The periodic table

How many elements are on the periodic table?

118. The IUPAC periodic table (IUPAC is the international body that decides chemical names), in its latest version dated 4 May 2022, ends at element 118. Each element has a different atomic number, the number of protons in the nucleus of its atoms, and every number from 1 to 118 is now filled.

The seventh row was completed most recently. On 30 December 2015, IUPAC announced that the discoveries of elements 113, 115, 117 and 118 had been verified, crediting teams in Japan, Russia and the USA. Their permanent names were approved on 28 November 2016:

These newest elements don't occur in nature. They are made in laboratories by smashing lighter nuclei together, and they are so unstable that they decay within seconds.

How is the periodic table organized?

In one sentence, from the AQA GCSE specification: the elements are arranged in order of atomic (proton) number. Reading left to right along each row, the atomic number goes up by one each time. A new row starts whenever the pattern of properties repeats, so elements with similar properties end up in the same column.

That repeating pattern is where the name comes from. As AQA puts it, the table is called a periodic table "because similar properties occur at regular intervals".

What is a period on the periodic table?

A period is a horizontal row. There are 7 periods in the periodic table. The rows get longer as you go down, because there are more ways to arrange electrons in the outer shells of bigger atoms.

Add up the second column: 2 + 8 + 8 + 18 + 18 + 32 + 32 = 118. Periods 6 and 7 each include 14 elements, the lanthanides and actinides, that are usually printed in two separate rows underneath. They belong inside the table, but leaving them in would make it too wide to fit on a page.

Vertical columns on the periodic table are called groups

The vertical columns are called groups, and US textbooks often call them families too. There are 18 groups. Elements in the same group react in similar ways.

There are two numbering systems, and it helps to know both. IUPAC has recommended numbering the columns 1 to 18 since 1988. GCSE courses in the UK usually number only the main groups: 1 and 2 on the left, then 3 to 7 and 0 on the right, skipping the transition metals. So the halogens are group 7 at GCSE and group 17 in the IUPAC system, and the noble gases are group 0 or group 18.

Metals, non-metals and the blocks

• Metals are on the left and towards the bottom. They are most of the table.
• Non-metals are on the right and towards the top, plus hydrogen.
• Metalloids such as silicon sit along the staircase line between them and have properties of both.
• Transition metals fill the wide middle section, groups 3 to 12, and include iron, copper and gold.

AQA defines metals as elements that react to form positive ions, and non-metals as elements that don't.

Atomic structure and the periodic table

The arrangement isn't just a convenient list: it maps the electrons in each atom. For the first 20 elements, two rules hold:

• The period number is the number of electron shells in use.
• The group number (GCSE numbering) tells you how many electrons sit in the outer shell, with group 0 holding atoms whose outer shell is full.

Every element in a group has the same count of outer electrons as the others, and outer electrons decide how an atom reacts. That is why a group shares its chemistry.

It works backwards too. An element with arrangement 2,8,5 uses 3 shells and has 5 outer electrons, so it is in period 3, group 5: phosphorus. Build any of these yourself in our Atom Builder.

Try one: this question shows an electron arrangement and asks you to find the element on the table.

The main groups to know

The trends have one cause: atoms get bigger down a group. In group 1, each extra shell puts the lone outer electron further out, with more inner electrons screening it from the pull of the nucleus, so it is lost more easily and the metal reacts faster. In group 7, the atom needs to gain an electron, and a bigger atom attracts it less strongly, so reactivity falls.

Who created the periodic table?

The Russian chemist Dmitri Mendeleev is usually credited, for the table he published in 1869. Others had tried before him, mostly by listing elements in order of atomic weight (relative atomic mass), but their tables broke down because some elements landed in groups that didn't suit them.

Mendeleev made two bold choices. First, where an element's properties didn't fit its place in the weight order, he moved it to the group it matched. He put tellurium before iodine, for example, even though tellurium is heavier, because iodine clearly behaves like the other halogens. Second, he left gaps for elements he believed hadn't been discovered yet, and predicted their properties.

The predictions came true. For the gap below aluminium, he forecast an element with an atomic weight around 68 and a density of 5.9 g/cm³. Gallium was discovered in 1875, and the Royal Society of Chemistry lists its relative atomic mass as 69.723 and its density as 5.91 g/cm³. Discoveries like this convinced chemists his table was right.

The reason the weight order sometimes failed came later. Ordering by atomic number instead of atomic weight puts every element in the right place, and isotopes explain the exceptions: tellurium has heavier isotopes than iodine, so its average mass is higher even though it has one proton fewer.

The GCSE periodic table: what you get in the exam

You don't have to memorise the table. In AQA GCSE Chemistry and Combined Science, you get a copy as an insert showing each element's symbol, atomic number and relative atomic mass. The lanthanides and actinides are left out. Other exam boards give you a periodic table too.

What you do need to know for AQA is the names and symbols of the first 20 elements, and how to read the table: which number is which, where metals and non-metals are, and what the group and period tell you.

Common mistakes

• Mixing up groups and periods. Groups go down (columns); periods go across (rows).
• Saying elements are arranged by mass. The modern table uses atomic number. Mendeleev used atomic weight.
• Swapping the two numbers in a box. The atomic number is always the smaller one.
• Calling hydrogen an alkali metal. It often sits above group 1 but is a non-metal gas.
• Forgetting helium's outer shell holds 2. It is in group 0 because its shell is full, not because it has 8.
• Mixing the two numbering systems. Group 7 at GCSE is group 17 in the 1 to 18 system.

Read next

Chemistry · Atoms and the periodic tableElectronic structure GCSE: shells, the first 20 elementsElectronic structure for GCSE Chemistry: how electrons fill shells, a table for the first 20 elements, shell diagrams, ions and the periodic table.