1.4 The Periodic Table of the elements

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Taught 29 Sep – 6 Oct · Booklet pp. 27–31 · Homework week 5

By the end of this, you should be able to:

Appreciate how the periodic table organises the elements according to their atomic structure and that it can be used to predict their physical and chemical properties
Know the meaning of the terms 'group' and 'period' and understand that elements within a group have similar physical and chemical properties
Suggest key physical and chemical properties of an element given information about its position in
Describe the key differences between metallic and non-metallic elements and know that metallic elements are found toward the left and non-metallic elements toward the right of
Understand that the inert nature of the noble gases (group 8) is associated with their full outer shell

This chapter also builds on 1.2 Atomic structure and classification of matter — those notes are not repeated here.

How the table is organised

What you need to understand

The Periodic Table lists every element in order of proton number (atomic number). That order is not just tidy — it groups elements so that ones with similar properties line up.

Metals sit to the left and centre; non-metals to the right. The noble gases (He, Ne, Ar, Kr, Xe, Rn) fill Group 18 — they have complete outer shells, so they barely react. (Older books number this group as Group 0.)

Metals vs non-metals — typical physical properties

Why position predicts behaviour

Why it works

The whole power of the table is this: an element's place tells you how it behaves, because position reflects electron arrangement.

Same group → similar reactions. Chemical reactions are about outer-shell electrons. Every Group 1 atom has one outer electron, every Group 2 atom two, every Group 17 atom seven. Because reactions depend on those outer electrons, elements in a group react in the same way — which is why we can learn a group's behaviour once and apply it to all its members (1.6).

Group number → outer electrons. For the main groups, the group number gives the outer-electron count (Group 1 → 1; Group 17 → 7; Group 18 → a full 8, except helium's 2). Period number → number of shells. So sodium (period 3, group 1) has three shells with one electron in the outermost: 2, 8, 1.

Why the noble gases are inert. A full outer shell is already stable, so a noble-gas atom has no tendency to lose or gain electrons — and reacting means losing or gaining electrons. A complete shell is the "goal" that drives every other element's reactions.

Place chlorine (Cl, Z = 17). Its electron arrangement is 2, 8, 7 — three shells, so it is in period 3; seven outer electrons, so it is in group 17. Seven outer electrons (one short of a full shell) tells you it is a reactive non-metal that tends to gain one electron.

Going deeper

When Dmitri Mendeleev built his table in 1869 he left gaps for elements not yet discovered — and predicted their properties from the pattern. Gallium and germanium were later found almost exactly as he described: striking evidence that a good model predicts, not just describes. Today the table's true order — proton number — only made sense once the nucleus (1.3) was understood, so this chapter quietly depends on the one before it. Next year, the link between group position and electron arrangement becomes the basis of bonding.

Diagrams and worked examples are in the Knowledge Book PDF below — they don't reproduce well on a page.

Now test yourself

Say it out loud before you tap.

Words you need

group define it

a vertical column; same number of outer-shell electrons → similar properties

E.G.Li, Na and K are all Group 1 — one outer electron each

NOTa horizontal row — that is a period

period define it

a horizontal row; the period number equals the number of occupied shells

E.G.Na, Mg, Al … are all Period 3 — three occupied shells

NOTa vertical column — that is a group

metal / non-metal define it

metals (left and centre) conduct and are malleable; non-metals (right) do not and are brittle

E.G.metals Fe, Cu, Na — conduct, malleable, shiny · non-metals S, O, Cl — brittle, poor conductors

NOTa sharp dividing line — silicon sits between the two as a metalloid

noble gas define it

a Group 18 element with a complete outer shell, so very unreactive

E.G.He, Ne, Ar — balloons, lighting, welding shields

NOThydrogen — also a light gas, but very reactive and not in Group 18

proton number (atomic number) define it

the number of protons; sets each element's place in the table

E.G.the table runs 1 H, 2 He, 3 Li … in proton order

NOTordered by mass — argon (40) still comes before potassium (39)

Easy to get wrong

TRAP Elements in a group have the same number of shells. true or false?

TRUTH No — that is a period. A group shares the same number of outer-shell electrons, but each element down a group has one more shell.

TRAP The table is ordered by atomic mass. true or false?

TRUTH It is ordered by proton number. Mass mostly rises too, but a few pairs (e.g. Ar and K) only sit correctly when ordered by protons.

TRAP Noble gases are unreactive because they have no electrons. true or false?

TRUTH They are unreactive because their outer shell is full — the most stable arrangement, with no drive to gain or lose electrons.

TRAP There is a sharp line between metals and non-metals. true or false?

TRUTH The change is gradual; a "staircase" of metalloids (e.g. silicon) shows in-between behaviour.

Check yourself

A group shares the same number of: A. protons B. neutrons C. shells D. outer electrons answer

Try this

RSC History of the periodic table opens in a new tab RSC Periodic table opens in a new tab

Test yourself

Periodic table history

Table ladder early

Table ladder late

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