Before the discovery of subatomic particles (protons, neutrons and electrons), scientists attempted to arrange the elements based on their atomic mass. By using the atomic mass of the element, they were ignoring the element’s chemical properties. So, some elements were placed in the wrong groups.
In the earlier models, there were gaps. Scientists tried to force elements into these gaps during the sorting process. As a result, the earlier models of the periodic table were very incomplete.
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.
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.
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.
Mendeleev’s approach proved to be successful as the discovery of these missing elements confirmed his predictions.
Dmitri Mendeleev’s was flexible when arranging the elements, as he did not always strictly follow the rule of increasing relative atomic mass.
For example, despite tellurium (Te) having a higher relative atomic mass, Mendeleev placed iodine (I) after it. This is because its properties aligned more closely with its group elements (chlorine, fluorine, and bromine). Tellurium was placed after sulfur, oxygen, and selenium, which it closely resembled.