Higher and foundation tier
Following the work of early chemists such as Dimitri Mendeleev great strides were made in the development
of a periodic table of elements. With the discovery of new sub-atomic particles such as
electrons and protons
scientists began to understand the fundamental nature of atoms. The addition of an extra column to Mendeleev's periodic
table with the discovery by the Scottish chemist William Ramsay of four Noble gases (neon, argon, krypton and xenon)
and the development of the Rutherford-Bohr model of the atom lead to the
idea that electronic arrangements were important in explaining the chemistry of the
elements and how they react.
Mendeleev arranged the elements in his periodic table by their atomic mass but there were some nagging problems that he had with some elements in his periodic table; such as iodine and tellerium where he had to swap their position in his periodic table and break his own rule of arranging the elements by their atomic masses. However it was not until well after Mendeleev's death in 1907 following the work of the brillant physcist Henry Moseley that an explanation for this problem was provided. Moseley bombarded certian elements with "cathode ray" (high energy electrons) and by analysing the frequencies of the x-rays emitted realised that the atomic number of an element was due to the number of positive charges in the nucleus. This led to a reorganisation of the periodic table with the elements now being arranged by their atomic number and not their masses.
Following the discovery of isotopes by Frederick Soddy, who received the Nobel prize for his work, it became clear as to why Mendeleev had to swap around some elements in his periodic table, particularly the problem Mendeleev had with masses of the elements iodine and tellerium. If Mendeleev arranged these two elements according to their atomic masses then iodine and tellerium would have to swap places in the periodic table. Mendeleev swapped these two elements around in his table based on their chemical properties but he had no idea why they did not fit his pattern other than that the atomic masses may have been calculated wrongly.
Though it made sense to put iodine in group 7, the halogens, since its chemical reactions were similar to the other halogens. However iodine has a mass of 127 and tellerium 128. Following the discovery of isotopes it was found that tellerium has several isotopes with high atomic masses and high percentage abundances which means its average atomic mass is higher than that of iodine.