Anything which has mass and volume is called matter
 Matter is the stuff that makes up everything in the universe. Matter has mass and takes up space. Matter is made of atoms. Solids, liquids, gases, and plasma are all matter. When all atoms that make up a substance are the same, then that substance is an element. Elements are made of only one kind of atom. Because of this, elements are called "pure" substances. An atom is the smallest piece of an element that still has the properties of that element. For example, aluminum is a lightweight, shiny metal. If we took a piece of aluminum and cut it into small pieces, it would still be aluminum. It would still be a lightweight, shiny metal. The smallest piece would be called an atom. Atoms are so small they cannot be seen even with a microscope. Atoms consist of a nucleus that has protons and neutrons surrounded by electrons outside the nucleus.
2) Atoms of different elements can combine to make new substances. A molecule is formed when two or more atoms join together chemically. If atoms combine that are of two or more different elements, we call that a compound. All compounds are molecules, but not all molecules are compounds. When two hydrogen atoms combine with one oxygen atom, it becomes the compound water. The oxygen we breathe is actually two atoms of oxygen combined, so it is a molecule of oxygen. We use abbreviations for elements. These abbreviations for elements are called chemical symbols. When two or more atoms combine to form a molecule or compound, the abbreviation is known as a chemical formula. The chemical formula for an oxygen molecule is O2.