The periodic table and its groups
Chemistry · GCSE · The School
Why the arrangement works
Elements are ordered by atomic number, and a GROUP is a column sharing the same number of outer electrons — which is why the whole group behaves alike. A PERIOD is a row, filling up one shell. Mendeleev's achievement was leaving gaps for elements not yet found and predicting their properties, which is what turned a table into a theory.
Group 1, Group 7, Group 0
GROUP 1 alkali metals get MORE reactive down the group: the outer electron is further from the nucleus and lost more easily. GROUP 7 halogens get LESS reactive down the group, for the mirror-image reason: they need to GAIN an electron, and it is harder to attract one from further away. GROUP 0 noble gases have full outer shells and are almost inert.
Predicting
Position tells you behaviour. A more reactive halogen displaces a less reactive one from its salt — chlorine displaces bromine, but bromine cannot displace chlorine. This is what makes the table useful: you can say what an element will do before you have ever handled it.
Rubidium sits below potassium in Group 1. Without looking it up, predict: it has ONE outer electron, so it forms a 1+ ion. It is further down, so it is MORE reactive than potassium — it will react violently with water, giving hydrogen and an alkaline solution. Its melting point will be lower than potassium's. All correct, from position alone. That is what Mendeleev did with his gaps, before anyone had found the elements.
Why the trend happens: shielding and atomic radius
Going down a group, each element has one more full shell of electrons sitting between the nucleus and the outer electron. Those inner shells SHIELD the outer electron from the pull of the positive nucleus, and the outer electron also sits further away, so the nucleus's grip on it weakens. That is why Group 1 loses its outer electron more easily further down — not simply because the atom "is more reactive", but because shielding and distance make the electron easier to remove. The same weakening pull is why Group 7 finds it harder to pull a new electron in as you go down, which is the mirror-image reason for its opposite trend.
Transition metals, by contrast
They are harder, denser and less reactive than Group 1, and they do things Group 1 does not: variable oxidation states, coloured compounds, and use as catalysts. Comparing them against Group 1 is a standard question, and the answer needs the contrast rather than a list of properties.