2aGroup 1 (alkali metals) – lithium, sodium and potassium

Lithium, sodium and potassium sit together in Group 1 for a reason that you can watch happen: they all react with water in the same way.

The reaction

metal + water → metal hydroxide + hydrogen

So sodium gives sodium hydroxide and hydrogen. Lithium gives lithium hydroxide and hydrogen. Potassium gives potassium hydroxide and hydrogen.

Same two products every time. That is the evidence they belong together.

Why they are called the alkali metals

The hydroxide produced dissolves, giving an alkaline solution with a pH well above 7. Drop universal indicator into the water afterwards and it turns purple.

That alkaline solution is where the group's name comes from.

The balanced equation

2Na + 2H₂O → 2NaOH + H₂

Two sodium atoms are needed because hydrogen leaves as H₂ molecules, and each water molecule contributes only one hydrogen atom to the gas. Writing H rather than H₂ as the product leaves the equation unbalanced.

What you actually see

With sodium on water:

• It floats — the metal is less dense than water.
• It melts into a ball — the reaction releases enough heat to melt it.
• It fizzes and moves about as hydrogen is produced.
• It gradually disappears as it is used up.

Those are observations. "It is reactive" is a conclusion drawn from them, and a question asking what you see wants the list, not the conclusion.

Answering the family question

If a question asks how these reactions show the metals belong to one family, answer from the reactions, not from electronic structure. Same products, same kind of behaviour, alkaline solution each time.

Electronic configuration explains why they behave alike — but it is not the observed evidence the question asked for.

All three metals react the same way. What changes is how vigorously.

Reactivity increases down the group

The distinguishing observation for potassium is that the gas catches fire. That is the one to name if a question asks how potassium differs from sodium.

Group 1 and Group 7 run opposite ways

Reactivity increases down Group 1. It decreases down Group 7. Mixing these up is the most common error across both topics, so fix the Group 1 direction firmly: further down means more violent.

Reaction with air

Freshly cut sodium is shiny and dulls within seconds, because it reacts with oxygen in the air.

In ordinary air it also reacts with water vapour and then carbon dioxide, so the dull layer is a mixture rather than one clean compound. That is precisely why these metals are stored under oil — the oil keeps oxygen and moisture away from the surface.

Predicting for metals you have never seen

Rubidium sits below potassium; caesium below that; francium below that again. None is used in a school laboratory, and you can still say what each would do.

For rubidium and water:

• More vigorous than potassium, following the trend.
• Same products — rubidium hydroxide and hydrogen.
• Alkaline solution, pH well above 7.

A prediction must follow the trend and keep the products the same. Going down a group changes how fiercely a reaction happens, never what it produces.

This is what makes the Periodic Table useful rather than merely tidy: it lets you state the chemistry of an element nobody has safely tested.

Separate Chemistry only.

Every Group 1 metal loses exactly one electron

They all have one outer-shell electron, and reacting means losing it to form a 1+ ion.

That is worth stating plainly, because a common wrong answer is that potassium is more reactive because it "has more electrons to lose". It does not. Lithium loses one, potassium loses one. Both form 1+ ions.

The difference is not how many are lost, but how easily.

Why it gets easier down the group

Two things change as you go down, and both point the same way:

1. The outer electron is further from the nucleus. Each element down the group has one more occupied shell, so the outer electron sits further out. Attraction weakens with distance.

2. There are more inner shells shielding it. Those inner electrons sit between the nucleus and the outer electron, reducing the pull it feels.

So the outer electron is held less tightly, is lost more easily, and the metal is more reactive.

Writing the full chain

A complete answer runs all the way to the end:

More shells → outer electron further out and better shielded → attraction to the nucleus is weaker → the electron is lost more easily → the metal is more reactive.

Stopping at "the atom is bigger" leaves out why size matters. The marks are in the links.

The same reasoning, opposite result

Group 7 atoms gain an electron rather than losing one. The same weakening attraction that makes losing easier makes gaining harder — which is exactly why the two groups' trends run in opposite directions.

One underlying cause, two opposite effects, because the two groups need opposite things.