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8
14−6 = 8.
O Level · English Medium · Cambridge · Chemistry
Atomic structure and periodic patterns
Use atomic number, electron arrangement and periodic position to explain patterns.
Shared cross-level core and stretch unit. Start with the core questions, then use the stretch example for deeper work. The same unit is available in related class and board pathways; it is not a full grade-specific or examination-specification course. A teacher should select the appropriate depth and further topics.
Learn the idea
Atomic number counts protons; mass number counts protons plus neutrons in a specified isotope. Neutral atoms have equal protons and electrons. Elements in a group can share valence patterns, but trends need a reason involving electron structure rather than only a memorised direction.
Worked example
Sodium-23 has 11 protons and 12 neutrons. A neutral sodium atom has 11 electrons; a sodium ion Na+ has 10 after losing one electron.
- Identify isotope and charge.
- Count each particle separately.
- Connect outer electrons to a stated pattern.
Stretch example
Relative atomic mass is a weighted isotope mean. A fictional element with 75% isotope mass 35 and 25% mass 37 has mean 0.75×35 + 0.25×37 = 35.5.
Your written practice
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10
A positive unit charge means one electron fewer.
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protons
Proton number defines the element.
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They have the same electron structure when neutral, despite different neutron numbers.
Connect chemistry mainly to electrons.
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Increasing nuclear charge attracts electrons in the same principal shell more strongly, with limited additional shielding.
State that this is a general pattern and explain attraction.
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10.5
(10+11)/2 = 10.5.
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Mass number counts nucleons in one isotope; relative atomic mass averages isotope masses over an abundance distribution.
Distinguish a count from a weighted average.
Isotope comparison
Compare two fictional isotopes and their ions using labelled particle counts.
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- Keep proton number consistent.
- Adjust electrons for charge.
- Explain which properties depend on isotope or ion formation.
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