IB Chemistry SLTopic 3 — Classifying the ElementsPaper 1 & 2Core idea~9 min read
Structure of the Periodic Table
Think of the periodic table as an address book. Every element has an address — a row and a column — and that address tells you how many shells it has, how many outer electrons it has, and roughly how it will behave. Learn to read the address and you stop needing to memorise the elements.
📚 What you need to know
Elements are listed in order of increasing atomic number (proton number), from 1 to 118.
Rows are periods. The period number tells you how many electron shells are occupied.
Columns are groups, numbered 1 to 18. For the s and p blocks, the group tells you the number of valence electrons.
Elements in the same group have similar chemical properties, because they have the same outer arrangement.
The table divides into metals (left and centre), non-metals (upper right) and metalloids (the staircase between them).
It also divides into the s, p, d and f blocks, named after the sub-shell the outer electrons occupy.
Hydrogen and helium are the two special cases you need to be able to explain.
Ordered by protons, not by mass
The elements are arranged by atomic number — the number of protons. That matters, because the number of protons is what fixes the number of electrons in a neutral atom, and the electrons are what do the chemistry. Ordering by mass would put a few pairs in the wrong place; ordering by protons never does.
Arranged this way, two useful things fall out on their own:
Atoms with the same number of shells end up side by side in a row.
Atoms with the same outer-shell arrangement end up stacked in a column.
Periods and groups
A period is a row. The period number is the number of occupied electron shells — so every element in Period 2, from lithium to neon, has exactly 2 shells. A group is a column, numbered 1 to 18 in the IUPAC system. For the s-block and p-block, the group number tells you how many valence (outer) electrons the atom has.
Sodium’s address, read straight off its electron arrangement: three shells means Period 3, one outer electron means Group 1.
Group and period aren’t two facts to memorise separately — they’re two readings of the same electron arrangement. Period counts the shells; group counts what’s in the outermost one.
Because chemistry happens with the outer electrons, elements in the same group react in similar ways. That is why all the Group 1 metals fizz in water and all the Group 17 elements form salts — you are watching the same outer arrangement do the same thing, over and over.
💡 Group names worth knowing
Group 1 — the alkali metals
Group 17 — the halogens
Group 18 — the noble gases
Groups 3–12 — the transition elements
Metals, metalloids and non-metals
Draw a staircase down the right-hand side of the p-block and you have split the table into three. It is a rough split, not a hard border, but it predicts behaviour well.
Metals give electrons away, non-metals take them, and the metalloids on the staircase sit awkwardly between the two.
Where
Typical properties
In reactions
Metals
Left and centre (s, d, f blocks and part of p)
Good conductors of heat and electricity, malleable, ductile, shiny when cut
Lose electrons to form positive ions; oxides are basic
Metalloids
Along the staircase — B, Si, Ge, As, Sb, Te
A mixture of both; often semiconductors
Behave as either, depending on what they meet
Non-metals
Upper right of the p-block
Poor conductors, brittle as solids, lower melting points
Gain or share electrons; oxides are acidic
The four blocks
The blocks are named after the sub-shell that the outermost electrons occupy. The width of each block is simply how many electrons that sub-shell can hold.
Block
Groups
Outer electrons in…
Columns
s
1 and 2 (plus helium)
an s orbital
2
p
13 to 18
p orbitals
6
d
3 to 12
d orbitals
10
f
the strip below (lanthanides and actinides)
f orbitals
14
Why this is useful: knowing the block lets you predict reactivity, likely oxidation states and typical bonding behaviour before you know anything else about an element.
The two special cases
Hydrogen is usually drawn floating on its own. It has one outer electron like Group 1, but it is a non-metal gas and does not behave like the alkali metals, so no single group fits it properly.
Helium has only 2 electrons, so by outer arrangement it belongs with Group 2. It is placed in Group 18 instead, because its outer shell is full and it is completely unreactive — which is what the noble gases are about. Position follows behaviour.
WORKED EXAMPLE
An element sits in Period 4, Group 2. State the number of occupied shells and valence electrons, and predict whether it is a metal.
Period 4 → shells4 occupied shellsGroup 2 → valence electrons2 valence electronsGroup 2 is on the far left → s-blocka metal — calciumOnly 2 outer electrons to lose, and they’re far from the nucleus, so it loses them easily.
WORKED EXAMPLE
Explain why sodium and potassium have similar chemical properties, but sodium and magnesium do not.
Na and K are in the same GROUPBoth have 1 valence electron, so both react by losing that one electron — same chemistry.Na and Mg are in the same PERIOD, not the same groupNa has 1 outer electron, Mg has 2, so they form different ions and behave differently.group = similar chemistry, not period
💡 Exam tip
The data booklet table (Section 7) is not labelled — no group names, no blocks, no element types. Practise finding them yourself.
Say “Group 17“, not “Group 7”. IB uses the IUPAC 1–18 numbering.
If asked why two elements are alike, the answer is almost always about the number of valence electrons.
⚠️ Common mix-up
Period ≠ group. Period counts shells (rows), group counts valence electrons (columns).
Group number gives valence electrons only for the s and p blocks. It does not work for the transition metals.
Metalloids are not a separate row or block — they are a diagonal staircase inside the p-block.
Helium is s-block but sits in Group 18. Its position reflects its behaviour, not its block.
The table is ordered by atomic number, not relative atomic mass.
Up next: Electron Configuration and Periodicity — turning an address into a full electron configuration, and back again.
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