Cambridge O Level Chemistry · Syllabus 5070 · 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 O Level Chemistry.
Atom in context
An atom is the smallest particle of an element, made of a central nucleus containing protons and neutrons surrounded by electrons arranged in shells. Chapter 2 of Cambridge O Level Chemistry 5070 builds everything on this structure: the proton number fixes which element an atom is, the electronic configuration (for example 2,8,3) fixes how its outer electrons behave, and what those outer electrons do — transfer, share or delocalise — decides whether a substance forms a giant ionic lattice, simple molecules, a giant covalent structure or a giant metallic lattice. Each structure in turn explains the melting point, the electrical conductivity and the uses of the substance.
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.” 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.” 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.
- 3. “An ion forms when an atom gains or loses protons.” 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. Where three atoms contribute electrons, add a third symbol and say so.
- 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.
Questions students ask about Atom
What is the difference between an element, a compound and a mixture?
An element contains only one type of atom. Those atoms may be separate, as in argon, joined in molecules such as \(\mathrm{O_2}\), or joined in a giant structure such as diamond — it is still an element. A compound contains atoms of two or more different elements chemically bonded together, so it has its own properties and a fixed formula. A mixture contains two or more substances that are not chemically bonded, so each keeps its own properties and the substances can be separated by physical means.
What is the difference between mass number and relative atomic mass?
Mass number (nucleon number) belongs to one atom: it is the total number of protons and neutrons in that nucleus, so it is always a whole number. Relative atomic mass belongs to the element as a whole: it is the mean mass of its atoms, weighted by the abundance of each isotope, so it is usually not a whole number. Chlorine has atoms of mass number 35 and 37, but a relative atomic mass of 35.5 because the two isotopes occur in different proportions.
What is an isotope, and why do isotopes have the same chemical properties?
Isotopes are different atoms of the same element that have the same number of protons but different numbers of neutrons. Changing the number of neutrons does not make a different element, because identity is set by the proton number alone. Isotopes have the same chemical properties because they have the same number of electrons and therefore the same electronic configuration, and it is the outer electrons that decide how an atom bonds and reacts.
How does an ionic bond form?
An ionic bond forms in two separate steps. First, outer electrons are transferred from a metal atom to a non-metal atom: the metal atom loses electrons to become a positive ion (a cation) and the non-metal atom gains them to become a negative ion (an anion). Second, the oppositely charged ions attract one another. The ionic bond is that strong electrostatic attraction between oppositely charged ions, not the transfer itself. Only electrons move; the nucleus and the proton number are untouched.