An atom consists of a nucleus containing protons (positive charge) and neutrons (no charge), surrounded by electrons (negative charge) in energy levels or shells. The number of protons defines the element — this is the atomic number.
Electrons occupy specific energy levels: the first shell holds up to 2 electrons, the second up to 8, the third up to 18, and so on. Valence electrons (in the outermost shell) determine an element's chemical properties and reactivity.
The periodic table organizes elements by atomic number. Elements in the same group (column) have the same number of valence electrons and similar chemical behavior. Elements in the same period (row) have the same number of electron shells.
Isotopes are atoms of the same element with different numbers of neutrons, giving them different mass numbers. Ions form when atoms gain or lose electrons: cations (positive) lose electrons, and anions (negative) gain electrons.