Mendeleev's periodic table

Mendeleev arranged elements so that those with similar chemical properties lined up in the same groups. He used atomic weight to order the elements, but he did not let a strict weight order override the pattern in their properties.

As the elements were arranged, their chemical properties recurred. Sometimes that pattern suggested an element was missing. Rather than force a known element into a place where its properties did not fit, Mendeleev left a gap for an element that had not yet been discovered.

He also changed the order of some elements when their chemical properties matched different groups from those suggested by their atomic weights. This allowed similar elements to line up more consistently.

Mendeleev's table was therefore more than a list ordered by atomic weight: it was an organised pattern of recurring chemical behaviour, with deliberate spaces for missing elements.

Early periodic tables by atomic weight

Before scientists understood the internal structure of atoms, they arranged the known elements in order of increasing atomic weight: a measure of how heavy an element’s atoms are on average compared with those of other elements.

Chemists wanted to find an arrangement that placed elements with similar chemical properties together. Ordering the elements by atomic weight revealed that similar properties appeared again at intervals. This repeating pattern made the early periodic tables useful: the order was not simply a list of elements, but showed a relationship between atomic weight and chemical behaviour.

The pattern was not perfect. If chemists followed atomic-weight order strictly, some elements fell into groups whose other members did not have similar chemical properties. The tables were also incomplete because some elements had not yet been discovered.

The early tables therefore revealed a repeating pattern, but atomic weight alone could not produce a fully correct arrangement.

Predicted elements and the validation of Mendeleev's table

Mendeleev arranged the known elements in a table so that elements with similar properties appeared in the same columns. In some places, the pattern suggested that an element was missing, so he left a gap rather than forcing a known element into the wrong position.

Mendeleev used the properties of elements around each gap to predict the properties of the missing element. These were predictions about elements that had not yet been discovered, not just descriptions of elements already known.

When some of the predicted elements were later discovered, their properties matched his predictions and they fitted the gaps. This provided strong evidence that the patterns in Mendeleev’s arrangement were scientifically sound, helping his table gain acceptance.

The successful predictions showed that the periodic table could do more than organise known elements: it could also be used to predict new chemical knowledge.

Isotopes and atomic weight anomalies

Atoms of the same element do not all have to have the same mass. They always have the same number of protons, but they can have different numbers of neutrons. These different forms of an element are called isotopes.

An element usually exists as a mixture of isotopes. Its measured atomic weight is a weighted average of their masses: an isotope that is more abundant contributes more to the average than a rarer one. Atomic weight therefore depends on both the masses of the isotopes and their relative abundances.

Atomic weight and the order of elements

Because atomic weights are averages, they do not always increase in the correct sequence of elements. For example, tellurium has an atomic weight of about $$127.60$$, while iodine has an atomic weight of about $$126.90$$. Strict atomic-weight order would place iodine first. However, tellurium belongs before iodine in the periodic table. The correct sequence follows atomic number, the number of protons: tellurium has $$52$$ protons and iodine has $$53$$.

Mendeleev placed tellurium before iodine because that order put the elements with others of similar properties, even though it departed from strict atomic-weight order. The later discovery of isotopes helped explain why atomic weights could show such inconsistencies and why atomic weight alone was not always a reliable guide to the order of elements.