History of the Periodic Table Timeline: Complete Guide

The periodic table as we know it today is the result of work by many scientists over roughly two centuries. The key figure is Dmitri Mendeleev, who in 1869 published a table that arranged the known elements by atomic weight and left gaps for undiscovered ones. This guide walks through the major steps in the history of the periodic table timeline, from early lists of elements to the modern arrangement based on atomic number.

Early attempts to classify elements

Before the 19th century, only a few dozen elements were known. Ancient civilisations recognised gold, silver, copper, iron, tin, lead, mercury, and sulfur. By 1789, Antoine Lavoisier published a list of 33 chemical substances he considered elements, grouping them into gases, metals, non-metals, and earths. Lavoisier’s list was a start, but it had no underlying order.

In 1829, the German chemist Johann Wolfgang Döbereiner noticed that certain groups of three elements shared similar properties. He called these triads. For example, lithium, sodium, and potassium all react strongly with water. The atomic weight of the middle element in each triad was roughly the average of the other two. Döbereiner’s triads were the first real hint that elements could be grouped by a numerical pattern, but only a few triads were known.

The law of octaves

By the 1860s, about 60 elements had been identified. The English chemist John Newlands proposed in 1864 that when elements were arranged by increasing atomic weight, every eighth element had similar properties. He called this the law of octaves, comparing it to a musical scale.

Newlands’ idea was not widely accepted. His table had problems: some elements did not fit the pattern, and he had no room for undiscovered elements. The Chemical Society of London rejected his paper. Still, Newlands was one of the first to see a repeating pattern in element properties.

Mendeleev’s breakthrough

Dmitri Mendeleev, a Russian chemist, published his first periodic table in 1869. He arranged all 63 known elements by atomic weight and placed them into columns based on their chemical and physical properties. Mendeleev made two bold moves that set his table apart.

First, he left gaps. Where an element did not fit the pattern of its column, Mendeleev left an empty space and predicted the properties of the missing element. He predicted three elements: eka-aluminium, eka-boron, and eka-silicon. Within 15 years, all three were discovered — gallium, scandium, and germanium — and their properties matched Mendeleev’s predictions closely.

Second, Mendeleev reversed the order of a few pairs of elements where atomic weight did not match the chemical pattern. For example, tellurium has a higher atomic weight than iodine, but Mendeleev placed tellurium before iodine because its properties fit better. He assumed the atomic weights were measured incorrectly. Later work showed he was right to trust the pattern, though the reason was not atomic weight but atomic number.

The Royal Society of Chemistry provides a detailed account of Mendeleev’s table and its reception on their history page.

The discovery of atomic number

Mendeleev’s table worked well, but it had no explanation for why the pattern existed. That explanation came from atomic number.

In 1911, Ernest Rutherford’s gold foil experiment showed that atoms have a small, dense nucleus with a positive charge. In 1913, Henry Moseley, a young British physicist working in Rutherford’s lab, used X-ray spectroscopy to measure the positive charge (atomic number) of each element. Moseley found that atomic number — not atomic weight — determined the order of the elements.

Moseley’s work fixed the few remaining problems with Mendeleev’s table. The reversed pairs (like tellurium and iodine) now made sense: tellurium has atomic number 52, iodine has 53. Moseley also predicted new elements between aluminium and gold, all later discovered. His work gave the periodic table a solid physical foundation.

The modern periodic table

After Moseley, the periodic table was reorganised by increasing atomic number. The modern table has 118 confirmed elements, from hydrogen (atomic number 1) to oganesson (atomic number 118). The table is arranged in 18 groups (columns) and 7 periods (rows).

Elements in the same group have the same number of valence electrons, which gives them similar chemical behaviour. For example, group 1 (alkali metals) are highly reactive, while group 18 (noble gases) are mostly inert. The periodic table also includes two rows of elements below the main table: the lanthanides (atomic numbers 57 to 71) and the actinides (atomic numbers 89 to 103).

The International Union of Pure and Applied Chemistry (IUPAC) oversees the naming and confirmation of new elements. As of 2025, elements 113 to 118 were added between 2016 and 2022, completing the seventh period. IUPAC maintains the official table and rules for element names on their website.

How the table is used today

The periodic table is not just a reference list. Chemists use it to predict how elements will react. For example, an element’s position tells you its electron configuration, which determines bonding behaviour. The table also shows trends in properties like atomic radius, ionisation energy, and electronegativity.

• Atomic radius increases down a group and decreases across a period.
• Ionisation energy decreases down a group and increases across a period.
• Electronegativity follows a similar pattern to ionisation energy.

These trends let scientists estimate the behaviour of elements they have never seen. For instance, the properties of element 119 (not yet discovered) can be predicted from its expected position below francium in group 1.

Key takeaways

• The periodic table evolved from simple lists (Lavoisier) through triads (Döbereiner) and octaves (Newlands) to Mendeleev’s predictive table in 1869.
• Mendeleev left gaps for undiscovered elements and predicted their properties, which were later confirmed.
• Henry Moseley established atomic number as the organising principle in 1913, fixing the remaining inconsistencies.
• The modern table has 118 elements arranged by atomic number in 18 groups and 7 periods.
• The table is a practical tool for predicting chemical behaviour and properties based on an element’s position.