Electron Shells: The 2, 8, 8 Rule and Where It Comes From

One of the first models students learn for how electrons are arranged is the shell model: electrons sit in layers around the nucleus, filling the innermost layer first. The pattern “2, 8, 8” is easy to remember and remarkably powerful: with it, you can explain why sodium is reactive, why neon isn’t, and why the periodic table has the shape it does. This article explains the shell model, how to use it, and where it comes from.

What is an electron shell?

An electron shell (or energy level) is a group of electrons with similar energy and similar average distance from the nucleus. Shells are numbered outward from the nucleus: n = 1, 2, 3, 4…

• Inner shells are closer to the nucleus. Their electrons are held more strongly and have lower energy.
• Outer shells are further away. Their electrons are held less strongly and have higher energy.

Electrons fill the lowest-energy shells first, just as water fills a container from the bottom up.

How many electrons can each shell hold?

The maximum number of electrons in shell n is 2n²:

Shell (n)	Maximum electrons (2n²)
1	2
2	8
3	18
4	32

But for the first 20 elements, the practical filling pattern is 2, 8, 8, 2:

• The first shell fills with 2.
• The second fills with 8.
• The third takes 8, then the fourth shell starts (for potassium and calcium) before the third shell is completely full.

Filling shells: the first 20 elements

Element	Z	Shell arrangement
Hydrogen	1	1
Helium	2	2
Lithium	3	2, 1
Carbon	6	2, 4
Nitrogen	7	2, 5
Oxygen	8	2, 6
Fluorine	9	2, 7
Neon	10	2, 8
Sodium	11	2, 8, 1
Magnesium	12	2, 8, 2
Aluminium	13	2, 8, 3
Silicon	14	2, 8, 4
Phosphorus	15	2, 8, 5
Sulfur	16	2, 8, 6
Chlorine	17	2, 8, 7
Argon	18	2, 8, 8
Potassium	19	2, 8, 8, 1
Calcium	20	2, 8, 8, 2

How to work out an arrangement

Find the electron arrangement of sulfur.

1. Atomic number of sulfur = 16, so a neutral atom has 16 electrons.
2. First shell: 2 (14 left).
3. Second shell: 8 (6 left).
4. Third shell: 6.

Sulfur: 2, 8, 6.

Drawing shell diagrams

A shell (Bohr) diagram shows the nucleus as a small circle, with concentric rings for shells. Electrons are drawn as dots or crosses on the rings. For sodium (2, 8, 1): 2 electrons on the first ring, 8 on the second, and 1 on the third.

Tips:

• Draw electrons in the second and third shells in pairs around the ring for clarity.
• For ions, add square brackets and the charge: the sodium ion is [2, 8]⁺.

Why the third shell “pauses” at 8

This is the question every curious student asks. If the third shell can hold 18, why does potassium put its 19th electron into the fourth shell instead?

The answer comes from the more detailed model: shells are divided into subshells (s, p, d, f) with slightly different energies.

• Shell 3 contains a 3s subshell (2 electrons), a 3p subshell (6 electrons) and a 3d subshell (10 electrons).
• The 4s subshell of the fourth shell is actually lower in energy than 3d in potassium and calcium.
• So after 3s and 3p fill (2 + 6 = 8 electrons), the next electrons go into 4s, not 3d.

From scandium (element 21) onwards, electrons start filling the 3d subshell, and the third shell grows towards its full 18. These are the transition metals. See shells vs subshells vs orbitals and the Aufbau principle.

Shells and the periodic table

The shell model explains the layout of the periodic table:

• Period number = number of occupied shells. Sodium (2, 8, 1) has three shells, so it’s in period 3.
• Group number (for groups 1–2 and 13–18) relates to the number of outer electrons. Group 1 elements have 1 outer electron; group 17 elements have 7; group 18 elements have full outer shells (8, or 2 for helium).

So chlorine (2, 8, 7) is in period 3 and group 17. See why is the periodic table shaped like that?

Outer electrons control chemistry

The electrons in the outermost shell, the valence electrons, take part in bonding and reactions. See how to find valence electrons.

• Full outer shells are stable. The noble gases (2, 2-8, 2-8-8) are very unreactive. See noble gases.
• Metals with 1–3 outer electrons tend to lose them, forming positive ions with the arrangement of the previous noble gas. Sodium (2, 8, 1) → Na⁺ (2, 8).
• Non-metals with 5–7 outer electrons tend to gain electrons to fill the shell, forming negative ions. Chlorine (2, 8, 7) → Cl⁻ (2, 8, 8).
• Atoms can also share electrons in covalent bonds, reaching full outer shells together. See Lewis dot structures.

Reactivity trends

• Group 1 gets more reactive down the group: the outer electron is in a shell further from the nucleus and more shielded by inner shells, so it’s lost more easily. See alkali metals.
• Group 17 gets less reactive down the group: the incoming electron is further from the nucleus, so it’s attracted less strongly. See halogens.

Limitations of the shell model

The 2, 8, 8 model is excellent for the first 20 elements and for explaining basic bonding. It struggles with:

• Transition metals, where d subshells fill and arrangements become less regular.
• Exceptions such as chromium and copper. See electron configuration exceptions.
• The shapes of molecules, which depend on orbitals. See the shapes of s, p, d and f orbitals.
• Spectra, which show finer detail than shells alone can explain.

For these, chemists use full electron configurations such as 1s² 2s² 2p⁶ 3s¹. See electron configuration rules and try the electron configuration tool.

Practice questions

1. Write the electron arrangement for magnesium (Z = 12).
2. An element has the arrangement 2, 8, 5. Which group and period is it in? Name it.
3. Write the arrangement for the oxide ion, O²⁻.
4. Why is argon unreactive?

Answers: (1) 2, 8, 2; (2) group 15, period 3: phosphorus; (3) 2, 8; (4) its outer shell is full (2, 8, 8), so it has no tendency to gain, lose or share electrons.

Key takeaways

• Electrons occupy shells, filling the innermost first.
• For the first 20 elements, shells fill in the pattern 2, 8, 8, 2.
• The number of shells gives the period; the number of outer electrons relates to the group.
• Outer (valence) electrons control reactivity; full outer shells are stable.
• The third shell pauses at 8 because the 4s subshell is lower in energy than 3d; subshells explain this and the transition metals.