Chemistry is the scientific study of matter, its properties, composition, structure, and the changes it undergoes during chemical reactions. At its most basic level, matter is anything that has mass and occupies space. It exists in three primary states: solid, liquid, and gas. Solids have a definite shape and volume, liquids have a definite volume but take the shape of their container, and gases have neither a definite shape nor volume, expanding to fill whatever space is available.
All matter can be classified into pure substances and mixtures. A pure substance is either an element or a compound. An element is a substance that cannot be broken down into simpler substances by chemical means and is made of only one type of atom. A compound, in contrast, is composed of two or more different elements chemically bonded together in fixed proportions. A mixture, on the other hand, is a combination of two or more substances that are not chemically combined, allowing each to retain its individual properties. Mixtures may be homogeneous, having a uniform composition throughout, or heterogeneous, where the components remain visibly distinct.
The atom is the smallest unit of an element that retains the chemical properties of that element. Every atom consists of a nucleus surrounded by electrons. The nucleus contains two main subatomic particles: protons, which carry a positive charge, and neutrons, which carry no charge. Electrons, which carry a negative charge, orbit the nucleus. The atomic number of an element equals the number of protons in its nucleus and uniquely identifies the element, while the mass number is the total count of protons and neutrons. Because isotopes are atoms of the same element with the same number of protons but different numbers of neutrons, they share an atomic number but have different mass numbers.