# Basic electron arrangement in atoms

> IB Chemistry SL · IB Diploma Programme Chemistry SL
> Source: https://www.owlsprep.com/study/ib-chemistry-sl-u1-basic-electron-arrangement-in-atoms/

This topic covers the fundamental arrangement of electrons around an atomic nucleus, focusing on main energy levels (shells). You will learn to write basic electron arrangements for the first 20 elements and connect this to an element's position on the periodic table.

**Prerequisites:** [Basic atomic structure](https://www.owlsprep.com/study/ib-chemistry-sl-u1-basic-atomic-structure/)

## Learning objectives

- Describe the arrangement of electrons in main energy levels (shells) around the nucleus
- Write electron arrangements for the first 20 elements using main energy level notation
- Relate electron arrangement to an element’s position on the periodic table
- Identify valence electrons and explain their role in chemical behavior

## The Bohr Model of the Atom

**Bohr Model** — An early model of the atom that describes electrons moving in fixed circular main energy levels (shells) around a positively charged nucleus, with each shell having a discrete fixed energy.

*Example:* In ground-state hydrogen, the single electron occupies the lowest energy first shell.

Niels Bohr proposed this model in 1913 to explain the line emission spectrum of hydrogen. While the modern atomic model is more complex, the Bohr model is sufficient for describing basic electron arrangement for the first 20 elements at the SL level.

> **info**
>
> The Bohr model only works perfectly for one-electron atoms like hydrogen, but it is a very useful approximation for basic electron counting in multi-electron atoms.

**Worked example:** How many electrons are in the outermost shell of a neutral oxygen atom (atomic number $Z=8$) according to the Bohr model?

1. A neutral oxygen atom has $Z = 8$, so it has 8 total electrons.
2. The lowest energy (first) shell fills first, and holds a maximum of 2 electrons.
3. Calculate remaining electrons: $8 - 2 = 6$, which fill the second shell.
4. The outermost (second) shell has 6 electrons.

## Main Energy Levels and Maximum Electron Capacity

**Principal main energy level** — A discrete region around the nucleus where electrons are most likely to be found, numbered starting from 1 (closest to the nucleus, lowest energy) outwards.

*Notation:* $n$

Each main energy level has a maximum number of electrons it can hold, calculated by the formula:

$$2n^2$$

| Principal energy level ($n$) | Maximum number of electrons |
| --- | --- |
| 1 | 2 |
| 2 | 8 |
| 3 | 18 |
| 4 | 32 |

**Worked example:** Calculate the maximum number of electrons that can occupy the 3rd main energy level ($n=3$).

1. Recall the formula for maximum electron capacity:
2. $$2n^2$$
3. Substitute $n=3$ into the formula:
4. $$2(3)^2 = 2 \times 9 = 18$$
5. The 3rd main energy level can hold a maximum of 18 electrons.

## Writing Full Electron Arrangements

Electrons fill main energy levels starting from the lowest energy level closest to the nucleus, moving outwards (this is the Aufbau principle for basic electron arrangement). For the first 20 elements, the order of filling is 1st → 2nd → 3rd (8 electrons only) → 4th, before the 3rd shell fills to its maximum 18 electrons.

**Worked example:** Write the full basic electron arrangement for a neutral neon atom ($Z=10$).

1. $Z=10$, so neutral neon has 10 total electrons.
2. Fill $n=1$ first: maximum 2 electrons. Remaining electrons: $10 - 2 = 8$.
3. Fill $n=2$ next: maximum 8 electrons. Remaining electrons: $8 - 8 = 0$.
4. Write the arrangement as electrons per shell, separated by commas: $2,8$.

