Cambridge IGCSE Chemistry · Syllabus 0620 · Atoms, Elements and Compounds
Atom
What is Atom?
The smallest particle of an element that still carries the identity of that element. It has a very small central nucleus containing protons and neutrons, surrounded by electrons arranged in shells. Almost all of the mass is in the nucleus because protons and neutrons have a relative mass of 1 while an electron has a relative mass of about one eighteen-hundredth, and an atom is electrically neutral because it contains equal numbers of protons and electrons.
This definition is part of the Atoms, Elements and Compounds chapter in Cambridge IGCSE Chemistry.
Atom in context
Topic 2 is one chain of reasoning, not seven disconnected lists. You decide what a substance is made of, look inside its atoms, follow what the outer electrons do, and the structure that results tells you the melting point, the conductivity and the use. Read this map in 10–12 minutes before you start the lessons, and return to it whenever a question feels unfamiliar.
Common mistakes with Atom
- 1. “An element is a substance made of single, separate atoms.” Core 2.1.1 Repair An element contains only one type of atom. Those atoms may be separate (argon), joined in molecules (\(\mathrm{O_2}\), \(\mathrm{S_8}\)) or joined in a giant structure (diamond). It is still an element. Taught fully in Lesson 2.1.
- 2. “Mass number and relative atomic mass are the same thing.” Supplement 2.3.4 Repair Mass number belongs to one atom and is always a whole number. Relative atomic mass belongs to the element and is an abundance-weighted mean, so it is usually not a whole number. Taught fully in Lesson 2.3B, which is a Supplement — Extended candidates section. Core candidates still need to know that the two quantities are different (mass number is Core 2.2.4); only the calculation is Supplement.
- 3. “An ion forms when an atom gains or loses protons.” Core 2.4.1 Repair Only electrons move. The nucleus is untouched, so the proton number — and therefore the element — never changes when an ion forms. Taught fully in Lesson 2.4A.
- An inconsistent key. Repair If the key says dots are the metal's electrons, then every dot in the diagram is a metal electron. Switching part-way through — or leaving the key off altogether — makes the diagram unreadable, because the whole point of the two symbols is to show provenance. Extended candidates: where a third kind of atom contributes electrons, as in methanol, add a third symbol and say so in the key.
- Extra electrons added to make a shell “look full”. Repair Fill shells by drawing the correct bonds, never by adding marks. If an atom will not reach eight, the bonding is wrong, not the electron count — go back and check whether a double or triple bond is needed.