The alkali metals are soft and melt at low temperatures
Definition
Alkali metal
An element in Group 1 of the periodic table, which has one electron in its outer shell.
- Lithium, sodium and potassium are soft enough to be cut with a knife.
- A freshly cut surface is shiny, then dulls within seconds as it reacts with air.
- Their melting points are low for metals, and they fall down the group.
- Sodium melts at about 98 ∘C98\ ^{\circ}\text{C}98 ∘C, which is below the boiling point of water.
- They are stored under oil to keep air and moisture away from the metal.
Key Idea
Softness and low melting points set these metals apart from typical metals such as iron.
Lithium, sodium and potassium all react with water
- Each of the three gives an alkaline solution and a gas: metal+water→metal hydroxide+hydrogen\text{metal} + \text{water} \rightarrow \text{metal hydroxide} + \text{hydrogen}metal+water→metal hydroxide+hydrogen
- Lithium fizzes steadily, floats, and slowly disappears.
- Sodium melts into a ball that darts across the surface, fizzing rapidly.
- Potassium reacts so vigorously that the hydrogen ignites with a lilac flame.
- The solution left behind turns universal indicator purple, because a hydroxide has formed.
Example
- Sodium and water: 2Na+2H2O→2NaOH+H22\text{Na} + 2\text{H}_2\text{O} \rightarrow 2\text{NaOH} + \text{H}_22Na+2H2O→2NaOH+H2.
- The gas test: a lit splint gives a squeaky pop, confirming hydrogen.
Reactivity increases down the group
- The three reactions differ in vigour, not in the products they give.
- Lithium is the least vigorous of the three and potassium the most.
- The order lithium, sodium, potassium is therefore an order of increasing reactivity.
- The same order appears in how quickly each metal tarnishes in air.
- A pattern that holds for three members of a group can be extended to the rest of it.
Common Mistake
Rubidium and caesium react explosively with water, so neither is used in a school demonstration.
Predicting the reactivity of rubidium and caesium
- Rubidium lies below potassium, so it reacts with water more vigorously than potassium does.
- Caesium lies below rubidium, so it is more vigorous still.
- The products follow the same pattern, so rubidium gives: 2Rb+2H2O→2RbOH+H22\text{Rb} + 2\text{H}_2\text{O} \rightarrow 2\text{RbOH} + \text{H}_22Rb+2H2O→2RbOH+H2
- Their melting points continue to fall, so caesium melts a little above room temperature.
- A prediction of this kind is made by extending the trend, not by recalling a result.
Note
A prediction is worth stating together with the trend it comes from.
Electronic configuration explains the pattern
Definition
Electronic configuration
The arrangement of an atom's electrons in its shells, written as the number in each shell from the innermost outwards.
- Every alkali metal atom has one electron in its outer shell, and reacting means losing it.
- Down the group each atom has more shells, so the outer electron sits further from the nucleus.
- The inner shells also shield the outer electron from the nucleus's positive charge.
- Distance and shielding together mean the outer electron is held less strongly.
- An electron that is lost more easily makes the metal more reactive, which is the pattern observed.
Exam technique
- An explanation of the trend needs three parts: more shells, greater distance, and more shielding.
- The conclusion is that the outer electron is lost more easily, which is what reactivity means here.
- Naming the products is not an explanation, because all three metals give the same products.
Self review
- Name two physical properties that make the alkali metals unusual for metals.
- Write the word equation for an alkali metal reacting with water.
- How does the reaction of potassium differ from that of lithium?
- Predict what happens when rubidium is added to water.
- Why does reactivity increase down Group 1?
