| 41. | Anode | 1. In a cathode ray tube, the positive electrode; 2. Electrode at which oxidation occurs. |
| 42. | Antibonding Orbital | A molecular orbital higher in energy than any of the atomic orbitals from which it is derived; lends instability to a molecule or ion when populated with electrons; denoted with a star (*) More… |
| 43. | Aromatic Hydrocarbons | Benzene and its derivatives. |
| 44. | Artificial Transmutation | An artificially induced nuclear reaction caused by the bombardment of a nucleus with subatomic particiles or small nucei. |
| 45. | Aryl Group | Group of atoms remaining after a hydrogen atom is removed from the aromatic system. |
| 46. | Associated Ions | Short-lived species formed by the collision of dissolved ions of opposite charges. |
| 47. | Atmosphere | A unit of pressure; the pressure that will support a column of mercury 760 mm high at 0 °C. |
| 48. | Atom | The smallest particle of an element |
| 49. | Atomic Mass Unit (amu) | One twelfth of a mass of an atom of the carbon-12 isotope; a unit used for stating atomic and formula weights; also called dalton. |
| 50. | Atomic Number | Integral number of protons in the nucleus; defines the identity of element. |
| 51. | Atomic Orbital | Region or volume in space in which the probability of finding electrons is highest. |
| 52. | Atomic Radius | Radius of an atom. |
| 53. | Atomic Weight | Weighted average of the masses of the constituent isotopes of an element; The relative masses of atoms of different elements. |
| 54. | Aufbau ('building up') Principle | Describes the order in which electrons fill orbitals in atoms. |
| 55. | Autoionization | An ionization reaction between identical molecules. |
| 56. | Avogadro's Law | At the same temperature and pressure, equal volumes of all gases contain the same number of molecules. |
| 57. | Avogadro's Number | The number (6.022x10^23) of atoms, molecules or particles found in exactly 1 mole of substance. |
| 58. | Background Radiation | Ratiation extraneous to an experiment. Usually the low-level natural radiation form cosmic rays and trace radioactive substances present in our environment. |
| 59. | Band | A series of very closely spaced, nearly continuous molecular orbitals that belong to the crystal as a whole. |
| 60. | Band of Stability | Band containing nonradioactive nuclides in a plot of number of neutrons versus atomic number. |
