The history of the periodic table

In a nutshell

The periodic table is arranged in order of increasing atomic number. A scientist named Mendeleev organised the periodic table in this way.

Mendeleev's periodic table

How did Mendeleev organise the periodic table?

Placed elements in order of increasing atomic mass
Swapped the places of some of the elements depending on their properties
Left gaps where he predicted other elements should go based on predicted properties
Once elements were discovered they were placed into the gaps

Note: Mendeleev came up with an order for the periodic table before all of the elements had been discovered, hence why some gaps were left.

Pair reversals

Mendeleev did not know about isotopes. The discovery of isotopes allowed for pair reversals.

Example

The relative atomic mass of tellurium ( $Te$ ) and iodine ( $I$ ) are $128$ and $127$ respectively. However, Mendeleev placed $Te$ before $I$ in the periodic table (due to their properties). The most abundant isotopes of $Te$ are $Te-128$ and $Te-130$ . The high abundance results in a higher relative atomic mass. But the atomic number of tellurium is $52$ and iodine is $53$ , which supports the reverse order of elements in the periodic table.

Atomic number

The periodic table is organised in order of increasing atomic number. The atomic number is the number of protons in the nucleus of an atom. The mass number is the number of protons and neutrons in the nucleus of an atom.

The periodic table

Chemistry; Key concepts; KS4 Year 10; The history of the periodic table

The periodic table is filled with elements
Periods are the horizontal rows in the periodic table
Groups are vertical columns in the periodic table
Metals are located to the left and in the middle of the periodic table
Non-metals are located to the right of the periodic table
Elements with similar properties are placed in the same group