6.1, 6.3, 6.5Group 1
The elements in Group 1 of the periodic table are known as the alkali metals, and have characteristic properties because of the single electron in their outer shell.
In Group 1, the reactivity of the elements increases going down the group. Losing that single outer electron is what these reactions consist of, and the further down the group you go, the further that outer electron is from the nucleus and the more shells shield it — so it is lost more easily, and the element is more reactive.
You should be able to describe the reactions of the first three alkali metals with oxygen, chlorine and water. Two general equations cover most of them:
• metal + water → metal hydroxide + hydrogen, for example 2Na + 2H₂O → 2NaOH + H₂ • metal + chlorine → metal chloride, for example 2Na + Cl₂ → 2NaCl
You should also be able to explain how properties of the elements in Group 1 depend on the outer shell of electrons of the atoms, and predict properties from given trends down the group.
Common mistake
Group 1 reactivity increases down the group, Group 7 reactivity decreases. Both follow from the same cause — the outer shell gets further from the nucleus — but the direction is opposite because Group 1 loses an electron and Group 7 gains one. Learn the reason, not the two directions separately.
- Alkali metals
- the elements in Group 1, which have a single electron in their outer shell; reactivity increases down the group because the outer electron is further from the nucleus and more shielded, so it is lost more easily
6.2, 6.4Predicting properties down Group 1
Two physical properties of the alkali metals have to be recalled outright: they are soft — soft enough to cut with a knife — and they have relatively low melting points for metals.
Both trends continue down the group, and so does the reactivity. That is what lets you predict the properties of the members you never see in a laboratory. Rubidium and caesium sit below potassium, so they are softer still, melt at still lower temperatures, and react with water more violently.
Predicting from a trend is a skill in its own right, and it is marked as one. Say which way the property changes down the group, then apply it to the element you were asked about, and give a number that fits if you were given data. If potassium melts at 63 °C and sodium at 98 °C, a sensible prediction for rubidium is somewhere below 63 °C — around 40 °C — and the reasoning is what earns the mark, not the exact figure.
Common mistake
"It is more reactive" on its own predicts nothing. Name the trend, then apply it: reactivity increases down the group, so caesium reacts more violently than potassium does.
Before you practise
- Group 1 metals have one electron in the outer shell, which is what puts them in the same group.
- They are soft, and have low melting points and low densities for metals.
- With water they give a metal hydroxide and hydrogen — the solution turns universal indicator purple.
- Lithium fizzes, sodium melts into a ball, potassium ignites with a lilac flame: reactivity increases down the group.
- The reason is the outer electron: further from the nucleus and shielded by more shells, so it is lost more easily.
- Predict a property by naming the trend and then applying it to the element asked about.