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Navigating the periodic table
How to read the periodic table: atomic number order, what each box shows, groups and periods, the link between position and electron arrangement, the main regions and families, and where element names come from.
The periodic table works like a map with an address system. Tell a chemist "group 17, period 3" and they know you mean chlorine, a poisonous green gas that forms ions with a 1− charge, without looking anything up.
What each box tells you
Every element has one box. The elements are placed in order of atomic number, the number of protons in the nucleus, from hydrogen (1) to oganesson (118). Most tables show at least three things in each box:
- the atomic number, which tells you which element it is and how many electrons a neutral atom has
- the symbol, one capital letter or a capital followed by a lower-case letter (Co is cobalt, but CO is carbon monoxide, a compound of two elements)
- the relative atomic mass, the average mass of its atoms (1C)
Some symbols come from Latin names: Na from natrium (sodium), K from kalium (potassium), Fe from ferrum (iron), Cu from cuprum (copper), Ag from argentum (silver), Au from aurum (gold), Pb from plumbum (lead) and Hg from hydrargyrum ("liquid silver", mercury).
Groups and periods
The columns are called groups and are numbered 1 to 18 from left to right. The rows are called periods and are numbered 1 to 7 from the top.
- Elements in the same group have the same number of electrons in their outer shell, so they have similar chemical properties. Lithium (2,1), sodium (2,8,1) and potassium (2,8,8,1) all have one outer electron, and all react with water in the same way.
- Elements in the same period have the same number of occupied electron shells. Sodium, magnesium, aluminium and chlorine are all in period 3 because their electrons fill three shells. Going across a period, the outer shell fills up one electron at a time until the noble gas at the end has a full shell.
For the main groups this gives a quick rule. Groups 1 and 2 have 1 and 2 outer electrons. Groups 13 to 18 have 3 to 8 outer electrons (take away 10). Helium is the exception: it is in group 18 because its only shell is full, even though that shell holds just 2.
Worked example. Where is phosphorus (atomic number 15)? Its electron arrangement is 2,8,5. Three shells means period 3. Five outer electrons means group 15.
The main regions
- Metals fill the left and centre: more than three-quarters of all elements. Groups 1 and 2 are the alkali metals and alkaline earth metals. The wide block in the middle (groups 3 to 12) is the transition metals.
- Non-metals are on the right, plus hydrogen. Group 17 is the halogens ("salt makers") and group 18 is the noble gases.
- Metalloids sit along the staircase line between them: boron, silicon, germanium, arsenic, antimony and tellurium. They have some properties of each.
- The two rows at the bottom, the lanthanoids (57 to 71) and actinoids (89 to 103), really belong in periods 6 and 7. They are pulled out only so the table fits on a page; put back in, the table would be 32 columns wide.
Hydrogen is the odd one out. It has one outer electron like the alkali metals, so it is often drawn above group 1, but it is a non-metal gas. Many tables now float it on its own.
You will see a few different versions of the table. Older ones number the groups I to VIII and skip the transition metals, and some put lanthanum and actinium (rather than lutetium and lawrencium) under group 3. Chemists still debate these details, but the order of the elements never changes.
- Colour the table by state. Which two elements are liquid at 25 °C? Where are all the gases?
- Colour it by when the elements were discovered. Which part of the table was found most recently, and why do you think that is?
- Colour it by outer electrons. What do you notice about each column?
- Try the Find-it challenge five times without a mistake.
Where the names come from
Element names are a history lesson in themselves. They are named after:
- properties: chlorine (Greek chloros, pale green), bromine (bromos, stench), iodine (iodes, violet), hydrogen ("water maker")
- places: americium, francium, germanium, polonium (Marie Curie's homeland, Poland), nihonium (Japan), and four elements (yttrium, terbium, erbium, ytterbium) all named after the one Swedish village of Ytterby, where the mineral they came from was mined
- people: curium (Marie and Pierre Curie), einsteinium, mendelevium, rutherfordium, meitnerium (Lise Meitner), seaborgium, oganesson
- myths and space: titanium (the Titans), promethium (Prometheus), helium (Greek helios, the Sun), uranium, neptunium and plutonium (the planets, in order)
Since 2002 the International Union of Pure and Applied Chemistry (IUPAC) has set rules for naming new elements, and it approves every name. New elements get a temporary name from their number until then: element 119 would be "ununennium" (un-un-enn = 1-1-9).
Test yourself
Elements in the same group of the periodic table have the same
That's why they react in similar ways.
An element has the electron arrangement 2,8,3. Where is it in the table?
Three shells means period 3; three outer electrons means group 13. It's aluminium.
Which symbol stands for potassium?
K comes from the Latin kalium. P is phosphorus, Po polonium, Pt platinum.
Which of these is a metalloid?
Silicon sits on the staircase line between metals and non-metals.
Name each region of the periodic table.
Sort the elements into their groups.
The elements are arranged in order of atomic . The columns are called and the rows are called . Elements in the same period have the same number of electron . The elements along the staircase line are called .
Magnesium has atomic number 12. Without looking at a periodic table, work out its group and period, and explain your reasoning.
A magnesium atom has 12 electrons, arranged 2,8,2. The electrons occupy three shells, so magnesium is in period 3. It has 2 electrons in its outer shell, so it is in group 2, the alkaline earth metals.