The development of scientific explanations
Earth's early atmosphere
Obtaining evidence for the composition of the early atmosphereThe layers of gases that surround the Earth. The important gases in the atmosphere are nitrogen, oxygen and carbon dioxide. is challenging. Scientists cannot travel back in time to collect samples of gas.
Scientists use evidence from rocks and fossils which then has to be interpreted. Existing explanations about how and when levels of gases changed in the ancient atmosphere are challenged by:
- new discoveries of ancient rocks
- discoveries of fossils of new types of living organismLiving entity, eg animals, plants or microorganisms.
- discoveries of earlier fossils of a known organism
New explanations are not usually accepted until more evidence is found.
The periodic table
When Mendeleev developed the periodic tableA tabular representation of all known elements in order based on atomic number, eg all the noble gases are found on the right of the periodic table., he arranged the elementA substance made of one type of atom only. in order of increasing relative atomic massThe mean relative mass of the atoms of the different isotopes in an element. It is the number of times heavier an atom is than one-twelfth of a carbon-12 atom..
Mendeleev left gaps where he predicted that elements, not discovered at the time, would fit. These elements were discovered later and closely matched his predictions. This increased confidence in Mendeleev's table and it gradually became accepted by the scientific community.
Mendeleev also had to swap a few elements so that they were still grouped with elements with similar chemical propertiesA description of how a substance reacts with other substances. For example, flammability, pH, reaction with acid, etc.. The later discovery of protonSubatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1. led to scientists knowing the number of protons in the atoms of each element. This explained why Mendeleev had had to swap some elements around, improving upon his ideas. The modern periodic table lists elements in order of increasing atomic numberThe number of protons in the nucleus of an atom. Also called the proton number..