IB Chemistry SLTopic 3 — Classifying the ElementsPaper 1 & 2Core idea~10 min read
Structure of the Periodic Table
The periodic table is not a storage cupboard for facts. It is a map, and the position of an element on it tells you what that element will do before you have ever seen a sample of it. This page is about reading the map — what a row means, what a column means, and why the whole thing is shaped the way it is.
📘 What you need to know
Elements are arranged in order of increasing atomic number, from 1 to 118.
A period is a row. The period number tells you how many occupied electron shells an atom has.
A group is a column. Elements in a group have the same number of valence electrons, so they behave in similar ways.
Groups are numbered 1 to 18 in the IUPAC system. Group 1 = alkali metals, 17 = halogens, 18 = noble gases, 3–12 = transition elements.
The table splits into four blocks — s, p, d and f — named after the subshell the outermost electrons occupy.
Metals sit to the left and centre, non-metals to the upper right, and metalloids along the diagonal boundary between them.
The table in Section 7 of the data booklet is unlabelled, so you must be able to work out groups, periods, blocks and element types yourself.
Why the table has that shape
Everything about the layout comes from one idea: elements are placed so that atoms with similar outer electron arrangements line up underneath each other. Once you accept that, the odd staircase shape stops looking arbitrary.
Go along a row and you are adding electrons to the same outer shell, one at a time, until it is full. Start a new row and you have started a new shell. That is why period 2 has eight elements — eight is how many electrons fit in the 2s and 2p subshells together — and why period 4 suddenly has eighteen, because the 3d subshell joins in.
Students often try to learn the shape of the table as a picture. Learn the reason instead. The block widths are 2, 6, 10 and 14 — exactly the number of electrons that fit in an s, p, d and f subshell. The table is a picture of electron capacity.
Helium is the one element whose position and block disagree. Its outer electrons are in an s subshell, but it is unreactive like the noble gases, so behaviour wins and it is placed at the top of group 18.
What a position actually tells you
Any element’s address on the table gives you three separate pieces of information, and each one answers a different question.
For groups 13 to 18 the number of valence electrons is the group number minus 10. Chlorine is in group 17, so it has 7 — one short of a full outer shell, which is the entire reason it is so reactive.
Metals, non-metals and metalloids
Type
Where it sits
Typical properties
Examples
Metals
Left and centre — the s-block, d-block and lower left of the p-block
Good conductors of heat and electricity, malleable, form positive ions, form basic oxides
Sodium, iron, aluminium
Non-metals
Upper right of the p-block
Poor conductors, brittle when solid, form negative ions, form acidic oxides
Oxygen, chlorine, sulfur
Metalloids
Along the diagonal staircase between the other two
A mixture of both — often semiconductors
Boron, silicon, germanium, arsenic, antimony
The pattern to remember: metallic character increases as you go down a group and right to left across a period. The most metallic elements are bottom left, the most non-metallic top right (ignoring the noble gases).
The awkward cases
Hydrogen has one valence electron like group 1, but it is a gas that forms covalent molecules and does not behave like an alkali metal. It is usually shown on its own, above group 1.
Helium has only two electrons, which is an s configuration, but a full first shell makes it completely unreactive. It goes with the noble gases in group 18.
Group 12 (zinc, cadmium, mercury) sits in the d-block but does not show typical transition metal behaviour. There is more on that later in this topic.
Worked examples
WORKED EXAMPLE
An element is in period 4 and group 16. State its number of occupied shells, its number of valence electrons, and its block.
Step 1: use the period
Period 4 means the outermost electrons are in the 4th shell, so there are 4 occupied shells.
Step 2: use the group16 − 10 = 6 valence electronsStep 3: use the position within the row
Groups 13–18 are the p-block.
4 shells, 6 valence electrons, p-block — this is seleniumthe minus 10 rule only applies to groups 13 to 18; groups 1 and 2 give the count directly
WORKED EXAMPLE
Why do lithium, sodium and potassium behave so similarly, when their atoms are such different sizes?
Step 1: find what they share
All three are in group 1, so each has exactly one valence electron.
Step 2: link to chemistry
Chemical reactions involve the outer electrons, so all three react in the same way — by losing that one electron to form a 1+ ion.
Step 3: explain the difference that remains
Size affects how easily the electron is lost, so the reactions differ in vigour but not in kind.
Same valence electrons means same type of reaction; different size means different rategroups tell you what happens, position within the group tells you how fast
WORKED EXAMPLE
Explain why period 2 contains 8 elements but period 4 contains 18.
Step 1: period 2
Only the 2s and 2p subshells are being filled.
2 + 6 = 8 electrons, so 8 elementsStep 2: period 4
The 4s, 3d and 4p subshells are all filled during this row.
2 + 10 + 6 = 18 electrons, so 18 elementsThe length of a period equals the number of electrons the subshells being filled can holdthis is why the d-block only appears from period 4 onwards
💡 Exam tip
The booklet table has no labels — no group numbers, no block names. Practise counting columns so you are not caught out under time pressure.
Learn these four by heart: group 1 alkali metals, group 17 halogens, group 18 noble gases, groups 3–12 transition elements.
For groups 13–18, valence electrons = group number minus 10. Write that on your rough paper at the start.
If a question says “explain, using the periodic table”, your answer must mention valence electrons or shells, not just the position.
Say metalloid, not semi-metal, and be ready to name one: silicon is the safest example.
Never write that elements are arranged by relative atomic mass. It is atomic number.
⚠ Common mix-up
Mixing up group and period. Group is the column and controls chemistry; period is the row and controls size.
Using old group numbering. The IB uses 1–18. Group 7 in the old system is group 17 now.
Counting valence electrons for transition metals from the group number. That shortcut only works for the s- and p-blocks.
Calling hydrogen an alkali metal. It shares one valence electron with them and almost nothing else.
Thinking the f-block is a separate table. It belongs in the middle of periods 6 and 7 and is only moved for printing convenience.
Saying the noble gases have no valence electrons. They have eight (helium has two) — a full shell, which is why they are unreactive.
Up next: Electron Configuration and Periodicity — we turn the map into arithmetic, so you can go from an element’s position to its electron configuration and straight back again.
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