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1.4.2Principles of Chemistry

Electronic Configurations — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions

5 min read 15 parts Lesson 19 of 50

📚 Edexcel International GCSE Science Double Award 4SD0 - Chemistry Hub / Topical Revision
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Part 1 of 8Rules for filling shells

1
Electrons occupy energy levels (shells) around the nucleus. The shell closest to the nucleus has the lowest energy and fills first.
2
The first shell holds a maximum of 2 electrons.
3
The second and third shells each hold up to 8 electrons (for the first 20 elements).
4
A new shell begins only when the one before is full.
5
Write the configuration as numbers separated by commas, for example sodium (11 electrons) is 2,8,1.

Part 2 of 8Examples

ElementAtomic numberConfigurationGroup and period
Hydrogen11Group 1, period 1
Carbon62,4Group 4, period 2
Oxygen82,6Group 6, period 2
Neon102,8Group 0, period 2
Sodium112,8,1Group 1, period 3
Chlorine172,8,7Group 7, period 3
Calcium202,8,8,2Group 2, period 4

Part 3 of 8Link to the periodic table

Electrons and position
Group number
  • Group number The group number equals the number of electrons in the outer shell.
    • Chlorine, with 7 outer electrons, is in group 7.
Period number
  • Period number The period number equals the number of occupied shells.
    • Sodium has three shells, so it is in period 3.
Noble gases
  • Noble gases Group 0 elements have full outer shells and are very unreactive.
Similar reactions
  • Similar reactions Elements in the same group have the same outer-shell electrons, so they react in similar ways.

Part 4 of 8Drawing electron shell diagrams

Nucleus
  • Nucleus Draw a circle or write the symbol in the middle to represent the nucleus.
Shells
  • Shells Draw concentric circles for each shell occupied.
Electrons
  • Electrons Place crosses or dots on each circle, filling inner shells first. Spread them evenly.

Part 5 of 8More configurations to practise

ElementAtomic numberConfigurationGroup and period
Lithium32,1Group 1, period 2
Nitrogen72,5Group 5, period 2
Magnesium122,8,2Group 2, period 3
Silicon142,8,4Group 4, period 3
Argon182,8,8Group 0, period 3
Potassium192,8,8,1Group 1, period 4

Part 6 of 8Worked example: phosphorus

1
Phosphorus has atomic number 15, so a neutral atom has 15 electrons.
2
Put 2 electrons in the first shell, leaving 13.
3
Put 8 electrons in the second shell, leaving 5.
4
Place the remaining 5 electrons in the third shell, giving the configuration 2,8,5.
5
Three shells means period 3 and five outer electrons means group 5.

Part 7 of 8Why configuration explains reactivity

Losing electrons
  • Losing electrons Metals have few outer electrons, so they tend to lose them to reach a full outer shell and form positive ions.
Gaining electrons
  • Gaining electrons Non-metals with nearly full outer shells tend to gain electrons and form negative ions.

Part 8 of 8Stable arrangements

Full outer shells
Helium and neon
  • Helium and neon Helium (2) and neon (2,8) have full outer shells, which makes them very unreactive.
Why atoms react
  • Why atoms react Other atoms react by losing, gaining or sharing electrons to obtain the same stable arrangement as the nearest noble gas.
Predicting ions
  • Predicting ions Sodium (2,8,1) loses one electron to become Na+ with the arrangement 2,8, while chlorine (2,8,7) gains one to become Cl− with 2,8,8.

Key terms

TermMeaning
Electronic configurationThe arrangement of electrons in the shells of an atom.
ShellAn energy level in which electrons are found around a nucleus.
Outer shellThe highest-energy occupied shell; its electrons decide chemical behaviour.
Noble gasAn unreactive element in group 0 with a full outer shell.
Valence electronAn electron in the outermost shell, involved in bonding.

Avoid these mistakes

1
Putting 8 electrons in the first shell. It holds only 2.
2
Mixing up group and periodgroup counts outer electrons, period counts shells.
3
Starting the third shell before the second is full.
4
Writing the configuration of calcium as 2,8,10; the third shell holds 8 and the extra 2 go in a fourth.

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