Matter- anything that occupies space and has mass. Composed of elements- composed of identical atoms (building blocks of matter) Body weight- carbon (65%), oxygen (18. 5%), hydrogen (9. 5%), nitrogen (3. 2%) 3 subatomic particles- protons, neutrons, electrons (make electron cloud negative) Isotopes of hydrogen- deuterium (1 neutron), tritium (2 neutrons) Radioisotope- isotope that is unstable and it will break down (p 32) Neutral atom (protons = electrons) vs stable atom (electron shell filled, losing or gaining an electron) Shells first shell- 2 electrons second shell- 8 electrons third shell- max of 18 electrons but wants to fulfill octet rule. Valence shell- outermost electron shell of an atom. Valance electrons- electrons in the valence shell, chemically active. Inert elements (complete valence shells, unreactive, noble gases) vs reactive elements. Can be same or diff kinds of atoms: diff = compound. Nonpolar covalent bond: electrons are shared equally between atoms, complete distribution of charge, co2.