Atomic structure and the periodic table
Every atom is a tiny nucleus of protons and neutrons surrounded by electrons in shells. The number of protons (the proton number) defines the element and fixes its place in the periodic table — and that position predicts how the element behaves.
Key facts
| Topic | Particulate Nature of Matter |
| Proton number | = number of protons (defines the element) |
| Nucleon number | = protons + neutrons |
| Electron shells | 2, 8, 8 … |
Inside the atom
Protons (positive) and neutrons (neutral) sit in the nucleus; electrons (negative) occupy shells around it. The proton number equals the number of protons and identifies the element. The nucleon number is protons plus neutrons. A neutral atom has equal protons and electrons.
Isotopes
Isotopes are atoms of the same element with the same proton number but different numbers of neutrons — so the same chemistry, different mass. Chlorine, for example, exists as two isotopes, which is why its relative atomic mass is not a whole number.
Electron shells
Electrons fill shells from the inside out, holding up to 2, then 8, then 8 for the elements you meet at this level. The number of electrons in the outer shell is what drives bonding. (K324 does not require the s, p, d, f orbital classification — shell arrangement is enough.)
Reading the periodic table
Groups (columns) share the same number of outer electrons and so behave similarly — Group I metals are reactive and get more reactive down the group; Group VII non-metals get less reactive down the group; Group 0 (noble gases) are unreactive with full outer shells. Periods (rows) show electron shells filling across. The transition metals sit in the middle, often coloured and useful as catalysts.Frequently asked questions
What does the proton number tell you?
It equals the number of protons and defines the element — it also fixes the element's position in the periodic table, which predicts its behaviour.
What are isotopes?
Atoms of the same element with the same proton number but different numbers of neutrons — same chemistry, different mass.
How are electrons arranged?
In shells filling from the inside out, holding up to 2, then 8, then 8 at this level. The outer-shell electrons drive bonding.
What do groups and periods mean?
Groups are columns of elements with the same number of outer electrons and similar behaviour; periods are rows showing shells filling across.
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