Cambridge O Level Chemistry · Syllabus 5070 · Atoms, Elements and Compounds
Electronic configuration
What is Electronic configuration?
The arrangement of the electrons of an atom or ion in its shells, written as a list of numbers separated by commas, such as 2,8,3. Shells are filled from the innermost outwards, holding up to 2 electrons in the first shell and up to 8 in the second and third for proton numbers up to 20. The number of electrons in the outer shell gives the group number for Groups I to VII, and the number of occupied shells gives the period number.
This definition is part of the Atoms, Elements and Compounds chapter in Cambridge O Level Chemistry.
Electronic configuration 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.
Questions students ask about Electronic configuration
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.