periodic table heading

Higher and foundation tiers

The modern periodic table

Following the work of early chemists such as Dmitri 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.

Iodine and tellurium problem that Mendeleev faced.  These elements had to be swapped around in the perioodic table. 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 tellurium where he had to swap their positions 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 brilliant physicist Henry Moseley that an explanation for this problem was provided. Moseley bombarded certain elements with "cathode rays" (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 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 tellurium. If Mendeleev arranged these two elements according to their atomic masses then iodine and tellurium 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 tellurium has several isotopes with high atomic masses and high percentage abundances which means its average atomic mass is higher than that of iodine.

Patterns and properties

The elements in the modern periodic table are arranged according to their atomic number. The image below shows the columns and periods in the periodic table of elements. In the modern periodic table the elements are arranged according to their atomic number and not their atomic masses.  Outline of the modern periodic table showing all the periods and groups. The periodic table makes it easy to predict and explain many of the properties of the elements. You should be aware of the following facts:

Key Points

Practice questions

Check your understanding - Questions on the periodic table

Check your understanding - Additional questions on the periodic table