Constituent of atom
An atom consists of a nucleus (protons and neutrons) surrounded by an electron cloud. Protons and neutrons together form the nucleus; electrons orbit in shells around it. Protons are positively charged (+1.60 × 10⁻¹⁹ C), neutrons are neutral, and electrons are negatively charged (−1.60 × 10⁻¹⁹ C), with equal magnitude to the proton charge.
Real World
Rutherford's 1911 gold foil experiment at the University of Manchester proved atoms have a tiny, dense nucleus by firing alpha particles at gold and observing unexpected large-angle scattering.
Exam Focus
State relative charges and masses to three significant figures — mark schemes require exact values like 1.60 × 10⁻¹⁹ C.
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