Cambridge O Level Chemistry · Syllabus 5070 · Atoms, Elements and Compounds
Proton number
What is Proton number?
The number of protons in the nucleus of an atom, also called the atomic number and given the symbol Z. It is what defines the element: every atom with the same proton number is the same element, and changing it would change the element. In a neutral atom the proton number also equals the number of electrons.
This definition is part of the Atoms, Elements and Compounds chapter in Cambridge O Level Chemistry.
Proton number 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 Proton number
- 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 Proton number
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.