Atomic Models and Nuclear Energy

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1Summary 重點摘要

Atomic models evolved from Thomson's plum pudding through Rutherford's nuclear model to Bohr's energy levels; mass number and atomic number balance across nuclear equations; fission and fusion convert mass defect into vast energy via E = mc², and nuclear power has both merits and risks.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: Atomic structure and nuclide notationAn atom consists of a central nucleus with electrons outside: the nucleus holds positively charged protons and uncharged neutrons, together called nucleons; tiny in volume, it carries almost all the mass. Nuclide notation writes the mass number A top-left and the atomic number Z bottom-left. For example, uranium-238 is written with A = 238, Z = 92, so neutrons = 238 − 92 = 146. The nuclear charge equals the proton number, and a neutral atom has as many electrons as protons; these basics underpin every nuclear equation and calculation, so drill them until automatic.
2. Step 2: Thomson and the plum pudding modelIn 1897 Thomson discovered the electron in cathode-ray experiments: the rays deflected in electric and magnetic fields, proving they were negatively charged particles far lighter than atoms. Since the atom is neutral, positive charge had to live inside it, and Thomson proposed the plum pudding model: positive charge spread evenly through the atom with electrons embedded like plums in a pudding. For example, imagine a sphere of positively charged 'batter' with electrons suspended and vibrating within. The model explained neutrality and ionization, but it would not survive the alpha-scattering test of the next step.
3. Step 3: Rutherford scattering and the nuclear modelRutherford had α particles fired at an extremely thin gold foil: the vast majority sailed straight through, showing the atom is mostly empty; a few deflected through large angles, and some rebounded almost straight back, as if striking something massive and positively charged. The plum pudding model cannot explain large-angle rebounds — evenly spread charge could never repel an α particle hard enough. Rutherford therefore proposed the nuclear model: all positive charge and nearly all mass concentrate in a tiny central nucleus. For example, if an atom were enlarged to a stadium, its nucleus would be a single bean at the centre.
4. Step 4: Bohr's model and energy levelsBy classical physics an orbiting electron should radiate energy continuously and spiral into the nucleus — stable atoms could not exist. In 1913 Bohr proposed that electrons can occupy only specific energy levels, so atomic energy is quantised; absorbing exactly the gap between two levels jumps the electron upward, and dropping back it emits light of the matching energy. For example, the hydrogen spectrum consists of discrete bright lines at fixed wavelengths, each matching a transition between a pair of levels; if energy were continuous the spectrum would be a rainbow band. Energy levels underpin atomic emission and lasers alike.
5. Step 5: Isotopes and balancing nuclear equationsIsotopes share an atomic number but differ in neutron count; their chemical properties are essentially identical while their nuclear properties can differ completely — some stable, some radioactive. Nuclear equations must balance: the mass numbers on both sides add up equally, and so do the atomic numbers. For example, the α decay of uranium-238: the nucleus with A = 238, Z = 92 emits an α particle (A = 4, Z = 2) and becomes thorium with A = 234, Z = 90 — 238 = 234 + 4 and 92 = 90 + 2, both rows balanced. Unbalanced equations are the most common loss in radioactivity questions; check every term after writing.
6. Step 6: Fission and the chain reactionWhen a neutron strikes uranium-235, the nucleus splits into two medium-mass nuclei (for example barium-141 and krypton-92), releasing 2 to 3 fast neutrons and a great deal of energy; the fresh neutrons strike other uranium nuclei, and the reaction spreads tenfold and hundredfold — a chain reaction. Nuclear plants absorb surplus neutrons with control rods to hold the rate steady, and use moderators to slow fast neutrons so they trigger fission more readily. For example, the complete fission of 1 kg of uranium-235 releases energy comparable to burning about 2700 tonnes of coal — an extraordinary energy density. Fission products are mostly radioactive, which is what makes waste so troublesome.
7. Step 7: Fusion, E = mc² and the nuclear debateLight nuclei — such as the hydrogen isotopes deuterium and tritium — fuse into heavier nuclei at extreme temperature and pressure, releasing energy by the same mass-energy relation; the Sun's core converts about 4 million tonnes of matter to energy every second through fusion. E = mc² shows that a minuscule mass defect carries enormous energy: for example, 1 gram of mass defect releases about 9 × 10¹³ J, the heat of thousands of tonnes of coal. Nuclear power's strengths: little fuel, vast energy, no carbon dioxide in generation; its weaknesses: waste isolated for millennia, high construction and decommissioning costs, accident and proliferation concerns. Discussion questions want both sides, with a measured conclusion.

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8Recap Recap

• An atom has a nucleus of protons and neutrons with electrons outside; nuclide notation gives A and Z, and neutrons = A − Z
• The model evolved from Thomson's plum pudding, through Rutherford's scattering proof of the nuclear model, to Bohr's discrete energy levels
• Isotopes share protons but differ in neutrons; nuclear equations balance in both mass number and atomic number
• Fission (uranium-235 chain reaction; power plants) and fusion (the Sun) both release vast energy from mass defect via E = mc²
• Nuclear energy is fuel-thrifty and carbon-free in operation, but waste isolation and accident risk fuel the debate — present both sides in discussion answers