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 that are always in motion according to kinetic theory. Heavier atoms move slower than lighter ones. - The periodic table organizes all known elements by their atomic structure.