Atomic structure and the periodic table — section overview
Section C1 is the foundation of all chemistry. It covers how atoms are structured, how elements are arranged in the periodic table, and how electrons determine an element's properties.
Key ideas in C1
| Sub-topic | Core content |
|---|---|
| C1.1 | Atomic structure: protons, neutrons, electrons; atomic/mass number |
| C1.2 | Electronic structure: shells, configuration; link to period and group |
| C1.3 | Development of the periodic table: Mendeleev's contribution |
| C1.4 | Metals and non-metals; properties and their position in the table |
| C1.5 | Group 0: noble gases — full outer shells, unreactive |
| C1.6 | Group 1: alkali metals — reactivity increases down group |
| C1.7 | Group 7: halogens — reactivity decreases down group; halogen displacement |
| C1.8 | Period 3 and transition metals |
The atom
An atom contains a tiny, dense nucleus (protons + neutrons) surrounded by electrons in shells.
| Particle | Relative mass | Relative charge | Location |
|---|---|---|---|
| Proton | 1 | +1 | Nucleus |
| Neutron | 1 | 0 | Nucleus |
| Electron | ~1/2000 | −1 | Shells |
Atomic number = number of protons = number of electrons (in a neutral atom) Mass number = protons + neutrons Isotopes: atoms of the same element with different numbers of neutrons.
Electronic structure
Electrons fill shells from the inside out: first shell holds 2; second and third hold 8 each.
Period = which outer shell is being filled (Period 2 → second shell) Group = number of electrons in the outermost shell (Group 1 → 1 outer electron)
Group 1 metals all have 1 outer electron → react similarly. Noble gases (Group 0) have full outer shells → very stable → unreactive.
Periodicity — properties and trends
Group 1 (alkali metals): reactivity increases down group (outer electron further from nucleus → more easily lost).
Group 7 (halogens): reactivity decreases down group (outer shell further from nucleus → harder to gain an electron).
Halogen displacement: more reactive halogens displace less reactive ones from solutions. Cl₂ + 2KBr → 2KCl + Br₂ (chlorine displaces bromine).
Common exam mistakes in C1
- Confusing atomic number and mass number — atomic number = protons; mass number = protons + neutrons
- Isotopes have different mass numbers — same element, same number of protons, different neutrons
- Group number = outer electrons — Group 7 has 7 outer electrons; Group 0 has 8 (except He with 2)
- Mendeleev left gaps — he predicted undiscovered elements; modern table ordered by atomic number, not relative atomic mass
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