We can think of matter as being composed of particles. Particles are composed of elements or compounds. Elements and compounds are composed of atoms.
Substances whose atoms are held together by positive and negative charges are called ionic substances. Examples: NaCl, `~MgCl_2`
Substances whose atoms are held together by shared electrons are called covalent substances. The term molecule is another name used to describe a covalent substance. Examples: `~H_2O` , `~CH_4`
Atoms: Atoms are the building blocks of elements and compounds. Atoms contain tiny subatomic particles. The main subatomic particles in atoms are: protons, neutrons, and electrons. Protons and neutrons are located in the nucleus of an atom. Electrons are located in the electron cloud surrounding the nucleus of the atom.
Therefore, a molecule is an element or compound that is composed of two or more covalently bonded atoms. Example: `~H_2O` is a molecule composed of two hydrogen (H) atoms and one oxygen (O) atom.