, 2 or 3 valence electrons lose them: $\mathrm{Na^+}$, $\mathrm{Mg^{2+}}$, $\mathrm{Al^{3+}}$. Non-metals with 5, 6 or 7 valence electrons gain enough to reach 8: $\mathrm{N^{3-}}$, $\mathrm{O^{2-}}$, $\mathrm{F^-}$.

### Watch out: Do not overfill shell 1
Shell 1 holds only **2** electrons. Lithium is 2, 1, not 3.

**Mind map**

```text
Electron configuration
  Shells fill inside out
    1st: 2
    2nd: 8
    3rd: 8 (first 20)
  Writing it
    Sodium 2, 8, 1
    Calcium 2, 8, 8, 2
  Valence electrons
    Outer shell
    Gives the group
    Shells give the period
  Stability
    Full outer shell
    Noble gases
  Ions
    Metals lose e⁻
    Non-metals gain e⁻
```

### Year 11: Subshells: s, p, d and f
Each shell is made of **subshells**. Each subshell holds a set number of electrons.

| Subshell | Holds |
|---|---|
| s | 2 |
| p | 6 |
| d | 10 |
| f | 14 |

Shell 1 has 1s. Shell 2 has 2s and 2p (2 + 6 = 8). Shell 3 has 3s, 3p and 3d (2 + 6 + 10 = 18).

Subshells fill in order of energy: **1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p...** Notice that 4s fills **before** 3d. That is why potassium is 2, 8, 8, 1 and not 2, 8, 9.

- Sodium: $1s^2\, 2s^2\, 2p^6\, 3s^1$
- Chlorine: $1s^2\, 2s^2\, 2p^6\, 3s^2\, 3p^5$
- Iron (26): $1s^2\, 2s^2\, 2p^6\, 3s^2\, 3p^6\, 4s^2\, 3d^6$

### Year 12: Orbitals, Hund's rule and exceptions
Each subshell is made of **orbitals**, and each orbital holds 2 electrons with opposite spins. An s subshell has 1 orbital, p has 3, d has 5.

**Hund's rule**: electrons fill orbitals in the same subshell **one at a time** before pairing up. Nitrogen's three 2p electrons sit in three separate orbitals.

Two famous exceptions: **chromium** is $[\mathrm{Ar}]\,4s^1\,3d^5$ and **copper** is $[\mathrm{Ar}]\,4s^1\,3d^{10}$. A half-full or completely full d subshell is extra stable, so one 4s electron moves into 3d.

When transition metals form ions, they lose **4s electrons first**: $\mathrm{Fe^{2+}}$ is $[\mathrm{Ar}]\,3d^6$.

## Quick summary

- Electrons fill shells from the inside out: 2, then 8, then 8, for the first 20 elements.
- Write configurations like 2, 8, 1 (sodium).
- **Valence electrons** are in the outer shell. They decide how an atom reacts and its group.
- Number of shells = period number.
- Atoms gain, lose or share electrons to get a full outer shell like a noble gas.

## Quiz

Try these questions from the chapter quiz.

1. How many electrons can the first shell hold?
   - A. 2
   - B. 8
   - C. 18
   - D. 1
   - Answer: A. 2. The first shell is small and holds only 2.

2. What is the electron configuration of nitrogen (7 electrons)?
   - A. 2, 4, 1
   - B. 2, 5
   - C. 7
   - D. 2, 8
   - Answer: B. 2, 5. 2 in shell 1, the remaining 5 in shell 2.

3. What is the electron configuration of magnesium (12 electrons)?
   - A. 2, 10
   - B. 2, 8, 1
   - C. 2, 8, 2
   - D. 8, 2, 2
   - Answer: C. 2, 8, 2. 2 + 8 + 2 = 12.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/chemistry/electron-configuration/#quiz