From Newlands to the modern table

Chemists tried to order the elements long before atoms were understood:

• Newlands arranged the known elements in order of atomic mass ("law of octaves"). Every eighth element seemed similar — but he left no gaps, forced everything into rows, and ended up grouping elements that did not belong together.
• Mendeleev (1869) also ordered by atomic mass, but crucially he left gaps for undiscovered elements and even swapped a few pairs out of strict mass order so that elements with similar properties lined up. He used the gaps to predict the properties of elements not yet found (e.g. "eka-silicon", later germanium).
• The modern table is arranged in order of atomic number (proton number), not mass. This fixed the few pairs Mendeleev had to reverse — because isotopes mean atomic mass doesn't always rise in step with atomic number.

Why Mendeleev's table worked

Groups, periods, metals & non-metals

The modern table is arranged by atomic number. Its layout reflects electronic structure:

• A period is a row. Across a period, atoms are filling the same outer shell — the period number tells you which shell.
• A group is a column. Elements in the same group have the same number of electrons in their outer shell — which is why they have similar chemical reactions. The group number = number of outer-shell electrons.
• Metals (the left and middle) lose electrons to form positive ions; non-metals (the top-right) gain or share electrons. A "staircase" separates them.

What a group tells you

The alkali metals

Lithium, sodium and potassium are soft, low-density metals with one electron in their outer shell. They are very reactive and are stored under oil.

• They react vigorously with water → a metal hydroxide (an alkali) + hydrogen gas. The metal fizzes and floats; the solution turns universal indicator blue/purple.
• They react with oxygen to form metal oxides, and with Group 7 elements to form white ionic salts (e.g. sodium + chlorine → sodium chloride).

The trend: reactivity increases DOWN Group 1 (Li → Na → K → …). Going down, the outer electron is in a higher shell, further from the nucleus and shielded by inner shells, so it is lost more easily — and losing that electron is what makes these metals react.

Reactivity increases down Group 1

Predict the Group 1 trend

The halogens

The halogens are reactive non-metals with seven electrons in their outer shell. They exist as diatomic molecules (Cl₂, Br₂, I₂) and their appearance changes down the group:

The trend: reactivity DECREASES down Group 7. To react, a halogen atom gains one electron. Lower down, the outer shell is further from the nucleus and more shielded, so the incoming electron is attracted less strongly — making the lower halogens less reactive. (This is the opposite direction to Group 1.)

Displacement reactions

Because reactivity falls down Group 7, a more reactive halogen displaces a less reactive one from a solution of its salt:

Key idea: the displacement happens because chlorine is more reactive than bromine. A less reactive halogen cannot displace a more reactive one — e.g. iodine will not displace chlorine from KCl.

Will it react?

Halogen colour & state

Tap a halogen on the left, then tap its matching appearance on the right.

The noble gases

Helium, neon, argon and the rest are in Group 0. Eduqas says they are completely unreactive — and the reason is their electronic structure:

• They have a full outer electron shell (helium has 2; the others have 8). A full shell is very stable, so these atoms have no tendency to gain, lose or share electrons — so they do not react.
• They exist as single atoms (monatomic), not molecules.
• Trends in physical properties: going down Group 0, boiling point and density increase (helium is the lightest, lowest boiling point).

Why are they unreactive?

The transition metals

The transition metals sit in the central block of the table (e.g. titanium, vanadium, iron, copper). Eduqas wants their general properties:

• High melting points and high density; hard and strong.
• They form coloured compounds/ions (e.g. copper(II) blue, iron(II) pale green, iron(III) orange-brown).
• They are useful as catalysts (e.g. iron in the Haber process).
• They form ions with different charges — i.e. variable oxidation states (e.g. Fe²⁺ and Fe³⁺).

Transition metals vs Group 1: compared with the alkali metals, transition metals are much harder, stronger and denser, have higher melting points, and are far less reactive. They also form coloured compounds, several oxidation states and act as catalysts — none of which Group 1 metals do (Group 1 form white compounds, one charge only, and react fiercely).

Transition metal or Group 1?

Which way does reactivity go?

Tap a statement, then tap the group whose trend it describes as you go DOWN the group.

The facts to lock in

You've now covered the whole of Eduqas Topic 4 — The Periodic Table and properties of elements. Press Finish to see your score.