Mendeleev’s Periodic Table   Recently updated !

Mendeleev's Periodic Table
Mendeleev’s periodic table organizes elements by atomic weight and leaves gaps for undiscovered elements.

Dmitri Mendeleev‘s periodic table is the forerunner to the modern periodic table. It is a “periodic” table because it groups elements in rows and columns that showcase recurring properties, such as valence, electronegativity, and ionization energy.

Key Points

  • Mendeleev’s first periodic table in 1869 included the 63 known elements and spaces for three predicted, undiscovered elements. He revised and refined this table multiple times, as new data came to light.
  • Dmitri Mendeleev did not invent the first periodic table. Instead, he devised a table that organizes elements by atomic weight and periodic properties and predicts properties of undiscovered elements.
  • The key difference between Mendeleev’s table and the modern table is that the modern table orders elements by increasing atomic number. To be fair, the proton and atomic number were unknown in Mendeleev’s time and the difference only changes the position of a few elements.
Mendeleev at a Conference
Dmitri Mendeleev


The periodic table is one of the most important tools in chemistry, organizing elements based on their properties and atomic structures. Its development has been a long and complex process, with contributions from many scientists over the centuries. One of the most significant figures in this history is Dmitri Mendeleev, a Russian chemist who first proposed his version of the periodic table in 1869.

Before Mendeleev

Dmitri Mendeleev did not invent the first periodic table, but his table organizes elements much like the table we use today. Mendeleev’s work built on the discoveries of earlier chemists such as John Dalton and Antoine Lavoisier, who laid the foundations of modern chemistry. In the early 19th century, scientists began to investigate the properties of different elements and how they reacted with one another. This led to the development of atomic theory, which proposed that all matter was made up of tiny particles called atoms. By the mid-19th century, scientists had discovered around 60 elements, but there was no systematic way to organize them.

In 1863, John Newlands published a letter in Chemical News, organizing the elements by atomic weight according to his “Law of Octaves”, which is comparable to the modern octet rule. Lothar Meyer’s 1864 book pointed out the recurring or periodic properties in a periodic table organizing elements by their valences.

Mendeleev’s Periodic Table

The story goes that Mendeleev saw the arrangement for his periodic table in a dream. His table orders the elements by increasing atomic weight, in rows and columns, with each column representing a group of elements with similar properties. Mendeleev’s 1869 table surpassed the work of Newlands and Meyer by including all of the 63 known elements and holding spaces for predicted undiscovered elements. The chemical and physical properties of undiscovered elements could be predicted based on periodicity. Specifically, Mendeleev’s periodic table predicted the properties of germanium, gallium, and scandium. Mendeleev also dealt with the lanthanides and actinides by placing them in two separate rows, which is the format that continues in the modern table.

Importance of Mendeleev’s Periodic Table

Mendeleev’s periodic table was a significant breakthrough in chemistry, providing a systematic way to organize the elements and predict their properties. For example, if you have a sample of sodium and understand its properties, you know the properties of lithium (above sodium on the table) and potassium (below sodium) without ever seeing them. All elements in that group (the alkali metals) are highly reactive shiny metals that burn in water and have a +1 oxidation state.

Mendeleev’s table was also a major step forward in understanding atomic structure, as it revealed patterns in the behavior of atoms that had not been noticed before. These patterns reflect the electron shells of atoms. Ultimately, chemical reactions involve interactions between electrons.

Mendeleev’s periodic table allowed scientists to make predictions about elements that had not yet been discovered. Knowing the properties of an unknown elements makes identifying and verifying it a simpler process.

Mendeleev’s Periodic Table vs the Modern Periodic Table

The difference between Mendeleev’s periodic table and the modern periodic table is that Mendeleev’s table ordered elements by increasing atomic weight while the modern table orders elements by increasing atomic number. The atomic weight is the sum of the protons and neutrons in an atom. In contrast, the atomic number is the number of protons. In a few cases, using atomic weight changes the order of elements. This is because of the isotope ratio of naturally occurring elements. But, the concepts of the proton and atomic number were not known in Mendeleev’s time.

Of course, the modern periodic table contains more elements than Mendeleev’s periodic table. The current table has 118 elements, while Mendeleev’s first table had 63.


Mendeleev’s periodic table was a significant breakthrough in chemistry because it offered a systematic way to organize the elements and predict their properties. Further, it allowed scientists to make predictions about elements that had not yet been discovered.

The modern periodic table is based on the concept of atomic number, instead of atomic weight. It includes more elements than Mendeleev’s table, yet still allows for predicting properties of undiscovered elements.


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