Reading the Periodic Table: Groups, Periods and Blocks

The periodic table looks like a wall of letters and numbers, but it is really a map. Every element’s position tells you how many electron shells it has, how many electrons sit in its outer shell, and roughly how it will behave in a reaction. Once you know how to read three things, groups, periods and blocks, you can work out a surprising amount about an element without memorising anything about it.

Below, each one is explained, with the UK and US numbering differences and a step-by-step method for placing any element from its atomic number alone.

What one tile on the periodic table tells you

Each square (or tile) holds at least three pieces of information: the chemical symbol, the atomic number and the relative atomic mass. The atomic number is the number of protons in the nucleus, and it is what defines the element. In a neutral atom the number of electrons equals the number of protons.

The relative atomic mass is the weighted average mass of the element’s naturally occurring isotopes, which is why it is usually not a whole number. One practical tip: tables are not laid out the same way everywhere. Many US tables print the atomic number at the top of the tile, while UK GCSE data sheets (AQA, for example) put the relative atomic mass on top and the atomic number underneath. You never need to guess which is which, because the atomic number is always the smaller of the two numbers.

Worked example: reading the chlorine tile

The chlorine tile shows Cl, 17 and 35.45. Here is what you can read from it, line by line:

• Atomic number = 17, so chlorine has 17 protons and, as a neutral atom, 17 electrons.
• Electrons fill shells of 2, then 8, then the rest: 17 = 2 + 8 + 7, so the arrangement is 2, 8, 7.
• Three shells in use means chlorine is in period 3.
• Seven outer electrons means it is in group 17 (group 7 on many UK GCSE tables), the halogens. It needs one more electron to fill the shell, so it tends to form the Cl^– ion.
• Neutrons = mass number minus atomic number. For chlorine-35: 35 – 17 = 18 neutrons. For chlorine-37: 37 – 17 = 20 neutrons.
• Why 35.45 and not 35 or 37? About 75.8% of natural chlorine is chlorine-35 (34.969) and 24.2% is chlorine-37 (36.966): 0.758 x 34.969 = 26.506, plus 0.242 x 36.966 = 8.946, gives 35.452.

That last step is exactly how the masses used in our guide on how to calculate molecular weight are built up.

Periods: the rows

The table has seven horizontal rows called periods. The period number equals the number of electron shells an atom uses. Hydrogen and helium (period 1) use one shell; sodium to argon (period 3) use three.

The rows are different lengths because the shells hold different numbers of electrons once subshells come into play. The seven periods contain 2, 8, 8, 18, 18, 32 and 32 elements, which adds up to 118. Period 7 was completed in 2016, when IUPAC approved the names nihonium (113), moscovium (115), tennessine (117) and oganesson (118).

Moving left to right along a period, each element has one more proton pulling on the same number of shells. The result is a clear trend: atoms get smaller, hold their electrons more tightly, and become more electronegative. The electronegativity and bond type calculator uses the gap between two elements’ values to predict whether a bond is ionic or covalent.

Groups: the columns

The 18 vertical columns are groups. Elements in the same group have the same number of outer-shell electrons, so they react in similar ways.

The confusing part is numbering. IUPAC (the international chemistry body) numbers the groups 1 to 18, and that is the safest system to learn. UK GCSE courses often number only the main groups 1 to 7 plus “group 0” for the noble gases. Older US textbooks use Roman numerals with A and B, where the A groups are the main groups.

IUPAC group	UK GCSE label	Older US label	Family name	Outer electrons	Example
1	Group 1	IA	Alkali metals	1	Sodium (Na)
2	Group 2	IIA	Alkaline earth metals	2	Calcium (Ca)
3 to 12	Transition metals	IIIB to IIB	Transition metals	Varies	Iron (Fe)
13	Group 3	IIIA	Boron group	3	Aluminium (Al)
14	Group 4	IVA	Carbon group	4	Silicon (Si)
15	Group 5	VA	Pnictogens	5	Nitrogen (N)
16	Group 6	VIA	Chalcogens	6	Oxygen (O)
17	Group 7	VIIA	Halogens	7	Chlorine (Cl)
18	Group 0	VIIIA	Noble gases	8 (helium 2)	Neon (Ne)

A quick rule for the main groups: in groups 1 and 2 the outer electrons equal the group number; in groups 13 to 18, subtract 10.

