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# The Atom ## Development of the Atomic Model **Dalton's Model:** * John Dalton thought of the atom as a solid sphere that could not be divided into smaller parts. * His model did not include protons, neutrons, or electrons. **Plum Pudding Model:** * Scientists' experiments discovered negativel...

# The Atom ## Development of the Atomic Model **Dalton's Model:** * John Dalton thought of the atom as a solid sphere that could not be divided into smaller parts. * His model did not include protons, neutrons, or electrons. **Plum Pudding Model:** * Scientists' experiments discovered negatively charged particles called electrons. * The plum pudding model describes the atom as a positively charged cloud, with negatively charged electrons embedded within it. **Alpha Scattering Experiment:** * Scientists fired alpha particles at a thin gold foil. * Most of the particles passed straight through. * Some particles were deflected at large angles. * Some particles bounced back. **Observations from Alpha Scattering:** 1. Most alpha particles passed straight through the foil. 2. A few alpha particles were deflected at large angles. 3. Some alpha particles bounced back. **Nuclear Model:** * Scientists replaced the plum pudding model with the nuclear model. * They proposed that the positive charge and mass of an atom are concentrated in a small space at the centre called the nucleus. * Electrons orbit the nucleus but not at set distances. **Electron Shell (Bohr) Model:** * Niels Bohr calculated that electrons orbit the nucleus at fixed distances. * These orbits are called shells or energy levels. ## Properties of Atomic Particles **Relative Mass:** * Protons and neutrons have approximately the same mass (relative mass of 1). * Electrons have a much smaller mass (relative mass of almost 0). **Size:** * The radius of an atom is approximately 1 × 10⁻¹⁰ m. * Nuclei (plural of nucleus) are approximately 10,000 times smaller than atoms, with a radius around 1 × 10⁻¹⁴ m. **The Proton:** * Further experiments revealed that the nucleus contains smaller particles called protons. * A proton has an opposite charge to an electron. **The Neutron:** * James Chadwick's experiments provided evidence for a neutral particle in the nucleus called a neutron.