Knowledge of chemical reactivity and properties may be approached on both the macroscopic and microscopic levels. Macroscopically this involves what is called descriptive chemistry. The person who first carries out a chemical reaction describes what happened, usually in terms of a balanced equation, and lists properties of any new substances. This enables other scientists to repeat the experiment if they wish. Even if the work is not carried out again, the descriptive report allows prediction of what would happen if it were repeated.
- 3.8: Radiation
- Just prior to the turn of the twentieth century, additional observations were made which contradicted parts of Dalton’s atomic theory. The French physicist Henri Becquerel discovered by accident that compounds of uranium and thorium emitted rays which, like rays of sunlight, could darken photographic films. Although themselves invisible to the human eye, the rays could be detected easily because they produced visible light when they struck phosphors such as impure zinc sulfide.
- 3.12: Isotopes
- The presence of neutrons in atomic nuclei accounts for the occurrence of isotopes— samples of an element whose atoms contain different numbers of neutrons and hence exhibit different "nuclidic masses". The nuclidic mass is the mass of a "nuclide", where a nuclide is the term used for any atom whose nuclear composition (Number of protons and neutrons) is defined.