Electronic Structure

How electrons arrange themselves around the nucleus — the rule that explains every column of the periodic table. You must be able to write the structure of any of the first 20 elements in two forms: as numbers (2,8,1) and as a diagram.

SPEC 4.1.1.7 • ATOMIC STRUCTURE

Why This Lesson

Electronic structure is the reason the periodic table works. The number of electrons in the outer shell decides whether an element is reactive, what kind of ion it forms, and why it sits in a particular column. Get the rules right here, and Groups 1, 7 and 0 in later lessons become straightforward.

What AQA tests

Three predictable tasks: (1) draw the electronic structure of a given element on a blank shell diagram; (2) write the structure of any of the first 20 elements as numbers; (3) compare the structures of two elements (e.g. sodium vs potassium) using the correct vocabulary.

Two examiner-flagged errors

Writing "more outer shells" (the outer shell is always singular — there's only ever ONE outermost shell). And replying with "different number of electrons" instead of giving the actual structures (2,8,1 vs 2,8,8,1). Both lose marks every year.

Shell Filling Rules

Two rules govern where every electron goes in a neutral atom:

Rule 1 — Lowest energy first

Electrons occupy the lowest available energy levels first — that is, the innermost available shells. You don't put an electron in shell 2 until shell 1 is full. You don't put one in shell 3 until shell 2 is full.

Rule 2 — Maximum electrons per shell

ShellMaximum electrons1st shell (innermost)22nd shell83rd shell (for first 20 elements)84th shell (only K and Ca need this)up to 2 (for first 20 elements)

Spec wording — both terms accepted

The spec says "students may answer questions in terms of either energy levels or shells." Both are equivalent and either earns the mark. We'll use "shells" throughout for consistency.

Two Notation Forms

AQA accepts both forms. You must be able to read and produce each. They contain identical information, just shown differently.

Form 1 — Numbers separated by commas

Write the number of electrons in each shell, starting from the innermost, separated by commas. Sodium = 2,8,1 means 2 in shell 1, 8 in shell 2, 1 in shell 3. Calcium = 2,8,8,2 means 2 in shell 1, 8 in shell 2, 8 in shell 3, 2 in shell 4.

Form 2 — Diagram (concentric circles)

Draw concentric circles for the shells around a central nucleus (typically a dot). Place electrons on the shells as crosses (×) or dots — AQA mark schemes accept "x, •, o, e⁻". Innermost shell filled first. Pair electrons up where possible (4 pairs of 2 around shell 2 looks neat).

Equivalence

For sodium (atomic number 11): "2,8,1" and a diagram with 2 + 8 + 1 = 11 crosses around a central nucleus describe the same atom. Choose whichever the question asks for. If unspecified, the diagram form is usually expected when the question provides a blank diagram (with empty shells already drawn).

Worked Example — Sodium (2,8,1)

Sodium has atomic number 11. In a neutral atom, that means 11 electrons. Apply the filling rules:

1
Fill shell 1: maximum 2 electrons → 2 in shell 1. (9 left to place.)
2
Fill shell 2: maximum 8 electrons → 8 in shell 2. (1 left.)
3
Place remaining electron in shell 3 → 1 in shell 3.
✓
Check: 2 + 8 + 1 = 11 ✓ Structure: 2,8,1

Drawing convention

Pair electrons up around each shell (one pair at top, right, bottom, left for shell 2). The outer shell often has lone electrons drawn singly. AQA accepts ×, •, o, e⁻ — pick one and use it consistently.

Worked Example — Aluminium (2,8,3)

Aluminium has atomic number 13 → 13 electrons. This is the exact diagram from AQA P1 June 2022 Q01.4 — a 1-mark "complete the diagram" question.

1
Shell 1: 2 electrons. (11 left.)
2
Shell 2: 8 electrons (full). (3 left.)
3
Shell 3: 3 electrons. Total = 2 + 8 + 3 = 13 ✓ Structure: 2,8,3

What earns the mark

The examiner checks: (1) correct number of electrons per shell (2 in inner, 8 in middle, 3 in outer); (2) innermost filled first (no gaps in inner shells); (3) total electrons matches atomic number (13). Drawing pairs neatly is good practice but the count is what scores.

