Chemistry 101: Elements & Compounds

Element

A pure substance consisting of only one type of atom, defined by its atomic number. There are 118 known elements on the periodic table.

Compound

A substance formed when two or more elements are chemically bonded together in fixed proportions (e.g., H2O, NaCl).

Atom

The smallest unit of an element that retains the chemical properties of that element, consisting of protons, neutrons, and electrons.

Proton

A positively charged subatomic particle found in the nucleus of an atom. The number of protons defines the element (atomic number).

Neutron

An electrically neutral subatomic particle found in the nucleus of an atom. It contributes to atomic mass but not charge.

Electron

A negatively charged subatomic particle that orbits the nucleus in energy levels (electron shells).

Atomic Number

The number of protons in the nucleus of an atom, which determines the element's identity and position on the periodic table.

Mass Number

The total number of protons and neutrons in the nucleus of an atom.

Isotope

Atoms of the same element that have different numbers of neutrons, resulting in different mass numbers (e.g., Carbon-12 and Carbon-14).

Ion

An atom or molecule that has gained or lost one or more electrons, giving it a net positive (cation) or negative (anion) charge.