One of the primary tools of any chemist is the periodic table, it enables chemists to understand many trends in the reactivity of elements and their chemical properties to be understood. To celebrate the International Year of the Periodic Table of Chemical Elements, which coincides with the centenary of the International Union of Pure and Applied Chemistry, we will be providing updates throughout the year on some of these elements.
An element is the simplest substance that cannot be broken down into any other substance using chemical reactions. There are 118 elements in the periodic table, each with its own atom made up of protons, neutrons and electrons.
In 1829, German scientist Johann Wolfgang Döbereiner recognised that some elements had similar properties and discovered these were in groups of three (e.g. chlorine, bromine and iodine) – which he called ‘triads’. However, these triads missed out important elements such as fluorine and magnesium.
In 1869, Dimitri Mendeleev built on these early
Independently of Mendeleev, a German chemist called Johan Lothar Meyer came up with a similar table, but stated his concerns over Mendeleev’s suggestion that some of the atomic weights were incorrect.
attempts by using the atomic mass of the elements and went on to formulate the periodic table. He was so successful that he was also able to predict new elements such as gallium, germanium and scandium shortly after. Mendeleev also correctly identified discrepancies in the calculation of the atomic weights ofsome of the elements.
The table was so successful that even though Mendeleev did not predict any other groups, there was space available should any more be discovered. Noble gases were later discovered and were slotted into this additional space called Group 0 (although this is now called Group 18 in the modern table).
Mendeleev used to play ‘element solitaire’ on the train so that he could match the chemical properties in his periodic table. He had cards of each element with their properties written on them so he could move them around.
The periodic table works well for most elements and is ordered in groups (vertical), periods (horizontal) and blocks (Group 1 and 2) of elements that show certain trends.
For instance, going down Group 1, the reactivity of metals increases – lithium fizzes steadily and slowly with water whereas caesium will explode on contact. The reverse is true for halogens (Group 17) where fluorine is the most reactive and, moving down the group, the other halogens decrease in reactivity .
The periodic table is one of the basic building blocks in a chemist’s education and this grouping of elements helps to classify similar organic and inorganic compounds. During 2019, we will be providing further insights on some of the elements – starting with hydrogen and helium.
Hydrogen is the only element that does not fit conveniently into the periodic table. While it is positioned above alkali metals, this cannot be justified scientifically because it is a gas. The next element in the table is helium, which is the most abundant gas contained within the Sun – it takes its name from the Greek Titan of the Sun, Helios. Fun fact – it is lighter than air and is often used in party balloons.
Wondering why hydrogen doesn’t sit with the other gases and how helium can be lighter than air? Find out now in our issue of
Hydrogen & Helium.