Summary
Highlights
Every atom has a nucleus containing positively charged protons and neutral neutrons, both with a relative mass of one. Electrons, which are much smaller and negatively charged, orbit the nucleus in shells.
The nuclear symbol on a periodic table represents an element. The elemental symbol identifies the element, the atomic number (bottom left) represents the number of protons (which defines the element), and the mass number (top left) indicates the total number of protons and neutrons. In a neutral atom, the number of protons equals the number of electrons.
Isotopes are atoms of the same element (same number of protons) but with different numbers of neutrons, leading to different mass numbers. Most isotopes are unstable and undergo radioactive decay by emitting radiation like alpha, beta, or gamma.
Electrons are arranged in energy levels or shells around the nucleus, with higher shells having higher energy. Electrons can jump to a higher energy level if they absorb the right amount of electromagnetic radiation, becoming 'excited'. They eventually fall back, re-emitting the energy as electromagnetic radiation.
If an outermost electron absorbs enough energy, it can completely leave the atom. This leaves the atom with more protons than electrons, resulting in a positive charge, and it becomes a positive ion. Ionizing radiation is radiation capable of knocking electrons off atoms, thus ionizing them.