Electron Configuration

Use proton number Z to find the electron total, fill the shells in order, then read the element’s group and period from the arrangement.

1. Definition

Electron configuration describes how electrons are arranged in electron shells (energy levels) around the nucleus, written as numbers in each shell (e.g., 2.8.1).

2. Key Ideas

• Electrons fill shells starting from the shell closest to the nucleus.
• For the first 20 elements, use the filling pattern 2, 8, 8, 2. This is not a statement that the fourth shell can hold only two electrons.
• In a neutral atom, number of electrons = proton number Z (see Atoms).
• Valence electrons are electrons in the outermost occupied shell.
• Group (main-group elements) = number of valence electrons.
• Period = number of occupied shells.

3. Detailed Explanations

A. Shell Filling Rules (First 20 Elements)

Shell capacities used at O-Level for the first 20 elements:

• 1st shell: maximum 2 electrons
• 2nd shell: maximum 8 electrons
• 3rd shell: maximum 8 electrons (for the first 20 elements)
• 4th shell: starts filling at potassium and calcium (so 19 = 2.8.8.1; 20 = 2.8.8.2)

B. Stable Outer Shell (Duplet/Octet)

Atoms are more stable when their outer shell is full:

• Duplet: 2 electrons in the first shell (helium)
• Octet: 8 electrons in the outer shell (e.g., neon, argon)

Noble gases have full outer shells, so they are generally unreactive.

C. Using Electron Configuration to Find Group and Period

• Group (main-group elements): number of valence electrons for Groups 1, 2 and 13–17 (e.g. 2.8.7 has 7 valence electrons → Group 17).
• Period: number of occupied shells (e.g., 2.8.7 has 3 shells → Period 3).

D. Electron Configuration Table (First 20 Elements)

4. Common Mistakes

• Writing 2.8.9 or 2.10.7 (wrong). The second shell max is 8 at this level.
• Using the wrong electron number (neutral atom has electrons = Z).
• Confusing Group and Period (Group is valence electrons; Period is occupied shells).
• Saying noble gases are unreactive “because they have 8 electrons” (helium has 2; say “full outer shell”).
• Trying to use orbital notation (1s²2s²…) — not required here.

5. Exam Tips

6. Worked Examples

Modelled example 1

Find the Element, Group, Period

Core

Problem

A neutral atom has electron configuration 2.8.7. Identify the element, its group and its period.

Study the worked solution

1. Count all electrons

   Method

   Add the shell populations.

   Reason

   A neutral atom has the same numbers of electrons and protons, so the total gives Z.

   Working

   2 + 8 + 7 = 17 electrons, so Z = 17 and the element is chlorine.

2. Read the group

   Method

   Use the outermost-shell population.

   Reason

   Main-group number is the number of valence electrons.

   Working

   7 valence electrons → Group 17.

3. Read the period

   Method

   Count the occupied shells.

   Reason

   Period number is the number of occupied electron shells.

   Working

   3 occupied shells → Period 3.

4. State the complete result

   Working

   Chlorine; Group 17; Period 3.

Guided practice 2

Write the Electron Configuration From Proton Number

About 5 min

Problem

Write the electron configuration of a neutral aluminium atom, Z = 13.

Fill each shell in order

First shell

Second shell

Third shell

Check my attempt

Hints

Hint 1: set the electron total

A neutral atom with Z = 13 has 13 electrons.

Hint 2: fill from the inside

Place 2 in the first shell and 8 in the second before putting the remainder in the third.

View solution step by step

1. Determine the electron count

   Method

   Use electrical neutrality.

   Reason

   A neutral aluminium atom has one electron for each of its 13 protons.

   Working

   Total electrons = Z = 13.

2. Fill shells from the nucleus outwards

   Method

   Fill 2, then 8, then place the remainder.

   Reason

   For the first 20 elements, inner shells fill before the next shell.

   Working

   13 = 2 + 8 + 3.

3. Write the configuration

   Working

   Aluminium: 2.8.3.

Common misconception 3

Error Analysis (Fix the Student)

Find and correct the mistake

Learner response

A student says: “An atom with configuration 2.8.2 is in Group 2 because it has two shells.” Locate the error and correct the explanation.

