How Did Mendeleev Arrange the Periodic Table? A Clear Guide

Dmitri Mendeleev arranged the periodic table by ordering the known chemical elements by increasing atomic weight and then grouping them into columns based on shared chemical properties. His key innovation was leaving blank spaces for elements that had not yet been discovered, predicting their properties with surprising accuracy. This structure, first published in 1869, gave the table its predictive power and led to its widespread adoption.

The Problem Mendeleev Faced

Before Mendeleev, several scientists had tried to organize the elements. John Newlands proposed the “law of octaves” in 1864, noting that every eighth element had similar properties, but his pattern broke down after calcium. Other chemists had grouped elements by atomic weight, but no system accounted for all the known elements in a useful way.

Mendeleev, a professor at the University of St. Petersburg, was writing a textbook called Principles of Chemistry in 1868. He needed a clear way to present the elements to his students. While preparing the book, he wrote each element’s name, atomic weight, and properties on a separate card. By arranging these cards, he noticed repeating patterns.

How He Built the Table

Mendeleev started by listing the elements in order of increasing atomic weight. He then looked for places where the properties of an element did not match the element next to it in the weight order. When that happened, he moved the element to a new row, creating vertical columns of elements with similar chemical behavior.

For example, lithium (atomic weight 7), sodium (23), and potassium (39) all react strongly with water and form similar compounds. Mendeleev placed them in the same column. Beryllium (9), magnesium (24), and calcium (40) also share properties and went into another column. The pattern held for most of the 63 known elements.

The Two Key Rules Mendeleev Followed

Mendeleev used two main principles to arrange his table. First, elements were placed in order of increasing atomic weight. Second, elements with similar chemical and physical properties were grouped together in vertical columns, which he called groups.

When Properties Overruled Weight

In a few cases, Mendeleev broke his own atomic-weight rule. Tellurium has a higher atomic weight (128) than iodine (127), but tellurium’s properties are similar to sulfur and selenium, while iodine’s properties match chlorine and bromine. Mendeleev placed tellurium before iodine in the table, keeping the property groups intact. He assumed the atomic weights must have been measured incorrectly. Later, work by Henry Moseley in 1913 showed that atomic number, not atomic weight, is the fundamental ordering principle, and that tellurium does indeed have a lower atomic number than iodine Royal Society of Chemistry.

This decision was controversial at the time. Many chemists thought Mendeleev was forcing the data to fit his pattern. But the success of his predictions soon silenced most critics.

Predictions and Corrections

Mendeleev left blank spaces in his 1869 table for elements he believed existed but had not been discovered. He predicted the properties of three missing elements: eka-aluminum, eka-boron, and eka-silicon. The “eka” prefix came from Sanskrit, meaning “one” or “first,” indicating the element one place below the known one in the same group.

The Discovery of Gallium

In 1875, French chemist Paul-Émile Lecoq de Boisbaudran discovered a new element, gallium. Its properties matched Mendeleev’s prediction for eka-aluminum almost exactly. Mendeleev had predicted an atomic weight of 68, and gallium’s measured weight was 69.7. He predicted the density would be 5.9 g/cm³, and the measured value was 5.94 g/cm³. This close match convinced many chemists that Mendeleev’s system was correct American Chemical Society.

Germanium and Scandium

Two more predicted elements were found in the following decade. Scandium (eka-boron) was discovered in 1879, and germanium (eka-silicon) in 1886. Germanium’s properties matched Mendeleev’s predictions for atomic weight, density, and the properties of its oxide and chloride compounds. These discoveries established the periodic table as the central organizing principle of chemistry.

What Mendeleev Got Wrong

Mendeleev’s table had limitations that later science corrected. He did not know about noble gases, which were discovered starting in the 1890s by William Ramsay and Lord Rayleigh. When argon was found, it did not fit into any existing group. Mendeleev initially resisted the idea of a new group, but Ramsay’s discovery of helium, neon, and other noble gases led to the addition of a whole new column.

Mendeleev also had no knowledge of atomic structure. He did not know about protons, neutrons, or electrons. The concept of atomic number, which is the number of protons in an atom’s nucleus, was not developed until the early 20th century. Moseley’s work showed that atomic number, not atomic weight, is the true basis for the order of elements. This explained why tellurium and iodine were in the order Mendeleev had chosen, even though their weights suggested otherwise.

The modern periodic table is arranged by increasing atomic number, not atomic weight. But the structure of groups and periods, and the placement of elements by shared properties, remains directly from Mendeleev’s original design.

Key Takeaways

• Mendeleev arranged elements by increasing atomic weight, then grouped them by shared chemical properties.
• He left blank spaces for undiscovered elements and correctly predicted their properties.
• He sometimes placed elements out of atomic-weight order to keep property groups intact, a decision later validated by atomic number.
• His predictions for gallium, scandium, and germanium were confirmed within 15 years, establishing the table’s credibility.
• The modern table uses atomic number instead of atomic weight, but Mendeleev’s group-and-period structure remains the standard.