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# The Atom ## Development of the Atomic Model **Dalton's Model:** John Dalton proposed the atom as a solid, indivisible sphere. This model did not include protons, neutrons, or electrons. **Plum Pudding Model:** Experiments revealed subatomic particles, particularly electrons. The plum pudding...

# The Atom ## Development of the Atomic Model **Dalton's Model:** John Dalton proposed the atom as a solid, indivisible sphere. This model did not include protons, neutrons, or electrons. **Plum Pudding Model:** Experiments revealed subatomic particles, particularly electrons. The plum pudding model depicted a positive cloud with negatively charged electrons embedded within. Protons and neutrons were not yet discovered. **Alpha Scattering Experiment:** * Scientists fired alpha particles (positively charged) at a thin gold foil. * Most particles passed straight through, but some were deflected or bounced back. * This unexpected result suggested that the positive charge and mass of an atom are concentrated in a small space at the center, called the nucleus. **Nuclear Model:** The plum pudding model was replaced with the nuclear model. Protons and neutrons were now part of the model. Electrons orbit this nucleus, but not at fixed distances. **Electron Shell (Bohr) Model:** Niels Bohr refined the model by suggesting that electrons orbit the nucleus in fixed energy levels or shells. ## Properties of Subatomic Particles | Particle | Relative Charge | Relative Mass | |-----------------|-----------------|----------------| | Proton | +1 | 1 | | Neutron | 0 | 1 | | Electron | -1 | ~0 | The mass of a proton is approximately equal to the mass of a neutron. The mass of an electron is significantly smaller than the mass of a proton or neutron. ## Size * Atoms have a radius of approximately 1 × 10-10 m. * Nuclei are much smaller, about 10,000 times smaller than atoms, with a radius around 1 × 10-14 m.