Group I: the alkali metals
Group I elements (lithium, sodium, potassium) each have one outer-shell electron. They are soft, have low densities and react vigorously with water to give an alkali (a metal hydroxide) and hydrogen gas.
Key idea
Going DOWN Group I, reactivity increases: the outer electron is further from the nucleus and lost more easily.
2Na + 2H2O → 2NaOH + H2
Group VII: the halogens
Group VII elements (fluorine, chlorine, bromine, iodine) each have seven outer electrons and exist as diatomic molecules (Cl2, Br2, I2). Down the group the colour darkens and the physical state changes: chlorine is a green gas, bromine a red-brown liquid and iodine a grey-black solid at room temperature.
Worked example
Chlorine is bubbled into potassium bromide solution. What happens?
Chlorine is more reactive than bromine, so it displaces bromine:
Cl2 + 2KBr → 2KCl + Br2
The solution turns orange as bromine is released.
Remember
- Group I: reactivity INCREASES down the group.
- Group VII: reactivity DECREASES down the group.