The periodic table of the elements

The periodic table is an arrangement of all 118 known chemical elements in order of increasing atomic number, laid out so that elements with similar chemical behaviour fall into the same vertical column. Its 7 rows are called periods and its 18 columns are called groups. Because the layout follows electron arrangement, an element's position tells you a great deal about how it will react before you have looked anything up.

Dmitri Mendeleev published the first widely accepted periodic table in 1869, ordering the elements by atomic mass and leaving gaps for elements he predicted but that had not yet been discovered — gallium, scandium and germanium were later found with close to the properties he forecast. In 1913 Henry Moseley showed that ordering by atomic number rather than atomic mass fixed the handful of pairs that sat in the wrong place, which is the modern table used today.

The modern periodic law states that the physical and chemical properties of the elements are a periodic function of their atomic numbers. It replaced Mendeleev's periodic law, which was stated in terms of atomic masses and could not explain why pairs such as cobalt and nickel, or tellurium and iodine, had to be swapped out of mass order to make the chemistry work.

There are 118 confirmed elements, numbered 1 (hydrogen) to 118 (oganesson). The first 94 occur naturally on Earth, although technetium, promethium and a few others are present only in traces. Elements 95 to 118 have only ever been produced in reactors and particle accelerators, and the heaviest of them exist for a fraction of a second before decaying.

A group is a vertical column and a period is a horizontal row. Elements in the same group have the same number of electrons in their outermost shell, which is why they react in similar ways — every group 1 metal reacts with water, every group 18 gas is unreactive. Elements in the same period have the same number of occupied shells, and their properties change steadily as you read across.

The blocks name the subshell that the last electron goes into. The s-block is groups 1 and 2 plus helium, the p-block is groups 13 to 18, the d-block is groups 3 to 12 — the transition metals — and the f-block is the two detached strips of lanthanoids and actinoids drawn below the main table. The block an element sits in is the quickest guide to its typical valency and bonding.

The atomic number is the number of protons in the nucleus, and in a neutral atom the number of electrons too. The atomic mass is the average mass of the element's atoms in unified atomic mass units, weighted by the natural abundance of each isotope — which is why chlorine reads 35.45 rather than a whole number. Subtracting the atomic number from the mass number of a particular isotope gives its number of neutrons.

Most of the table is metal: everything in the s-block below hydrogen, the whole d- and f-blocks, and the post-transition metals such as aluminium, tin and lead. The non-metals sit in the top right — hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur, selenium, the halogens and the noble gases. Between them runs a staircase of metalloids — boron, silicon, germanium, arsenic, antimony and tellurium — which behave partly like both.

Learn it group by group rather than element by element: one mnemonic per group, then the first 20 elements in order, then the period 4 transition metals. Attach the chemistry to each group as you go — group 1 loses one electron, group 17 gains one — so that the sequence carries meaning instead of being a list. Testing yourself on a few elements a day fixes it far faster than rereading the table.

Yes. The table itself is the same for every board and every exam — CBSE, ICSE and the state boards all teach the modern 118-element table, and NEET, JEE and KCET all examine it. What changes is the depth: Class 9 and 10 chemistry stays with atomic number, symbol, valency and electron arrangement, while Class 11 adds the blocks, periodic trends and the reasoning behind them, all of which are on this page.