First 10 Elements (H → Ne)

Hydrogen (atomic number 1) to neon (atomic number 10) — fills shells 1 and 2:

hydrogen (1)

helium (2)

lithium (3)

2,1

beryllium (4)

2,2

boron (5)

2,3

carbon (6)

2,4

nitrogen (7)

2,5

oxygen (8)

2,6

fluorine (9)

2,7

neon (10)

2,8

Pattern to notice

From hydrogen to helium, shell 1 fills (1 → 2). From lithium to neon, shell 2 fills (2,1 → 2,8). At neon, both shells 1 and 2 are full — that's why neon is unreactive (a noble gas).

Elements 11 → 20 (Na → Ca)

Sodium (atomic number 11) to calcium (atomic number 20) — fills shell 3 (and starts shell 4 for K and Ca):

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

Watch the shell change at K

Shell 3 fills up to 8 (at argon, 2,8,8). Then for potassium (atomic number 19), the 19th electron goes into shell 4, giving 2,8,8,1. The third shell can hold more than 8 in reality, but for the first 20 elements the GCSE rule is "max 8 in shell 3".

Group Number = Outer Shell Electrons

The most useful pattern in chemistry. The group number in the periodic table tells you exactly how many electrons are in the outer shell.

GroupOuter shell electronsExamples

1 (alkali metals)1Li (2,1), Na (2,8,1), K (2,8,8,1)

22Be (2,2), Mg (2,8,2), Ca (2,8,8,2)

33B (2,3), Al (2,8,3)

7 (halogens)7F (2,7), Cl (2,8,7)

0 (noble gases)8 ( except helium = 2 )Ne (2,8), Ar (2,8,8); He (2)

Group 0 — the helium exception

Helium is in Group 0 but has only 2 outer electrons (shell 1 is full at 2). All other Group 0 elements have 8. The mark-scheme phrase is: "8 outer shell electrons, except helium which has 2." Forgetting helium is a recurring error (P1 Jun 2024 Q02.2).

Period Number = Number of Shells

The other half of the pattern. The period number tells you how many shells the atom uses.

PeriodOccupied shellsExamples (with structures)

11H (1), He (2)

22Li (2,1), C (2,4), Ne (2,8)

33Na (2,8,1), Cl (2,8,7), Ar (2,8,8)

44K (2,8,8,1), Ca (2,8,8,2)

Practical use

If you know the group and period, you can write the electronic structure without memorising — just count shells (period) and place outer electrons (group). E.g. element in Period 3, Group 6 has 3 shells with 6 outer electrons → 2, 8, 6 → sulfur.

Comparing Two Elements — Sodium vs Potassium

This is the AQA P1 June 2024 Q02.1 question pattern. Students consistently lose marks on the wording — every word matters.

The structures

Sodium: 2,8,1 | Potassium: 2,8,8,1

Strong answer (full marks)

Similarity: both have 1 electron in their outer shell. (That's why they're both Group 1 and react similarly.)

Difference: sodium has 3 occupied shells (2,8,1); potassium has 4 occupied shells (2,8,8,1) — i.e. potassium has one more shell than sodium.

Weak answer (loses marks)

"They have a different number of electrons" — this isn't a structure, it's a count. Doesn't earn the mark.

"Potassium has more outer shells than sodium" — wrong. The outer shell is always SINGULAR. Should be "potassium has one more shell than sodium."

Mark-scheme phrases

Similarity: "same number of outer (shell) electrons" or "both have one outer electron". Difference: "sodium has 3 shells but potassium has 4 shells" or "potassium has one more shell". Both must reference the specific structure.

PEE Framework — Worked Chain

Q: Use electronic structures to explain why sodium and potassium have similar chemical properties. 2 MARKS

P — PointSodium and potassium react similarly because they have the same number of outer shell electrons.

E — EvidenceSodium is 2,8,1 — 1 electron in the outer shell. Potassium is 2,8,8,1 — also 1 electron in the outer shell. Both are in Group 1.

E — ExplainChemical reactions involve outer electrons. With the same outer electron count, both elements lose 1 electron in similar reactions, forming +1 ions and producing similar products.

How marks are scored

Mark 1: same number of outer electrons. Mark 2: a clear link to similar reactions or formation of +1 ions. The structures (2,8,1 and 2,8,8,1) aren't strictly required for the marks but make the answer concrete.

Examiner Alert

Errors flagged in P1 Jun 2024 Q02.1

Error 1: "Potassium has more outer shells than sodium" — loses the mark. There is only ever ONE outer shell (the outermost). Plural "outer shells" is meaningless. Correct: "potassium has one more shell than sodium".

Error 2: "They have a different number of electrons" — this isn't an electronic structure, it's a count. Electronic structure means the distribution: 2,8,1 vs 2,8,8,1.

