Mendeleev's Periodic Law

Background and core of the law

By the mid-19th century, scientists knew of the existence of 62 chemical elements. Chemists attempted to group them based on empirical data, but no unified classification existed. No one knew how many elements remained to be discovered. Dmitry Ivanovich Mendeleev was the only one who maintained a firm conviction in the existence of a single law connecting absolutely all elements.

In 1869, Mendeleev arranged the elements in a row by increasing atomic masses (then referred to as "atomic weight"). He discovered a fundamental pattern: substances with similar chemical properties appeared cyclically. Initially, recurrences were observed every 7 and 17 elements, but later, with the discovery of noble gases, these rows increased to 8 and 18.

This proved that elements are not just a random set of substances, but a coherent system built from the same more elementary particles.

Architecture of the Periodic Table

The graphical expression of the law became the table, where the resulting continuous series of elements is divided into segments—periods—placed one below another.

• Short periods (1–3): Contain up to 8 elements. They begin with an active alkali metal (e.g., lithium) and end with an inert noble gas. Within them, the transition of properties from metallic to non-metallic is most pronounced and rapid.
• Long periods (4–5): Include 18 cells each. Their feature is the presence of 10 additional transition metals (an insertion from scandium to zinc, from yttrium to cadmium) that wedge between typical elements.
• Super-long periods (6–7): Contain 32 elements each. In addition to transition metals, they feature groups of 14 analog elements—lanthanides and actinides. Their chemical properties are so similar that they are usually placed in a separate block in the table.

Vertical columns are called groups. Their number is strictly tied to the formula of the element's highest oxide (from $R^2O$ in Group I to $RO^4$ in Group VIII). Main groups (A) include elements of short periods and their analogs. Subgroups (B) consist exclusively of transition metals.

Error correction and scientific triumph

In the 19th century, gas methods for accurately measuring metal masses did not exist. Weight was calculated via chemical equivalent (reaction with 8 parts of oxygen) and expected valence. Any error in valence resulted in a proportional distortion of mass.

While creating the table, Mendeleev noticed that certain elements did not fit the logic of the law. Instead of changing the law, he hypothesized that the atomic masses of these substances were measured incorrectly.

1. Uranium (U): Considered trivalent with a mass of 120. Mendeleev proved that uranium is hexavalent, placed it in Group VI with chromium and tungsten, and doubled its mass to 240. Uranium became the heaviest known element at the time.
2. Beryllium (Be): Due to its similarity to aluminum, it was considered trivalent (mass 13.5). The scientist moved it to Group II, correcting its valence and weight.

The main triumph was the predictive function. In the first four periods, Mendeleev left empty cells, marking them with question marks. He detailed the properties of yet-unseen metals: eka-boron, eka-aluminum, and eka-silicon.

Already in 1875, the French chemist P. Lecoq de Boisbaudran discovered element no. 31, the properties of which perfectly matched those of eka-aluminum. The new metal was named gallium (from Lat. Gallia—France). Over the next 15 years, the predicted scandium and germanium were discovered, leading to the unconditional acceptance of the Periodic Law.

Class and exam questions

What else to study on this topic

• Formation of higher oxides and their formulas by groups (from $R^2O$ to $RO^4$)
• The concept of chemical equivalent and the complexities of calculating atomic masses in the 19th century
• Isolation of metal triads (e.g., Fe-Co-Ni) into a separate Group VIII
• Patterns of change in the chemical activity of halogens and alkali metals
• Structure and properties of elements of the lanthanide and actinide families
• Transuranic elements and the limits of atomic nucleus stability

Other topics in "Chapter 5. Structure of Matter"

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