Atomic number
The number of protons in the nucleus of an atom, denoted by Z. Atomic number uniquely identifies the chemical element; all atoms of the same element have the same atomic number regardless of the number of neutrons. In a neutral atom, the atomic number equals the number of electrons.
Real World
In PET scanners used by the NHS, the atomic number of oxygen-15 (Z = 8) determines how it bonds in water molecules injected into patients to trace blood flow in the brain.
Exam Focus
If asked to define atomic number, always state 'number of protons' — writing 'number of protons and electrons' loses the mark.
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