Structure of Atoms and Nuclei
Easy Overview
What's inside an atom? First we thought it was a plum pudding. Then Rutherford shot particles at gold foil and discovered a tiny nucleus. Then Bohr figured out why electrons don't spiral into the nucleus. This chapter traces that journey — from early atomic models to nuclear energy and radioactivity.
Bohr's Model of the Atom
Bohr said electrons orbit the nucleus in fixed circular paths (energy levels) and can only exist in certain allowed orbits. They don't radiate energy while in these orbits — which was revolutionary because according to classical physics, they should spiral in. Electrons jump between levels by absorbing or emitting photons of specific energies. The energy of the photon equals the difference between the two levels. That's why each element has a unique fingerprint of colors (spectral lines) — each jump produces a specific wavelength of light.
Atomic Spectra
When you pass light from a gas through a prism, you don't get a continuous rainbow — you get a handful of bright lines (emission spectrum) or dark lines (absorption spectrum). Each element has its own unique set of lines, like a barcode. Hydrogen has the simplest spectrum — Lyman (ultraviolet), Balmer (visible), and Paschen (infrared) series. The Balmer series is why hydrogen gas glows pinkish-purple. The spacing between lines decreases as you go to higher energy levels, eventually converging — that convergence point gives you the ionization energy of the atom.
Radioactivity
Some nuclei are unstable — they spontaneously break apart, emitting particles and energy. That's radioactivity. There are three types: alpha (helium nucleus, heavy and slow, stopped by paper), beta (fast electron, stopped by aluminum), and gamma (high-energy photon, needs thick lead). The decay is random — you can't predict which atom will decay next, but for a large sample, the rate follows a simple exponential law. Half-life is the time for half the atoms to decay. Carbon-14 has a half-life of 5730 years — that's how we date ancient artifacts.
Nuclear Energy
Nuclear fission is when a heavy nucleus (like uranium-235) splits into two smaller ones, releasing a huge amount of energy. It happens when the nucleus absorbs a neutron and becomes unstable. The fission fragments fly apart, and more neutrons are released — causing a chain reaction. That's how nuclear reactors and atomic bombs work. Nuclear fusion is the opposite — two light nuclei (like hydrogen isotopes) combine to form a heavier one, releasing even more energy. That's how the sun works. Fusion is cleaner and more powerful than fission, but we haven't figured out how to control it sustainably yet.
Key Points
- •Bohr's model: electrons in fixed orbits with quantized angular momentum. mvr = nh/2π.
- •Energy of hydrogen atom Eₙ = -13.6/n² eV. Transitions produce spectral lines.
- •Rutherford's gold foil experiment revealed a tiny, dense, positively charged nucleus.
- •Radioactive decay: N = N₀e^(-λt). Half-life T½ = ln2/λ.
- •Alpha, beta, and gamma radiation have different penetrating powers and ionizing abilities.
- •Nuclear fission splits heavy nuclei; fusion combines light nuclei. Both release enormous energy.
Practice Questions
- Explain Bohr's model of the hydrogen atom. Derive the expression for the radius of the nth orbit.
- What is radioactivity? Derive the law of radioactive decay.
- Distinguish between nuclear fission and nuclear fusion with examples.
- The half-life of a radioactive substance is 10 days. How much of a 100 g sample remains after 40 days?