It hangs on the wall of every chemistry classroom on earth: rows and columns of symbols that claim to organise all matter. The periodic table is arguably the most successful organisational scheme in science — and it began with a Russian chemist, a deck of cards’ worth of known elements, and a daring decision to leave blank spaces for elements nobody had ever seen.

Mendeleev’s gamble

On March 6, 1869, Dmitrii Mendeleev presented his first periodic table to the Russian Chemical Society. About 63 elements were known at the time. He arranged them by atomic weight and grouped them by chemical behaviour — and, crucially, he left gaps. Where the pattern demanded an element that hadn’t been discovered, he left an empty box and even predicted the missing element’s properties.

He was not the first to notice periodicity — others had spotted patterns, including Johann Döbereiner’s “triads” decades earlier — but Mendeleev’s version made bold, testable predictions. Within about seventeen years, three of his predicted elements were found: gallium (his “eka-aluminium”), germanium and scandium, with properties matching his forecasts remarkably closely. That predictive power is what won his table acceptance and made him the credited discoverer of the periodic law. Element 101, synthesised in 1955, was named mendelevium in his honour.

The fourth prediction took 68 years

Mendeleev’s fourth gap — “eka-manganese,” the element that should sit below manganese — was not found in his lifetime. It was identified in 1937, 68 years after his prediction, by Italian physicists Emilio Segrè and Carlo Perrier, who found it in molybdenum samples irradiated at Ernest Lawrence’s cyclotron in California. They named it technetium, from the Greek for “artificial”: it barely exists in nature because its longest-lived isotope has a half-life of about 4.2 million years, so nearly all of Earth’s original technetium has long since decayed away. Mendeleev had, without knowing it, predicted an element that 20th-century nuclear technology would have to manufacture before it could be observed.

From atomic weight to atomic number

Mendeleev ordered his table by atomic weight, which produced a few awkward inversions. The modern table is ordered by atomic number — the count of protons in the nucleus — a change that resolved the anomalies and revealed the deeper logic: an element’s position reflects its electron configuration, which determines how it bonds and reacts. Metals sit to the left, non-metals to the right, and the far-right column holds the noble gases, named for their reluctance to react with anything.

The noble gases weren’t in Mendeleev’s original scheme at all — helium and argon were only identified in the 1890s and were slotted into the table around 1902, a new column for a whole new family of elements.

The table keeps growing

The modern table holds 118 confirmed elements, in 7 rows (periods) and 18 columns (groups). Only 94 occur naturally on Earth; the rest are synthetic, created in particle accelerators and nuclear reactors, often existing for fractions of a second before decaying.

The most recent additions were named in 2016: nihonium (113), moscovium (115), tennessine (117) and oganesson (118), completing the table’s seventh row. Their names honour Japan, Moscow, Tennessee and physicist Yuri Oganessian — a reminder that element discovery remains an international, collaborative enterprise.

Unfinished business

Chemists still argue about the details. The placement of lutetium and lawrencium versus lanthanum and actinium at the top of the f-block remains disputed, with the Royal Society of Chemistry and IUPAC weighing electron configurations differently. And theorists speculate about an “island of stability” — superheavy elements beyond 118 that might, if ever made, live long enough to study properly. Laboratories in Russia, Japan, Germany and the United States continue the hunt, fusing lighter nuclei in the hope of creating a few atoms of element 119 or 120. Each success would extend the table into an eighth row that currently exists only in theory.

What endures is the table’s essential insight: matter is not a random catalogue but a patterned system, and the pattern itself can predict what we haven’t found yet. A chart drawn in 1869, from 63 elements and a few brave empty boxes, still organises everything chemistry knows.

FAQs

Who invented the periodic table?

Russian chemist Dmitrii Mendeleev, who presented his first version to the Russian Chemical Society on March 6, 1869. Others noticed periodic patterns, but his table’s successful predictions won it acceptance.

How many elements are on the periodic table?

118 confirmed elements. Ninety-four occur naturally; the rest are synthetic, made in laboratories.

What were the newest elements added?

Nihonium, moscovium, tennessine and oganesson (elements 113, 115, 117 and 118), named in 2016 — completing the seventh row.

Why are there gaps and disputes in the table?

Mendeleev deliberately left gaps for undiscovered elements — some took decades to fill. Modern disputes, like the placement of certain f-block elements, concern which electron configurations should define the columns.

Compiled by the Khabar 24h Editorial Desk from publicly available sources.