Atom: The defining structure of an element, which cannot be broken by any chemical means.

Examples: hydrogen, carbon-14, zinc, cesium, Cl- (a substance can be an atom and an isotope or ion at the same time)

Atomic Mass: The average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally-occurring element.

Also Known As: Atomic Weight

Examples: The atomic mass of carbon is 12.011; the atomic mass of hydrogen is 1.0079

Atomic Number: The number of protons in an element.

Examples: The atomic number of hydrogen is 1; the atomic number of carbon is 6

Buffer: Definition: A solution containing either a weak acid and its salt or a weak base and its salt, which is resistant to changes in pH.

Examples: blood

Carbohydrate: A class of organic compounds having the general formula C_m(H₂O)_n.

Examples: Glucose, fructose, sucrose, starch, cellulose

Covalent Bond: A chemical link between two atoms in which electrons are shared between them.

Examples: There is a covalent bond between the oxygen and each hydrogen in a water molecule (H2O). Each of the covalent bonds contains two electrons - one from a hydrogen atom and one from the oxygen atom. Both atoms share the electrons.

Electrolyte: A substance which forms ions in an aqueous (water) solution.

Example: NaCl forms Na⁺ and Cl^- in water.

Electron: a negatively charged component of an atom. Electrons exist outside of and surrounding the atom nucleus. Each electron carries one unit of negative charge and has a very small mass as compared with that of a neutron or proton.

Isotope: Atoms with the same number of protons, but differing numbers of neutrons. Isotopes are different forms of a single element.

Examples: Carbon 12 and Carbon 14 are both isotopes of carbon, one with 6 neutrons and one with 8 neutrons (both with 6 protons).

Mass number: an integer equal to the sum of the number of protons and neutrons of an atomic nucleus.

Examples: ³⁷₁₇Cl has a mass number of 37. Its nucleus contains 17 protons and 20 neutrons.

Neutron: The particle in the atomic nucleus with a mass = 1 and charge = 0.

Molecular Equation - Equation for a chemical reaction in which all formulas are written as if all substances existed as molecules; only complete formulas are used.

Molecular Formula - Formula that indicates the actual number of atoms present in a molecule of a molecular substance.

Molecule -The smallest particle of an element or compound capable of a stable, independent existence.

Element - A substance that cannot be decomposed into simpler substances by chemical means.

Enzyme - A protein that acts as a catalyst in biological systems.