Cambridge O Level Chemistry · Syllabus 5070 · Atoms, Elements and Compounds
Element
What is Element?
A substance that contains only one type of atom, meaning every atom in it has the same proton number. An element cannot be broken down into simpler substances by chemical means. Its atoms may exist separately, as in argon, joined in molecules, as in oxygen, or joined in a giant structure, as in diamond, and in every one of those cases the substance is still an element.
This definition is part of the Atoms, Elements and Compounds chapter in Cambridge O Level Chemistry.
Element 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.
Common mistakes with Element
- 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.
- 4. “Change the number of neutrons and you get a different element.” Repair You get a different isotope of the same element. Identity is set by the proton number alone. Taught fully in Lesson 2.3A.
Questions students ask about Element
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.