**Worked example:** Write the full basic electron arrangement for neutral potassium ($Z=19$).

1. $Z=19$, so neutral potassium has 19 total electrons.
2. Fill $n=1$: 2 electrons, remaining = $19 - 2 = 17$.
3. Fill $n=2$: 8 electrons, remaining = $17 - 8 = 9$.
4. For elements up to $Z=20$, the 3rd shell holds 8 electrons before starting the 4th shell. Fill $n=3$: 8 electrons, remaining = $9 - 8 = 1$.
5. Remaining 1 electron goes to $n=4$. Final electron arrangement: $2,8,8,1$.

**Check your understanding**

Test your understanding

1. What is the electron arrangement of neutral calcium ($Z=20$)?

   - 2,8,10
   - 2,8,8,2
   - 2,10,8

   *Answer:* 2,8,8,2

   *Why:* Correct! Calcium follows the filling rule for the first 20 elements, giving 2,8,8,2.

## Valence Electrons and Periodic Table Position

**Valence electrons** — Electrons located in the outermost (highest energy) main energy level of an atom. These are the electrons that participate in chemical bonding and determine an element's chemical reactivity.

For main group elements, the number of valence electrons directly corresponds to an element's group number. The number of occupied main energy levels corresponds to the element's period number. This creates a direct link between electron arrangement and the structure of the periodic table.

**Worked example:** For neutral sulfur ($Z=16$), state the number of valence electrons, group number and period number.

1. First write the full electron arrangement: $2,8,6$.
2. The outermost shell is $n=3$, which has 6 electrons, so there are 6 valence electrons.
3. There are 3 occupied main energy levels, so sulfur is in period 3.
4. 6 valence electrons corresponds to group 16.

## Common pitfalls

- **Wrong:** Writing calcium's electron arrangement as 2,8,10 instead of 2,8,8,2
  - Why it fails: You assumed the 3rd shell fills completely to 18 electrons before starting the 4th shell, which is incorrect for basic arrangement of the first 20 elements
  - Correct: For Z ≤ 20, fill the 3rd shell with 8 electrons, then start filling the 4th shell, so calcium is 2,8,8,2
- **Wrong:** Counting all electrons to find the number of valence electrons
  - Why it fails: Inner shell core electrons are not valence electrons, so this gives an incorrect count
  - Correct: Only count electrons in the highest energy (outermost) main energy level to get valence electrons
- **Wrong:** Using mass number instead of atomic number to count electrons for a neutral atom
  - Why it fails: Mass number counts protons + neutrons, not the number of protons/electrons
  - Correct: Always use atomic number Z to get the number of electrons in a neutral atom
- **Wrong:** Claiming all group 18 elements have 8 valence electrons
  - Why it fails: Helium only has one main energy level, which holds a maximum of 2 electrons
  - Correct: Remember the exception: helium has 2 valence electrons, all other group 18 elements have 8

## Cheatsheet

| Concept | Key Rule/Value |
| --- | --- |
| Max electrons per shell | $2n^2$, $n$ = principal level |
| Filling order (Z ≤ 20) | 1 → 2 → 3 (8 e⁻) → 4 |
| Valence electrons = group number | For all main group elements |
| Occupied shells = period number | For all elements |
| Potassium (Z=19) | 2,8,8,1 |
| Calcium (Z=20) | 2,8,8,2 |

## What's next

Understanding basic electron arrangement is the foundation for more advanced topics in IB Chemistry SL, including subshells, orbitals, and full spdf electron configuration. This topic also underpins core concepts across the syllabus: periodic trends, chemical bonding, and reactivity all depend on the arrangement of electrons in atoms. Mastering the basic rules here will make learning more complex electron configurations and chemical behavior much easier as you progress through the course.

- [Structure 2: Electron Configuration](https://www.owlsprep.com/study/ib-chemistry-sl-u2-overview/)
- [Atomic orbitals](https://www.owlsprep.com/study/ib-chemistry-sl-u2-atomic-orbitals/)
- [Electron configuration rules](https://www.owlsprep.com/study/ib-chemistry-sl-u2-electron-configuration-rules/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-chemistry-sl-u1-basic-electron-arrangement-in-atoms/