The atomic nucleus is a tiny, incredibly dense core at the center of every atom. It contains protons and neutrons, collectively called nucleons, packed into a space roughly 100,000 times smaller than the atom itself. If an atom were the size of a football stadium, the nucleus would be a marble at the center. Despite this size difference, the nucleus contains 99.9% of the atom’s mass.
The strong nuclear force holds nucleons together. It is about 100 times stronger than electromagnetism but has a very short range, about 1 to 3 femtometers (a femtometer is 10 to the negative 15 meters). Protons repel each other electrostatically because they are all positively charged. In small nuclei, the strong force easily overcomes this repulsion. But as nuclei get larger, the repulsion grows faster than the strong attraction, which is why no stable elements exist beyond lead (element 82) without being artificially created.
Isotopes are atoms of the same element with different numbers of neutrons. Carbon-12 has 6 protons and 6 neutrons. Carbon-14 has 6 protons and 8 neutrons. Both are carbon, but carbon-14 is unstable and undergoes beta decay with a half-life of about 5,730 years. This is the basis of radiocarbon dating, which allows archaeologists to determine the age of organic materials up to about 50,000 years old.
Radioactive decay occurs in several forms. Alpha decay emits a helium nucleus (2 protons, 2 neutrons). Beta decay converts a neutron to a proton (or vice versa) and emits an electron (or positron). Gamma decay emits high-energy photons. Each type changes the nucleus in a different way. Alpha decay reduces the atomic number by 2. Beta decay changes it by 1. Gamma decay leaves the atomic number unchanged.
Half-life is the time it takes for half of a radioactive sample to decay. It is a statistical property of large populations. A single atom might decay in the next second or in a thousand years. But for a billion atoms, half will have decayed after one half-life, three quarters after two, and so on. Our Half-Life Calculator (adapted from nuclear chemistry) computes remaining quantities for any isotope.
Nuclear fission splits heavy nuclei into lighter ones, releasing energy. A uranium-235 nucleus absorbing a neutron splits into two smaller nuclei, several neutrons, and a large amount of energy. The released neutrons can trigger further fissions, creating a chain reaction. Controlled chain reactions power nuclear reactors. Uncontrolled ones produce nuclear explosions. A single kilogram of uranium-235 releases about 8.2 times 10 to the 13th joules, equivalent to about 20,000 tons of TNT.
Nuclear fusion combines light nuclei into heavier ones. The Sun fuses hydrogen into helium, releasing energy in the process. Fusion releases more energy per unit mass than fission and produces less radioactive waste. The challenge is achieving the extreme temperatures and pressures needed for fusion to occur. Experimental reactors like ITER aim to demonstrate sustained fusion power, but commercial fusion remains a work in progress.