Atoms and Nuclei

An ATOM is the smallest unit of an element that retains its properties. Inside every atom is a tiny dense NUCLEUS made of PROTONS (positively charged) and NEUTRONS (neutral). ELECTRONS orbit the nucleus in clouds. The nucleus is INCREDIBLY small — if an atom were a stadium, the nucleus would be a marble in the middle. Yet it contains 99.9% of the atom's mass.

Nuclear science basics. The number of PROTONS determines what ELEMENT it is (1 proton = hydrogen, 6 = carbon, 92 = uranium). The number of NEUTRONS can vary, creating ISOTOPES of an element. Some isotopes are STABLE; others are UNSTABLE (radioactive) and decay over time, releasing energy and particles. Nuclear reactions involve changes in the nucleus — fission (splitting), fusion (joining), or radioactive decay.

Forces in the nucleus. Why don't protons (all positive) fly apart from each other? The STRONG NUCLEAR FORCE — much stronger than electric repulsion at very short ranges — holds nuclei together. But it has limits. Very heavy nuclei (like uranium) are barely stable; they're prone to splitting (fission) or decaying. Tiny imbalances in mass become huge energies through E = mc².

Nuclear physics is the science of the smallest, densest, most powerful core of matter. The principles power stars, atomic energy, and medicine.