Groups and Periods of the Periodic Table
All 18 groups, what they have in common, and why the columns predict chemistry.
The periodic table is a grid, and both directions mean something. Groups are the eighteen vertical columns; periods are the seven horizontal rows. Group membership predicts how an element behaves, because every element in a column has the same number of outer electrons. Period membership tells you how many shells those electrons are spread across.
That is the whole trick of the table: it is arranged so that elements which behave alike end up stacked on top of each other. Mendeleev built it that way before anyone knew electrons existed, purely from observed chemistry.
Reading down a group
As you descend a group, the outer electrons sit in shells further from the nucleus and are screened by more inner electrons. So atoms get bigger, ionisation energy falls, electronegativity falls, and metals become more reactive while non-metals become less so. Caesium reacts far more violently with water than lithium for exactly this reason.
Reading across a period
Moving right, protons pile up while the outer shell stays the same, so the nucleus grips more tightly. Atoms shrink, ionisation energy and electronegativity climb, and the character shifts from metallic through metalloid to non-metallic — ending each row at a noble gas with a full shell.
The eighteen groups
Group 1Alkali metals
H HydrogenLi LithiumNa SodiumK PotassiumRb RubidiumCs CesiumFr Francium
1 valence electron · typically forms 1+ ions
Group 2Alkaline earth metals
Be BerylliumMg MagnesiumCa CalciumSr StrontiumBa BariumRa Radium
2 valence electrons · typically forms 2+ ions
Group 3Group 3
Group 4Titanium group
Group 5Vanadium group
Group 6Chromium group
Group 7Manganese group
Group 8Iron group
Group 9Cobalt group
Group 10Nickel group
Group 11Coinage metals
Group 12Zinc group
Group 13Boron group
B BoronAl AluminumGa GalliumIn IndiumTl ThalliumNh Nihonium
3 valence electrons · typically forms 3+ ions
Group 14Carbon group
C CarbonSi SiliconGe GermaniumSn TinPb LeadFl Flerovium
4 valence electrons · typically forms 4+ ions
Group 15Pnictogens
N NitrogenP PhosphorusAs ArsenicSb AntimonyBi BismuthMc Moscovium
5 valence electrons · typically forms 3− ions
Group 16Chalcogens
O OxygenS SulfurSe SeleniumTe TelluriumPo PoloniumLv Livermorium
6 valence electrons · typically forms 2− ions
Group 17Halogens
F FluorineCl ChlorineBr BromineI IodineAt AstatineTs Tennessine
7 valence electrons · typically forms 1− ions
Group 18Noble gases
He HeliumNe NeonAr ArgonKr KryptonXe XenonRn RadonOg Oganesson
2 valence electrons · forms no ions in ordinary chemistry
The two rows underneath
The lanthanides (57–71) and actinides (89–103) have no group numbers in the 18-column layout, because they are not columns — they are the f-block, which belongs inside periods 6 and 7 between groups 2 and 3. Printing them there would make the table 32 columns wide, so convention pulls them out and leaves a placeholder. The interactive table has a 32-column view that puts them back.
More on the periodic table
Related Calculators
Frequently Asked Questions
What is the difference between a group and a period?
A group is a vertical column and a period is a horizontal row. Elements in the same group share the number of electrons in their outer shell, which is why they behave alike chemically. Elements in the same period share the number of shells, and their properties change steadily from metallic on the left to non-metallic on the right.
How many groups and periods are there?
There are 18 groups and 7 periods. The lanthanides and actinides are usually printed as two separate rows below the table; they belong inside periods 6 and 7, and are not given group numbers in the standard 18-column layout.
What is group 17 on the periodic table?
Group 17 is the halogens — fluorine, chlorine, bromine, iodine, astatine and tennessine. Each is one electron short of a full outer shell, which makes them aggressive oxidisers that form −1 ions and react directly with metals to form salts. The name means "salt-former".
Why do elements in the same group behave the same way?
Because chemistry is done by the outermost electrons, and every element in a group has the same number of them. Lithium, sodium and potassium each have one loose outer electron, so each reacts with water in the same way — just more vigorously further down, because the electron is held less tightly the further it sits from the nucleus.