Element: the simplest chemical substance that can exist, comprised of atoms with the same number of protons (and thus electrons).
Compound: a substance formed by the chemical combination of two or more elements. A mixture of elements (e.g. nitrogen and oxygen in the air) is not a compound because those elements are not chemically combined.
Atom: the smallest part of an element that can exist. Consists of a nucleus of neutrons and protons, which is orbited by electrons in fixed energy levels (shells). Atoms are uncharged because they have the same numbers of protons and electrons.
Protons: subatomic particles, charge +1, relative mass 1.
Neutrons: subatomic particles, charge 0, relative mass 1.
Electrons: subatomic particles, charge -1, relative mass 1/1836.
Atomic Number: The number of protons in the nucleus of an atom of a particular element.
Molecule: the smallest part of most elements and all compounds that exists alone under ordinary conditions. Molecules are uncharged because they have the same numbers of protons and electrons.
Isotopes: Atoms of the same element (same atomic number), which have different numbers of neutrons (different atomic mass). Hydrogen has 3 isotopes- regular hydrogen has 1 proton in its nucleus, deuterium has one proton and one neutron and tritium has one proton and 2 neutrons.
Relative atomic mass (RAM)/mass number: the average mass of the atoms of an element (depends on masses and abundance of the isotopes), relative to 1/12 the mass of a carbon-12 atom, i.e. 1/12 of a carbon-12 atom is assigned a mass of exactly 1.
The relative atomic mass in grams of any element contains one mole of atoms.
See Spectroscopy techniques for info on calculating RAM from mass spectra.
Relative molecular mass (RMM)/Molar mass: as above, but for molecules of a compound instead of atoms of an element.
Relative isotopic mass: the mass of one isotope of an element, relative to 1/12 the mass of a carbon-12 atom.
Relative isotopic abundance: the proportions/percentage abundance of the different isotopes of an element; e.g. chlorine-35 has an abundance of 75%, and chlorine-37 has an abundance of 25%.
Mole: the amount of substance that contains as many specified entities as there are in 12 g of carbon-12, i.e. the amount of substance that contains 6.02×1023 entities. 6.02×1023 is known as Avogadro’s number or the Avogadro constant.
Empirical formula: the simplest whole number ratio of the atoms of different elements in a compound.
Molecular formula: the actual number of atoms of different elements in one molecule of a compound.
Ion: A charged particle. Ions can be monatomic (charged atom) or polyatomic (charged molecule). Positively charged ions are called cations; negatively charged ions are called anions.
Species: A generic term used to describe any particle/entity- molecule, atom, ion, etc. A pure chemical species contains identical entities.