The Gaps He Left

The motion: “The periodic table is a way of organising the elements we know, not a source of new knowledge.”

Your opponent: Dmitri Mendeleev · Course: Pearson Edexcel International GCSE Chemistry (4CH1) · Assessment objectives: AO1, AO2

Mendeleev left holes in his table and said what would fill them. He will ask what your version of the table has ever predicted.

What you are asked to argue

Defeat the motion by showing that periodicity has explanatory and predictive force — and be ready to say why it works.

The arrangement of the modern table: groups, periods, atomic number
Group 1 and Group 7 trends and their explanations
Electronic configuration and why it produces the group behaviour
The difference between Mendeleev's ordering by atomic weight and the modern ordering by atomic number

Disagreeing with the figure is not an error. Any of these can score full marks if argued well:

That the table's predictive power comes from electronic structure, which Mendeleev did not have, so the motion is true of HIS table and false of the modern one
That prediction and organisation are not opposites — a good classification predicts because it captures a real cause

Wrong: Group 1 metals get more reactive down the group because they get bigger.
The correction: Size is part of it. The outer electron is further from the nucleus AND shielded by more inner shells, so it is held less strongly and lost more easily. Both halves are needed for the marks.
Wrong: Group 7 gets more reactive down the group for the same reason as Group 1.
The correction: The trend runs the OTHER way: halogens become less reactive down the group, because they gain an electron, and a larger, more shielded atom attracts an incoming electron less strongly.
Wrong: The modern table is ordered by atomic weight.
The correction: It is ordered by atomic number. Mendeleev used weight and had to break his own order in places to keep chemically similar elements together — which was a remarkable act of judgement.

At least one group trend explained by mechanism rather than stated, plus a prediction reasoned from position.

Read the life before you argue with it — the case you are about to meet was built by a real person, over a real career:

The life of Dmitri Mendeleev — Author of the Periodic Table · Chemist · 1834–1907