Electronic Structure

Electrons are arranged around the nucleus in energy levels, also called shells. The key rule is that electrons occupy the lowest available energy level first — the innermost shell fills before any electron goes into the next one out.

This happens because electrons are most stable when they are closest to the positive nucleus. Only once a shell is full does the next shell begin to fill.

AQA accepts answers written in terms of energy levels or shells. The two words mean the same thing here, so use whichever you prefer.

The arrangement of electrons across these shells is called the atom's electronic structure (or electron configuration). It controls how an atom behaves chemically, so it underpins bonding and the whole layout of the periodic table.

For the first 20 elements, the shells fill in a fixed pattern:

Shell (energy level)	Maximum electrons (first 20 elements)
1st (innermost)	2
2nd	8
3rd	8
4th	begins after 3rd has 8 (for K and Ca)

So the filling order for the first 20 elements is 2, then 8, then 8, and then the fourth shell starts. Potassium and calcium are the reason the third shell stops at 8 for these elements: their last electrons go into the fourth shell once the third holds 8.

Worked example — potassium (19 electrons). Fill the first shell with 2, the second with 8 (total 10), the third with 8 (total 18), and the last 1 electron goes into the fourth shell. The structure is 2,8,8,1. The third shell does not take a ninth electron here; the fourth shell opens instead.

An electronic structure is written as numbers separated by commas, from the innermost shell outwards. Each number is the count of electrons in that shell, and the numbers must add up to the total number of electrons (the atomic number for a neutral atom).

Worked example — sodium (atomic number 11, so 11 electrons).

• First shell: 2 (full)
• Second shell: 8 (full)
• Third shell: the remaining 1

So sodium is written 2,8,1. Check: $2+8+1=11$, matching the atomic number.

Element	Atomic number	Electronic structure
Hydrogen	1	1
Carbon	6	2,4
Oxygen	8	2,6
Neon	10	2,8
Sodium	11	2,8,1
Chlorine	17	2,8,7
Argon	18	2,8,8
Calcium	20	2,8,8,2

Always check your numbers total the atomic number. That single check catches most slips.

You must be able to give the electronic structure of every one of the first 20 elements, in both numbers and diagram form. The full list:

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

Read down the pattern: each shell fills up neatly, and the outer number climbs by one across a row before a new shell starts. That regularity is exactly what the periodic table is built on.

The diagram form shows the nucleus in the centre with circles (shells) around it, and the correct number of electrons drawn on each circle as crosses or dots. Fill the inner shells first, and spread the outer electrons around the circle.

Sodium, structure 2,8,1, is drawn like this:

The two inner electrons sit on the first shell, eight fill the second, and a single electron sits alone on the outer shell. That lone outer electron is why sodium is a very reactive Group 1 metal.

Chlorine, structure 2,8,7, has seven electrons on its outer shell:

When you draw these by hand, keep the electron counts exact: an examiner counts the electrons on each shell, so 2,8,7 must show exactly seven on the outer circle.

Electronic structure explains an atom's position in the periodic table, which is the single most useful thing it gives you.

• The number of occupied shells equals the period (row) number.
• The number of electrons in the outer shell equals the group number (for the main groups, Groups 1 to 7 and Group 0).

Worked example — chlorine, 2,8,7. It has 3 occupied shells, so it is in Period 3. It has 7 outer-shell electrons, so it is in Group 7. Both are correct: chlorine is a Period 3, Group 7 halogen.

Element	Structure	Occupied shells → period	Outer electrons → group
Lithium	2,1	2 → Period 2	1 → Group 1
Carbon	2,4	2 → Period 2	4 → Group 4
Calcium	2,8,8,2	4 → Period 4	2 → Group 2

Group 0 (the noble gases) is the exception to the outer-electron rule: helium has 2 outer electrons but sits in Group 0 because its outer shell is full. Every other noble gas has 8 outer electrons.

1. Overfilling the first shell

The first shell holds a maximum of 2 electrons. Writing hydrogen as anything but 1, or helium as anything but 2, is wrong. Only the first shell has this small limit.

2. Putting a ninth electron in the third shell for element 19 or 20

For the first 20 elements the third shell stops at 8. Potassium is 2,8,8,1 and calcium is 2,8,8,2; the extra electrons open the fourth shell rather than making the third hold 9.

3. The numbers not adding up to the atomic number

The electron counts must total the atomic number for a neutral atom. If 2,8,2 is written for an element with atomic number 11, the total is 12 and the structure is wrong.

4. Drawing the wrong number of outer electrons

Diagrams are marked by counting electrons on each shell. Draw exactly the right number on the outer shell, because it is the outer shell that determines group and reactivity.

5. Reading the structure from the outside in

Electronic structures are written innermost shell first. For sodium that is 2,8,1, not 1,8,2. Always start counting from the shell nearest the nucleus.