From Relative Atomic Mass to Atomic Number

Mendeleev’s arrangement: mass and chemical patterns

Before scientists understood protons, neutrons and electrons, they could compare the relative masses of atoms and investigate the chemical properties of elements. These were the clues available for organising the elements.

In 1869, the Russian chemist Dmitri Mendeleev produced a periodic table based mainly on increasing relative atomic mass, then often called atomic weight. He noticed that similar chemical properties appeared at regular intervals and grouped similar elements together.

However, a strict order of increasing relative atomic mass put some elements in the wrong groups. Mendeleev recognised that chemical properties mattered: he sometimes reversed the mass order rather than placing an element alongside others with very different properties.

For example, mass order would put iodine before tellurium. Mendeleev instead placed tellurium before iodine, keeping iodine with the similar halogens, including chlorine and bromine. This fitted the chemical evidence, but he could not yet explain why the mass order was misleading.

Isotopes explain why mass is not a reliable guide

An element is identified by its atomic number: the number of protons in each atom’s nucleus. Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Their masses therefore differ.

Carbon-12 and carbon-13 make this distinction concrete:

Both are carbon because both have six protons. Their mass numbers differ because carbon-13 has one more neutron. Neutral atoms of both isotopes also have six electrons, with the same electron arrangement, so they have the same chemical properties.

Most elements occur naturally as mixtures of isotopes. An element’s relative atomic mass is a weighted mean: more abundant isotopes contribute more to the average than less abundant ones. It is measured relative to the carbon-12 standard, rather than being a proton count or the mass number of one particular atom.

This explains the weakness of mass-based ordering. An element’s average mass depends on its isotopes and their abundances, not just its number of protons. Relative atomic mass therefore does not always increase as atomic number increases. A heavier average does not necessarily mean an element should come later in the table.

Atomic number gives a consistent order

The modern periodic table arranges elements in order of increasing atomic number, not relative atomic mass. Every isotope of an element has the same atomic number, so all its isotopes belong in the same position.

For a neutral atom, the proton number also determines the number of electrons. The arrangement of those electrons, especially the outer electrons, explains the element’s chemical properties. Ordering by atomic number therefore connects the table to atomic structure and keeps chemically similar elements in the appropriate groups.

Tellurium comes before iodine in atomic-number order. The modern rule thus explains the placement Mendeleev had chosen using chemical properties, without needing an exception to the ordering rule.

Scientific explanations change with evidence

Mendeleev’s work was useful even before its underlying explanation was understood. He left gaps for undiscovered elements and predicted their properties. The later discovery of elements such as germanium, matching his predictions, strengthened confidence in his table.

New methods then provided evidence unavailable to Mendeleev. In 1913, Henry Moseley’s measurements of X-rays from elements established an experimental basis for ordering them by atomic number. Knowledge of isotopes explained why relative atomic mass could give a different order.

The development of the table shows how science progresses: scientists identify patterns, test predictions and revise explanations when new evidence becomes available. The modern table retained Mendeleev’s successful chemical patterns while replacing mass-based ordering with a rule explained by atomic structure.

Why mass order caused problems

• Mendeleev’s 1869 table was based mainly on increasing relative atomic mass, while keeping chemically similar elements together.
• Strict mass order put some elements in the wrong groups. He reversed some pairs, including tellurium and iodine, to fit their properties.

How isotopes resolve the problem

• Isotopes have the same proton number but different neutron numbers and masses.
• Most elements are mixtures of isotopes; is a weighted mean.