Matter is anything that has mass and takes up space. It is made of elements like atoms and molecules. Atoms are the smallest particle of an element and contain protons, neutrons, and electrons. Molecules are formed when two or more atoms join together. Compounds contain different elements joined as molecules, while mixtures can separate their substances but compounds cannot. Matter can be a solid, liquid, or gas depending on its state, and undergo physical or chemical changes that alter its properties without or with changing its composition.