How to Draw a Bohr Model (Step-by-Step, with Practice)

Asked to “draw a Bohr model / Bohr diagram,” or for “electrons in each shell / energy level.”

Not this type?

• Asked for 1s² 2s² 2p⁶…? · 3.2That’s an electron configuration.How to write electron configurations →
• Only the outer electrons? · 3.2That’s valence electrons.How to find valence electrons →
• Subshells and orbitals? · 3.1Count by s, p, d instead.How to count electrons in shells, subshells and orbitals →

The idea

Element	Electrons	Rings
H	1	1
He	2	2
C	6	2, 4
Na	11	2, 8, 1
Ar	18	2, 8, 8
Ca	20	2, 8, 8, 2

On the periodic table: the atomic number is the electron count for a neutral atom: H 1, He 2, C 6, Na 11, Ar 18, Ca 20.

Try first, then check. Hints if you need them.

Problem 1 of 4 · warm-up

Draw a Bohr model for nitrogen-14. How many electrons go in each ring?

Electrons per ring

Check my answer

Hint 1 · the clue

Nitrogen’s atomic number on the periodic table = electrons.

Hint 2 · the next move

7 electrons: 2 in ring 1, the rest in ring 2.

Your path:

1. 7 electrons
2. Ring 1: 2
3. Ring 2: the rest

Answer

Answer: 2, 5

Full worked solution

What you know: nitrogen-14, neutral.

1. Nitrogen has 7 protons and 7 electrons; 14 − 7 = 7 neutrons.
2. Rings: 2, then 7 − 2 = 5 → 2, 5.

7N14.01On the periodic table: nitrogen (N) is 7.

Answer: 2, 5

Problem 2 of 4

Draw a Bohr model for magnesium-24. How many electrons go in each ring?

Electrons per ring

Check my answer

Hint 1 · the clue

Magnesium’s atomic number on the periodic table: 12 electrons.

Hint 2 · the next move

2, then 8, then what’s left.

Your path:

1. 12 electrons
2. 2 + 8 = 10
3. 12 − 10 = 2

Answer

Answer: 2, 8, 2

Full worked solution

What you know: magnesium-24, neutral.

1. 12 protons, 12 electrons; 24 − 12 = 12 neutrons.
2. Rings: 2, 8, 2.

12Mg24.31On the periodic table: magnesium (Mg) is 12.

Answer: 2, 8, 2

Problem 3 of 4

Draw a Bohr model for calcium-40. How many electrons go in each ring?

Electrons per ring

Check my answer

Hint 1 · the clue

20 electrons (periodic table). Ring 3 stops at 8 here.

Hint 2 · the next move

2, 8, 8, then the rest.

Your path:

1. 20 electrons
2. 2 + 8 + 8 = 18
3. 20 − 18 = 2

Answer

Answer: 2, 8, 8, 2

Full worked solution

What you know: calcium-40, neutral.

1. 20 protons, 20 electrons; 40 − 20 = 20 neutrons.
2. Rings: 2, 8, 8, 2.

20Ca40.08On the periodic table: calcium (Ca) is 20, the last element of the simple pattern.

Answer: 2, 8, 8, 2

Problem 4 of 4 · watch for the trap

Draw a Bohr model for the chloride ion, ³⁵Cl⁻. How many electrons go in each ring?

Electrons per ring

Check my answer

Hint 1 · the clue

The 1− charge means one extra electron.

Hint 2 · the next move

Chlorine’s 17 (periodic table) + 1 = 18 electrons.

Your path:

1. 17 + 1 = 18 electrons
2. 2, 8, …
3. 18 − 10 = 8

Answer

Answer: 2, 8, 8

Full worked solution

What you know: ³⁵Cl⁻.

1. 17 protons; 35 − 17 = 18 neutrons; 17 + 1 = 18 electrons.
2. Rings: 2, 8, 8.

Watch out: 2, 8, 7 is neutral chlorine, not the ion.

17Cl35.45On the periodic table: chlorine (Cl) is 17.

Answer: 2, 8, 8

• 8 electrons in ring 1.Fix: ring 1 holds 2.
• Electrons = mass number.Fix: electrons = atomic number (neutral). Mass number is for the nucleus.
• Ignoring an ion’s charge.Fix: Cl⁻ has 18 electrons: 2, 8, 8.
• Putting 9+ in ring 3 for K or Ca.Fix: first 20 elements: ring 3 stops at 8, then ring 4.

• With · 3.1Shells, subshells, orbitals, and electronsHow to count electrons in shells, subshells and orbitals.
• Next · 3.2Valence electronsHow to find valence electrons.
• Then · 3.2Writing an electron configurationHow to write electron configurations.
• Before · 1.3Protons, neutrons and electronsHow to find protons, neutrons and electrons.
• Hub · Ch 3Chapter 3: Electrons and periodic patternsAll topics in this chapter.