Error 3: Wrong shell counts — saying sodium has 2 shells (it has 3) or potassium has 3 shells (it has 4). Count occupied shells in the structure.

Weak student

"They have a different number of electrons. Potassium has more outer shells."

Two errors: "different electrons" isn't a structure; "more outer shells" is grammatically and chemically wrong.

Strong student

"Sodium is 2,8,1; potassium is 2,8,8,1. They have the same number of outer electrons (1) but potassium has one more shell."

Hits both marks: specific structures + correct vocabulary.

Common Errors

Students often writeCorrect versionWhy it matters

"More outer shells "" One more shell " (or specify the count)Outer shell = the SINGLE outermost; plural is nonsense

"They have different numbers of electrons""Sodium is 2,8,1; potassium is 2,8,8,1"Structure = distribution, not count

"Group 0 has 8 outer electrons" (no exception)"8 outer electrons, except helium which has 2 "Helium fills shell 1 with just 2 — exception every year

Wrong shell count (e.g. sodium has 2 shells)Sodium has 3 shells (2,8,1)Period number = number of occupied shells

Filling outer shells before inner onesAlways fill innermost first (lowest energy)Spec rule — no gaps in inner shells

Drawing the wrong total number of electronsTotal = atomic number for a neutral atomCount crosses against atomic number to verify

Practice Question 1

Complete the diagram below to show the electronic structure of an aluminium atom (atomic number 13). Use the periodic table.1 MARK

(Pattern from AQA 8462 P1 June 2022 Q01.4)

1

Aluminium has atomic number 13 → 13 electrons in a neutral atom.

2

Apply filling rules: 2 (shell 1) + 8 (shell 2) + 3 (shell 3) = 13 ✓

3

Place 2 crosses on shell 1, 8 on shell 2, 3 on shell 3. See diagram on slide 6.

Mark scheme note: the 1 mark requires all 13 electrons correctly placed (2, 8, 3). AQA accepts ×, •, o, e⁻ as electron symbols.

Practice Question 2

(a) Write the electronic structure (in numbers) of an atom of sulfur (atomic number 16). 1 MARK

(b) An element has electronic structure 2,8,7. Identify the element and state its group number. 2 MARKS

(a) Sulfur, atomic number 16:

1

16 electrons → 2 (shell 1) + 8 (shell 2) + 6 (shell 3) = 16. Structure: 2,8,6. [1 mark]

(b) Element with structure 2,8,7:

1

Total electrons = 2 + 8 + 7 = 17 → atomic number 17 → chlorine. [1 mark]

2

Outer shell has 7 electrons → group number = 7 (the halogens). [1 mark]

Practice Question 3

Sodium and potassium are both in Group 1 of the periodic table.

(a) Give the electronic structure of sodium and potassium. 2 MARKS

(b) Give one similarity and one difference between the electronic structures of sodium and potassium. 2 MARKS

(Pattern from AQA 8462 P1 June 2024 Q02.1)

(a)

1

Sodium (atomic number 11): 2,8,1. [1 mark]

2

Potassium (atomic number 19): 2,8,8,1. [1 mark]

(b)

S

Similarity: both have the same number of outer shell electrons (1). [1 mark]

D

Difference: sodium has 3 shells, potassium has 4 shells (potassium has one more shell). [1 mark]

Examiner notes: "potassium has more outer shells" loses the mark. "They have a different number of electrons" isn't an electronic structure. State the specific shell count or use the phrase "one more shell".

Summary

Shell Filling Rules

Fill the innermost available shell first (lowest energy)

Maximum: shell 1 = 2 ; shell 2 = 8 ; shell 3 = 8 (for first 20 elements)

Total electrons = atomic number (in a neutral atom)

Two Notation Forms

Numbers: 2,8,1 (sodium) — comma-separated, innermost first

Diagram: concentric circles with crosses (or •, o, e⁻) for electrons; pair them up where possible

Both are equivalent and AQA accepts either

Periodic Table Rules

Group number (1–7) = number of outer shell electrons

Group 0 = full outer shell (8 electrons, except helium = 2 )

Period number = number of occupied shells

Three things never to write

(1) "More outer shells" — outer shell is singular. (2) "Different number of electrons" — that's not a structure. (3) Forget that helium has 2 outer electrons, not 8.

Up Next — Lesson 1.06: The Periodic Table, Its Development & Metals vs Non-Metals

How the periodic table was built — Mendeleev's gaps, the move from atomic weight to atomic number, and the dividing line between metals and non-metals.