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Chemistry Study Guide Unit 1: Measurement and the Methods of Chemistry Counting significant figures: 1) leading zeros never count 2) Trailing zeros only count if there is a decimal point written 3) All numbers that aren?t leading or trailing zeros count Unit 2: Atomic Structure, Periodicity, and Nuclear Energy Proton: +1 (charge) Neutron: 0 (charge) Electron: -1 (charge) Atomic number= number of protons within an atom Atomic mass= total weight of the atom. P+N If there is a power to the left of the element, that implies the mass. Isotope: same atom with same atomic number, but different number of neutrons. Different weight. Neutrons and protons are INSIDE nucleus. Electrons on orbitals. Valence electrons= electrons on outermost shell. The electrons that do all reactions.