1.1.1—Atomic structure and nucleus
- Syllabus
- 9701–2028–2029
- Objective
- 1.1.1
- Level
- AS
Atomic structure explains how nuclear identity and electrons fit together
An atom is mostly empty space. A very small, dense, positively charged nucleus contains protons and neutrons, with electrons arranged in shells around it.
The nucleus contains nearly all of an atom’s mass because protons and neutrons are much more massive than electrons. In a neutral atom, the positive charge of the protons is balanced by the negative charge of the electrons.
The number of protons identifies the element; changing the number of neutrons gives an isotope, while changing the number of electrons gives an ion. These changes do not move the nucleus from the centre of the atom.
Shells describe allowed electron energy levels or regions, not fixed miniature planetary orbits. Detailed electron configurations, orbital shapes and atomic-radius trends belong to the neighbouring objectives.
ConceptA-Level CAIE Chemistry AS