This document defines key concepts in matter, including: - Matter is anything that has mass and occupies space, and can be made of elements, molecules, or compounds. - Elements are the simplest forms of matter and cannot be broken down further into simpler substances. Atoms are the smallest particle of an element. - Atoms bond together to form molecules or bond with different elements to form compounds like salt or water. - All matter is made of atoms in constant motion according to kinetic theory. Atoms are arranged in the periodic table by atomic number. - Atoms contain a nucleus of protons and neutrons, with electrons orbiting the nucleus in shells. The number of protons determines the element.