Going down a group, atoms gain a whole shell each time, so they get larger and their outer electrons are held more loosely. That explains why reactivity increases down group 1 (potassium reacts more violently than sodium) but decreases down group 17 (fluorine is the most reactive halogen). Density varies hugely too: lithium has a density of 0.534 g/cm³ (about 33.3 lb/ft³), so it floats on water, while osmium, deep in the transition metals, is about 22.6 g/cm³ (roughly 1,410 lb/ft³).

Blocks: s, p, d and f

Blocks sort elements by the type of subshell their last electron enters, and they explain the table’s odd shape.

• s-block: groups 1 and 2, plus helium. Fourteen elements, all with their outermost electrons in an s subshell.
• p-block: groups 13 to 18 (except helium). Thirty-six elements, including every non-metal apart from hydrogen and helium.
• d-block: groups 3 to 12, the transition metals. Ten per row across periods 4 to 7, so forty elements.
• f-block: the lanthanides and actinides, the two rows usually printed below the main table. Fourteen per row, twenty-eight in total.

Check the arithmetic: 14 + 36 + 40 + 28 = 118. The f-block rows are pulled out only to stop the table being 32 columns wide.

How to find any element’s position from its atomic number

This method uses only the atomic numbers of the noble gases: 2, 10, 18, 36, 54, 86 and 118. It follows the common layout where lanthanum and actinium start the f-block rows and lutetium and lawrencium sit in group 3 (some tables do it the other way round, which only changes those four elements).

1. Find the largest noble-gas number that is smaller than your atomic number.
2. The period is one more than that noble gas’s period. (Below 2 is period 1, between 2 and 10 is period 2, between 10 and 18 is period 3, and so on.)
3. Subtract that noble-gas number from your atomic number. Call the result n.
4. Periods 1 to 3: n = 1 or 2 gives group 1 or 2; n = 3 to 8 gives group n + 10.
5. Periods 4 and 5: the group is simply n.
6. Periods 6 and 7: n = 1 or 2 gives group 1 or 2; n = 3 to 16 means the element is in the f-block; n = 17 to 32 gives group n – 14.
7. Read the block from the group: 1 to 2 is s, 3 to 12 is d, 13 to 18 is p.

Worked examples

Iron, atomic number 26. The largest noble gas below 26 is argon (18), in period 3, so iron is in period 4. n = 26 – 18 = 8. In period 4 the group equals n, so iron is in group 8, d-block.

Gold, atomic number 79. The largest noble gas below 79 is xenon (54), in period 5, so gold is in period 6. n = 79 – 54 = 25. That falls in the 17 to 32 range, so the group is 25 – 14 = 11, d-block, directly below copper and silver.

Lead, atomic number 82. Again xenon is the reference, so period 6. n = 82 – 54 = 28, and 28 – 14 = group 14, p-block, in the same column as carbon, silicon and tin.

Neodymium, atomic number 60. Period 6 again, n = 60 – 54 = 6. That is between 3 and 16, so neodymium is in the f-block, one of the lanthanides used in strong magnets.

Frequently asked questions

What is the difference between a group and a period?

A group is a vertical column and tells you the number of outer electrons, so elements in a group behave alike. A period is a horizontal row and tells you the number of electron shells. Chlorine is in group 17 and period 3: seven outer electrons, three shells.

Why is group 18 called group 0 in UK schools?

It comes from an older convention where the noble gases were thought to have a valency of zero, because they do not normally form bonds. Group 0 and group 18 mean the same column; IUPAC numbering uses 18.

Is hydrogen a metal because it is in group 1?

No. Hydrogen is placed above group 1 because it has one electron, but under normal conditions it is a non-metal gas.

Explore the table yourself

Pick any atomic number, run it through the method above, then check your answer on our free interactive periodic table, where you can look up any element and check its position. When you are ready to put those masses to work, the molecular weight calculator adds them up for any formula, and the wider science tools collection has converters for the rest of your homework or lab work.