States and Properties of Matter

Matter is anything that has mass and occupies space. At the most fundamental level, matter is composed of atoms — the smallest units that retain the chemical identity of an element. Each atom consists of a nucleus containing positively charged protons and neutral neutrons, surrounded by a cloud of negatively charged electrons. The number of protons (atomic number) defines the element; the total of protons and neutrons gives the mass number. Elements are arranged in the periodic table according to increasing atomic number. Elements in the same group share similar chemical properties because they have the same number of valence (outer-shell) electrons. Isotopes are atoms of the same element with different numbers of neutrons — for example, carbon-12 and carbon-14. Matter exists in three states — solid, liquid, and gas — depending on temperature and pressure. Changes between states (melting, boiling, condensation, freezing) involve energy transfer but no change in chemical composition. Understanding atomic structure underpins all of chemistry: bonding, reactions, and the behaviour of substances all trace back to electron arrangement.