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Overview
The atomic nucleus contains most of an atom's mass and all of its positive charge. It is made of protons and neutrons bound together by the strong nuclear interaction. Nuclear structure determines isotopes, radioactivity, nuclear reactions, and many properties of matter.
Protons and Neutrons
Protons carry positive electric charge, while neutrons are electrically neutral. Together they are called nucleons.
Atomic Number
The number of protons determines the element. Carbon has six protons, oxygen has eight, and uranium has ninety-two.
Mass Number
Mass number is the total number of protons and neutrons in a nucleus. Isotopes of an element have the same proton number but different neutron numbers.
Strong Nuclear Force
The strong nuclear force binds nucleons together over very short distances and overcomes electric repulsion between protons.
Nuclear Stability
Nuclear stability depends on the balance of protons, neutrons, forces, and energy. Unstable nuclei can decay radioactively.
Nuclear Size
The nucleus is tiny compared with the whole atom but contains nearly all the atom's mass.
Isotopes
Isotopes are atoms of the same element with different neutron numbers. Some isotopes are stable, while others are radioactive.
Common Mistakes
A common mistake is thinking electrons are inside the nucleus. Electrons occupy quantum states around the nucleus, while the nucleus contains protons and neutrons.
Why This Matters in Physics
The atomic nucleus is central to radioactivity, fission, fusion, binding energy, medicine, power generation, stars, and particle physics.