Electron Shell – IGCSE Chemistry Definition

Electron shells (also called energy levels) are the regions around the nucleus where electrons are found. Writing correct electron arrangements is an essential skill on the 0620 syllabus — it underpins bonding, the periodic table, and ion formation. Examiners award separate marks for correct electron arrangements, so getting the rules right is worth practising.

The 0620 definition

An electron shell is an energy level around the nucleus of an atom in which electrons are found. Electrons fill shells starting from the innermost shell (lowest energy level) outward.

Shell capacity rules

Shell number	Maximum electrons	Distance from nucleus
1st shell	2	Closest
2nd shell	8	Middle
3rd shell	8 (at IGCSE level)	Furthest (for elements up to calcium)

Note: the 3rd shell can actually hold up to 18 electrons, but for the 0620 syllabus (up to calcium, atomic number 20), it fills to 8 before the 4th shell begins. Potassium (19) is 2, 8, 8, 1 and calcium (20) is 2, 8, 8, 2.

How to write electron arrangements

1. Find the atomic number (= number of electrons in a neutral atom)
2. Fill the 1st shell first (up to 2)
3. Fill the 2nd shell next (up to 8)
4. Fill the 3rd shell (up to 8)
5. Any remaining electrons go into the 4th shell

Examples:

Element	Atomic number	Electron arrangement
Helium	2	2
Nitrogen	7	2, 5
Neon	10	2, 8
Silicon	14	2, 8, 4
Argon	18	2, 8, 8
Potassium	19	2, 8, 8, 1

Electron shells and the periodic table

• Group number = number of electrons in the outer shell (for main groups)
• Period number = number of occupied electron shells
• Noble gases have full outer shells — this is why they are unreactive

Electron shells and ion formation

When atoms form ions, they gain or lose outer shell electrons to achieve a full outer shell:

• Sodium (2, 8, 1) loses 1 electron to become Na+ (2, 8)
• Chlorine (2, 8, 7) gains 1 electron to become Cl- (2, 8, 8)

Worked exam question

Write the electron arrangements for: (a) an atom of sulfur (atomic number 16) (1) (b) a sulfide ion, S²- (1) (c) State which noble gas has the same electron arrangement as S²-. (1)

Mark scheme

(a) 2, 8, 6 [1]

(b) 2, 8, 8 [1] (sulfur gains 2 electrons)

(c) Argon (Ar) [1]

Common exam mistakes

• Putting more than 2 electrons in the first shell. The maximum is always 2.
• Writing electron arrangements for ions as if they were neutral atoms. Remember: cations have fewer electrons than the atomic number; anions have more.
• Confusing the 3rd shell limit. At IGCSE level, treat the 3rd shell as holding 8 electrons maximum. Potassium starts a 4th shell rather than putting a 9th electron in the 3rd.