Basic electron arrangement in atoms

IB Chemistry SL· Structure 1: Atomic Structure, Topic 4· 15 min read

1. The Bohr Model of the Atom★☆☆☆☆⏱ 3 min

📘 Definition

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.

📐 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.

2. Main Energy Levels and Maximum Electron Capacity★★☆☆☆⏱ 4 min

📘 Definition

Principal main energy level

$n$

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.

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.

3. Writing Full Electron Arrangements★★☆☆☆⏱ 5 min

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: $2, 8, 8, 1$ .

✓ Quick check

Test your understanding

What is the electron arrangement of neutral calcium ( $Z = 20$ )?
- 2,8,10
- 2,8,8,2
- 2,10,8
Reveal answer
1 —
Correct! Calcium follows the filling rule for the first 20 elements, giving 2,8,8,2.

4. Valence Electrons and Periodic Table Position★★☆☆☆⏱ 3 min

📘 Definition

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.

5. Common Pitfalls

Wrong move:

Writing calcium's electron arrangement as 2,8,10 instead of 2,8,8,2

Why:

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 move:

For Z ≤ 20, fill the 3rd shell with 8 electrons, then start filling the 4th shell, so calcium is 2,8,8,2

Wrong move:

Counting all electrons to find the number of valence electrons

Why:

Inner shell core electrons are not valence electrons, so this gives an incorrect count

Correct move:

Only count electrons in the highest energy (outermost) main energy level to get valence electrons

Wrong move:

Using mass number instead of atomic number to count electrons for a neutral atom

Why:

Mass number counts protons + neutrons, not the number of protons/electrons

Correct move:

Always use atomic number Z to get the number of electrons in a neutral atom

Wrong move:

Claiming all group 18 elements have 8 valence electrons

Why:

Helium only has one main energy level, which holds a maximum of 2 electrons

Correct move:

Remember the exception: helium has 2 valence electrons, all other group 18 elements have 8

6. Quick Reference Cheatsheet

Concept	Key Rule/Value
Max electrons per shell	$2 n^{2}$ , $n$ = 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

7. Frequently Asked

Do I need to know subshells for this topic?

No, this topic covers only basic main energy level (shell) arrangement. Subshells and spdf notation are covered in a later topic.

How many electrons can each main energy level hold?

The maximum number is calculated by $2 n^{2}$ , where $n$ is the principal energy level number. For the first 20 elements, the 3rd level only fills with 8 electrons before the 4th level starts filling.

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 1
Electron arrangement of calcium
2021 · 1
Count valence electrons for sulfur
2023 · 2
Relate arrangement to group/period

Going deeper

webPhET Bohr Model Interactive SimulationPractice building atoms with this free tool

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.