Structure and bonding · GCSE Chemistry
Electronic structure
GCSE Chemistry revision on electronic structure: shells 2, 8, 8, how group and period follow the outer electrons, and why noble gases are unreactive.
Fill 2, then 8, then 8. Group number is the outer electrons. Period number is the number of shells. Bonding is the story of the outer shell.
The important bits
What you need to know
- 1
Electrons occupy shells (energy levels) around the nucleus. At GCSE, the first shell holds 2, the second 8 and the third 8 for the first 20 elements.
- 2
Write electronic structure as numbers, for example sodium 2,8,1, chlorine 2,8,7, oxygen 2,6, calcium 2,8,8,2 and argon 2,8,8.
- 3
The outer shell (valence electrons) decides bonding and the group number for main-group elements. Sodium has one outer electron, so it is Group 1.
- 4
Period number equals the number of occupied shells. Sodium has three shells, so it is in Period 3. Potassium (2,8,8,1) is Period 4.
- 5
Atoms react to achieve a full outer shell. Metals in Groups 1, 2 and 3 lose outer electrons. Non-metals in Groups 5, 6 and 7 gain or share electrons.
- 6
Noble gases already have a full outer shell (He is 2; Ne is 2,8; Ar is 2,8,8), which is why they are very unreactive and exist as monatomic gases.
- 7
Bohr’s model placed electrons in shells to explain line spectra. Rutherford had shown a tiny positive nucleus; shells explain why atoms only absorb or emit certain energies.
- 8
Ions keep the same electronic structure as the nearest noble gas: Na⁺ is 2,8 like neon; Cl⁻ is 2,8,8 like argon; O²⁻ is 2,8; Mg²⁺ is 2,8.
Quotations worth analysing
Short evidence. Real method.
“Na 2,8,1 Cl 2,8,7 O 2,6 Ca 2,8,8,2”
Read the last number as the group and the count of numbers as the period. Sodium is Group 1, Period 3. Chlorine is Group 7, Period 3.
“A full outer shell is a stable electronic structure.”
This is why noble gases barely react, why Na forms Na⁺, and why Cl forms Cl⁻ or a shared pair in Cl₂. Do not say atoms “want” electrons; say they achieve a full outer shell.
“Electrons in shells: 2, then 8, then 8.”
Do not invent a third-shell capacity of 18 for potassium at this level unless a question goes beyond the first 20 elements. Calcium is 2,8,8,2, not 2,8,10.
Go deeper
The periodic table is electronic structure drawn as a grid
Once you can write 2,8,1 you already know sodium’s chemistry: one outer electron, lost to make Na⁺, vigorous with water, ionic compounds with non-metals. Chlorine 2,8,7 tells you it will gain one electron or share one pair. Carbon 2,4 tells you four covalent bonds, not a C⁴⁺ ion in ordinary chemistry. Group 0 is the full-shell column. If a six-mark bonding question starts with “explain in terms of electronic structure”, write the two structures, say how electrons are transferred or shared, and name the full-shell ions or molecules that result. A diagram of shells with dots is worth the thirty seconds it takes.
Go deeper
Ions are atoms with a different electron count, not a different nucleus
Aluminium is 2,8,3. Al³⁺ has lost the three outer electrons, so 2,8, which matches neon. The nucleus still has 13 protons; that is why the ion is 3+. Oxide is O²⁻: oxygen 2,6 gains two electrons to become 2,8. Students draw Al³⁺ with three extra electrons, or they empty the inner shells. Inner shells do not take part in ordinary GCSE reactions. When you write a half equation for electrolysis, the electrons you add or remove are these outer ones: Na⁺ + e⁻ → Na puts the outer electron back and restores 2,8,1.
Go deeper
History is three models, not a story about famous men
Thomson’s plum pudding had electrons in a ball of positive charge. Rutherford’s alpha-particle scattering showed a small dense positive nucleus and mostly empty space. Bohr put electrons in shells at fixed energies. In the exam, pair each model with the evidence that changed it. Most alpha particles went through: empty space. Some deflected: positive nucleus. A few bounced back: nucleus is dense and massive. Shells then explained why atoms emit only certain frequencies of light. “Rutherford discovered the atom” is too vague to score. Write observation, then conclusion, as a matched pair.
See the idea in action
Potassium has atomic number 19. Electronic structure is 2,8,8,1. It is Group 1, Period 4. The atom has 19 electrons. K⁺ has lost the outer electron, so 2,8,8, matching argon, with 18 electrons and 19 protons. Chlorine, atomic number 17, is 2,8,7. In KCl the outer electron from potassium transfers to chlorine: Cl⁻ is 2,8,8. The ionic bond is attraction between K⁺ and Cl⁻, not a shared pair.
Exam technique
Turn knowledge into marks
Write the structure as numbers first, then state group and period. For ions, show the electrons lost or gained and the noble-gas structure that remains. Never change the proton number.
Common mistakes
Do not give these marks away
- 01
Writing calcium as 2,8,10 instead of 2,8,8,2, or putting 18 electrons in the third shell at GCSE.
- 02
Giving ions a different number of protons, or drawing Na⁺ with an extra electron.
- 03
Saying noble gases are unreactive because they are gases, rather than because they have a full outer shell.
What is the electronic structure of a Ca²⁺ ion? Calcium is atomic number 20.
A2,8,8,2
B2,8,10
C2,8,8
D2,8,8,2,2
Show the answer
2,8,8. The calcium atom is 2,8,8,2. Ca²⁺ has lost the two outer electrons, leaving 2,8,8. The proton number is still 20, which is why the charge is 2+.
Quick questions
If this is the bit you searched
How do you write electronic structure GCSE Chemistry?
Fill shells 2, then 8, then 8 for the first 20 elements. Sodium is 2,8,1; oxygen is 2,6; calcium is 2,8,8,2. Write the numbers separated by commas.
How is electronic structure linked to group and period?
The number of outer electrons is the group number for main-group elements. The number of occupied shells is the period number.
Why are noble gases unreactive?
They already have a full outer shell (He 2; Ne 2,8; Ar 2,8,8), so they do not need to lose, gain or share electrons in ordinary reactions.
What is the electronic structure of Cl-?
Chlorine is 2,8,7. Cl⁻ has gained one electron, so 2,8,8, matching argon. Protons stay at 17, which is why the ion is 1−.