| A | B |
|---|
| element | Substance that cannot be broken down into simpler substances and whose atoms are exactly alike. |
| electron | Tiny, negatively charged particle that is present in all atoms and that has almost no mass. |
| proton | Positively charged, heavy particle contained in the nucleus of all atoms |
| neutron | Electrically neutral particle that is present in the nucleus of all atoms; has the same mass as a proton. |
| electron cloud | Region surrounding the nucleus of an atom, where electrons are more likely to be found. |
| atomic number | Number of protons in the nucleus of an atom of a specific element. |
| isotope | Atoms of the same element that have different numbers of neutrons in their nuclei. |
| mass number | Number of neutrons plus the number of protons in the nucleus of an atom. |
| radioactive decay | Release of nuclear particles and energy from unstable atomic nuclei. |
| half-life | Time needed for one-half the mass of a sample of radioactive isotope to decay; can vary from fractions of a second to billions of years. |
