History of the Periodic Table

Early Attempts to Classify the Elements

Before the discovery of subatomic particles (protons, neutrons and electrons), scientists attempted to arrange the elements based on their atomic mass. By focusing mainly on the atomic mass of the element, the chemical properties of some elements were overlooked. This led to inconsistencies, as some elements were placed in the wrong groups.

Earlier models of the periodic table had gaps. Scientists sometimes placed elements in the wrong spots when trying to fill these gaps. As a result, the earlier models of the periodic table were very incomplete.

Mendeleev’s Periodic Table

In 1869, a Russian chemist called Dmitri Mendeleev created the first modern periodic table. He did this as a way to organise elements by their properties. Mendeleev arranged the elements in order of increasing atomic mass. However, he also took into account the chemical and physical properties of the elements and their compounds.

Mendeleev identified patterns with chemically similar elements, as they naturally fell into the same columns. This made it easier to predict the properties of new elements. He also changed the order of elements when possible to fit the pattern. This made it much easier to find elements with similar properties.

  • At that time, protons had not been discovered, so Mendeleev made predictions based on the properties of the known elements.
A simplified periodic table chart with rows and groups labelled. The table presents select elements from rows 1 to 6 and groups I to VIII. Some positions have question marks indicating missing or unidentified elements, while others have dashes denoting empty spaces.

Elements in the modern periodic table are arranged in order of increasing atomic number, which is the number of protons in the nucleus of an atom.

This arrangement provides a more accurate representation of the elements, as it eliminates the confusion caused by the existence of isotopes.

Gaps in Periodic Table

Mendeleev left gaps for the elements that had not yet been discovered at that time. He realised that he could predict the properties of the undiscovered elements by looking at trends and the properties of elements near the gap.

  • For example, he predicted the properties of ‘eka-silicon’, which is now known as germanium.

Mendeleev’s approach proved to be successful as the discovery of these missing elements confirmed his predictions.

Swapping elements

Dmitri Mendeleev was flexible when arranging the elements, as he did not always strictly follow the rule of increasing relative atomic mass.

For example, even though tellurium (Te) has a higher relative atomic mass than iodine (I), Mendeleev placed iodine after tellurium. This is because iodine’s properties aligned more closely with those of its group elements (chlorine, fluorine, and bromine). Tellurium was placed after sulfur, oxygen, and selenium, which it closely resembled.

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