C1 K) The Development of the Periodic Table
In the early 19th century, scientists were unaware that atoms were made out of protons, neutrons and electrons. This meant that scientists arranged the elements based on two different categories:
- The relative atomic mass
- The chemical and physical properties
When they arranged the elements based on relative atomic mass and their properties, they noticed that there was a periodic pattern, hence why the periodic table is known as the periodic table.
The early periodic tables were incomplete because some elements were missing. Also, some elements were placed in the wrong groups if the scientists strictly followed the increasing relative atomic mass rule and ignored the properties of the elements (the increasing relative atomic mass rule is where the scientists placed the elements in order of increasing relative atomic mass).
Dimitri Mendeleev overcame these problems by leaving gaps in the periodic table for elements that he thought had not yet been discovered (only around 50 elements were known when Mendeleev was creating his periodic table in 1869). The gaps that Mendeleev left in the periodic table were for undiscovered elements that have similar properties to that group. His periodic table is shown below.
As new elements were discovered, they were added to the periodic table where they filled the gaps that Mendeleev left. The discoveries of new elements confirmed that Mendeleev was right in his thinking about the periodic table.
Isotopes are different forms of the same element that have different relative atomic masses; think carbon-12 and carbon-13. Isotopes have the same number of protons and a different number of neutrons (or, isotopes have the same atomic number and a different relative atomic mass). As isotopes are the same element, they have the same physical and chemical properties. Isotopes were discovered in the early 20th century and their discovery explained why ordering elements solely on increasing relative atomic mass was not always correct. Instead, the chemical and physical properties of the elements should also be taken into account (like what Mendeleev did).