Why do elements in the same group behave the same way chemically?

The alkali metals

Group 1 holds lithium, sodium, potassium and more below them. All have one valence electron, all are soft enough to cut with a knife, and all are shiny until the air dulls them.

When such a metal meets water it gives up its outer electron, and an alkali and hydrogen are produced. With lithium the reaction is brisk, with sodium it is vigorous, and with potassium it ignites the very hydrogen it releases.

That direction is the point: the further down the group, the fiercer the reaction. The same reaction exactly, at rising intensity.

Why intensity runs in opposite directions

For an alkali metal the reaction is giving an electron away. Going down the group, shells are added, so the outer electron is further from the nucleus and shielded from it by the inner shells. It is held less tightly, so it is easier to give away — and reactivity rises.

For the halogens the reaction is the reverse: they take an electron in. Going down the group the vacancy is further from the nucleus, so the pull on an incoming electron is weaker and capture is harder — and reactivity falls.

So fluorine is the most reactive halogen and caesium among the most reactive metals, at the two ends of the same row. The same physical explanation, two opposite conclusions, because the task is opposite.

The group tells you how many electrons move

An element in group 1 gives away one electron and becomes an ion with a charge of 1+. Group 2 gives away two. An element in group 17 — 7 in the older numbering — takes one electron and becomes 1−, and group 16 takes two.

Behind all of these is one rule: the atom moves to the nearest full outer shell. Whoever has one too many sheds it, whoever is one short completes it.

The noble gases do neither. Their outer shell is already full, so they barely react at all.

Worked examples

1. What is produced when sodium reacts with water?

   1. Sodium gives away its single valence electron
   2. Sodium hydroxide forms, dissolved in the water
   3. Hydrogen is released as a gas

   Answer: An alkali and hydrogen gas. The solution left behind is alkaline

2. Potassium reacts with water more violently than sodium. Why?

   1. Potassium has one more shell than sodium
   2. Its valence electron is further out and more shielded
   3. So it is easier to give away

   Answer: Because giving the electron away gets easier down the group

3. What charge does an ion of a group 16 element carry — 6 in the older numbering?

   1. Six valence electrons
   2. Two short of a full eight
   3. The atom takes in two electrons

   Answer: A charge of 2−, as in the oxide ion

Common mistakes

Saying reactivity always rises going down

True for the metals and false for the halogens. The question is whether the element gives an electron away or takes one, so the same descent helps one and hinders the other.

Expecting an alkali metal to release oxygen from water

The metal gives an electron away, and the gas released is hydrogen. What stays in solution is a hydroxide, which is why it is alkaline.

Thinking a noble gas is unreactive because it is heavy or rare

The only reason is a full outer shell. Helium is very light and unreactive for exactly the same reason argon is.

More in The Periodic Table

• What did Mendeleev actually notice when he arranged the elements?
• What does the atomic number count, and why does it fix an element's identity?
• Why does the periodic table have exactly the rows it has?
• What changes as you move across a single row of the periodic table?
• How do you predict a compound's formula from two positions in the table?