Separate group from period

Group

Group 2Group 13Period 2

Period

Period 2Period 3Group 13

Check my attempt

View solution step by step

1. Locate the reasoning error

   Method

   Reject the claim that 2.8.2 has two occupied shells.

   Reason

   Each number describes one occupied shell, so the configuration contains three shells.

   Working

   2 | 8 | 2 → three occupied shells.

2. Use the correct group rule

   Method

   Read the final shell population.

   Reason

   For a main-group element, group is determined by valence electrons, not shell count.

   Working

   2 valence electrons → Group 2.

3. Correct the full statement

   Working

   The atom is in Group 2 because it has 2 valence electrons, and in Period 3 because it has 3 occupied shells.

Examiner practice 4

Predict the Ion Charge From Configuration

3 marks

Examination question

A neutral atom has electron configuration 2.8.1. Identify the atom, state the ion it forms and give the ion’s electron configuration. [3 marks]

Write a complete three-part answer

Identify the atom, ion and ion configuration

View solution step by step

1. Identify the neutral atom

   1 mark

   Method

   Add the electrons and match the proton number.

   Reason

   The atom is neutral, so 11 electrons means Z = 11.

   Working

   2 + 8 + 1 = 11, so the atom is sodium.

2. Predict the ion

   1 mark

   Method

   Remove the single valence electron.

   Reason

   Losing one electron produces a full outer shell and a net 1 + charge.

   Working

   Na → Na⁺ + e⁻.

3. Give the ion configuration

   1 mark

   Method

   Write the two remaining occupied shells.

   Working

   Na⁺ has electron configuration 2.8.

Self-mark with the mark scheme

Compare your response with each mark point. Select a point only when your response contains that evidence.

Self-mark identification, ion charge and final electron configuration separately.

Mark points

1. 1 mark: Identifies the neutral atom as sodium.
2. 1 mark: States that sodium loses one electron to form Na+.
3. 1 mark: Gives the Na+ electron configuration as 2.8.

Calculate my mark

Challenge 5

Find Configuration of an Ion

Minimal support

Particle-count transfer

A particle has proton number 17 and 18 electrons. Deduce its symbol with charge and its electron configuration.

Infer charge before arranging electrons

Particle

Cl atomCl- ionCl+ ion

Electron configuration

2.8.72.8.82.8.9

Check my attempt

Hints

Hint 1: compare positive and negative charges

Compare 17 positive proton charges with 18 negative electron charges.

Hint 2: use every electron

The particle has 18 electrons to distribute using the first-20 filling pattern.

View solution step by step

1. Identify the element

   Method

   Use proton number rather than electron number.

   Reason

   Proton number fixes the element even when the particle is an ion.

   Working

   Z = 17 identifies chlorine.

2. Infer the charge

   Method

   Compare electron and proton counts.

   Reason

   One more electron than proton gives one net negative charge.

   Working

   Charge = 17-18 = -1, so the particle is Cl⁻.

3. Arrange the electrons

   Method

   Fill all 18 electrons from the inner shell outwards.

   Working

   18 = 2 + 8 + 8, so Cl⁻ has configuration 2.8.8.

Next step

Try Mind Stretcher 1 independently.

7. Mind Stretchers

Mind stretcher 1: Two Candidates TrapExtension

Question: A particle has electron configuration 2.8. Which element could it be, and what extra information would you need to decide?

Show Answer

2.8 means 10 electrons. That could be:

• neon atom ( Z = 10), or
• an ion with 10 electrons (e.g., Na⁺ or Mg²⁺).

You need to know whether it is a neutral atom or an ion (or be told the charge / proton number).

Mind stretcher 2: Group and Typical Ion ChargeExtension

Question: An element is in Period 3 and Group 16. (a) Write its electron configuration. (b) State the ion it commonly forms.

Show Answer

Period 3 means 3 shells. Group 16 means 6 valence electrons.

(a) Configuration: 2.8.6 (sulfur).
(b) It gains 2 electrons to reach 2.8.8 → S²⁻.

8. Quiz