1dThe Periodic Table

The Periodic Table is not a list of elements in alphabetical order or by weight. It is arranged so that elements which behave alike sit together, and that arrangement comes from one number.

Ordered by atomic number

Elements run in order of atomic number — the number of protons.

Early versions of the table used relative atomic mass instead, and it very nearly works. But a few pairs come out in the wrong place. Argon has a greater relative atomic mass (40) than potassium (39), yet argon must come first.

Ordering by atomic number puts argon at 18 and potassium at 19, which places argon with the unreactive noble gases and potassium with the violently reactive alkali metals — exactly where their behaviour says they belong. Ordering by mass would have swapped them and put each in a group whose properties it does not share.

That is the whole argument for atomic number: it groups elements by how they actually behave.

Groups and periods

Groups run down, periods run across. Swapping the two is a frequent and easily avoided error.

Metals and non-metals

Metals occupy the left-hand side and the centre. Non-metals sit towards the upper right. A diagonal boundary runs between them.

Most elements are metals, so an unfamiliar element on the left of the table is almost certainly one. The central block — the transition metals — is entirely metallic.

Electrons occupy shells around the nucleus, and they fill from the inside out.

Shell capacities

For the first twenty elements that is all you need. Sodium, atomic number 11, is 2,8,1: two in the first shell, eight in the second, one left over in the third.

Calcium is 2,8,8,2, not 2,8,10. On this specification the third shell takes eight and then the fourth begins, even though the third can eventually hold more. Writing 2,8,10 is a common slip.

A quick check on any configuration: the digits must add up to the atomic number, and no shell may exceed its maximum.

Reading the table from the configuration

For a main group element, the configuration tells you exactly where the element sits:

• Group number = number of outer-shell electrons
• Period number = number of occupied shells

Take chlorine, 2,8,7. Seven outer electrons puts it in Group 7. Three occupied shells puts it in period 3.

The two numbers come from different features of the same configuration — the last digit for the group, the count of digits for the period. Reading the wrong one is the usual error.

Why this matters

This is not bookkeeping. The outer-shell electrons are the ones involved in reactions, so knowing the group tells you how an element will behave — which is the point of the whole table.

Same group, same chemistry

Lithium, sodium and potassium all react with water in the same way. Fluorine, chlorine and bromine all form 1− ions.

The reason is simple: elements in the same group have the same number of outer-shell electrons, and chemical reactions involve losing, gaining or sharing those outer electrons. Same outer arrangement, same behaviour.

Be precise in the wording. It is not "the same number of electrons" — sodium has 11 and potassium has 19. It is the same number in the outer shell, which is one for both.

Chemical alike, physical different

Group members share chemical properties. Their physical properties change steadily down the group — melting point, density, and how vigorously they react.

That is why a question about similar chemistry and a question about a trend can both be about the same group and want opposite answers.

The noble gases

Group 0 elements are extremely unreactive, and the reason is that they already have a full outer shell.

Atoms react in order to reach a full outer shell. The noble gases start there, so they have nothing to gain by losing, gaining or sharing anything.

One detail worth getting right: helium's full shell holds only two electrons, not eight. The rule of eight applies from neon onwards. An answer claiming all noble gases have eight outer electrons is wrong for the first one.

What their uses tell you

Argon fills filament light bulbs so the filament does not burn. Helium fills balloons because it will not catch fire. Both uses depend on the gas doing nothing at all — which is the clearest evidence of an unreactive element you can get.

There are two reliable tests, and questions often give you both and expect you to use each.

Test 1: electrical conductivity

Metals conduct electricity. Most non-metals do not.

The word most is doing real work there. Graphite is a non-metal that conducts, which is why questions phrase this carefully. If a question offers graphite as an option, conductivity alone will not settle it.

Test 2: the acid-base character of the oxide

This is the test students forget, and it is often the more decisive one.

This is the rule 4CH1 examines, and it is the one to use. It is a generalisation: a few oxides are amphoteric (aluminium oxide reacts with both acids and alkalis) and some non-metal oxides are neutral. You will meet those later; they do not change the answer expected here.

So an element whose oxide gives a solution of pH 12 is a metal. One whose oxide gives pH 3 is a non-metal.

Worked reasoning

Sulfur burns to form sulfur dioxide, which dissolves to give an acidic solution. Sulfur is therefore a non-metal — and that acidic solution is also part of why sulfur dioxide contributes to acid rain.

Magnesium burns to form magnesium oxide, which gives an alkaline solution. Magnesium is a metal.

Using both together

When a question gives conductivity and oxide data, use both. Each is worth a mark, and an answer resting on one when two were provided has left marks on the table.

The pattern to write is: state the classification, then give the evidence. "P is a metal, because it conducts electricity and its oxide is basic."