Important Terms ATOM - Smallest unit of an element that keeps its chemical properties ELEMENT – Unique substance with the same number of protons in every atom, that cannot be broken down by chemical means MOLECULE - Smallest unit of a compound that keeps its chemical properties (made of two or more atoms) COMPOUND - Substance that can be broken into separate elements by chemical reactions ELECTRON (e-) - Particle moving around the nucleus of an atom with a negative charge; it does NOT have a definitive position in the energy level; (mass = 1/2000 amu) NEUTRON (N) - Particle in the nucleus of an atom with no charge; (mass = 1 amu) PROTON (P) - Particle in the nucleus of an atom with a positive charge; (mass = 1 amu) ION - Electrically charged atom (i.e., excess positive or negative charge) NUCLEUS - Dense, central core of an atom, does not have a definitive membrane ATOMIC MASS – total mass of the protons and neutrons in an atom ISOTOPES - atoms of an element each form having a different atomic mass. Isotopes of an element have the same number of protons but different numbers of neutrons – which ultimately alters the mass