Group 1: the alkali metalsIB MYP Chemistry: Revision notes
Section 1
Where are the alkali metals?
Group 1 is the first column of the periodic table. Its elements are lithium (Li), sodium (Na) and potassium (K), together with rubidium, caesium and francium further down. They are called the alkali metals because they react with water to form alkaline solutions.
All Group 1 atoms have one electron in their outer shell. This one fact explains almost all of their behaviour, and they all form ions with a charge of 1+.
Section 2
Physical properties
Compared with most metals, the alkali metals are unusual:
- They are soft and can be cut with a knife. The inside is shiny but quickly turns dull in air.
- They have a low density. Lithium, sodium and potassium float on water.
- They have low melting points for metals (sodium melts at about 98 °C).
Going down the group the metals get softer and their melting points fall.
Remember: lithium, sodium and potassium are less dense than water, which is why they float.
Section 3
Reactivity increases down the group
Alkali metals react by losing their one outer electron to form a 1+ ion. The easier this is, the more reactive the metal.
Going down the group:
- each atom has one more electron shell, so the outer electron is further from the nucleus
- there is more shielding from inner shells
- the attraction between the nucleus and the outer electron is weaker
- so the outer electron is lost more easily and reactivity increases
So potassium is more reactive than sodium, and sodium is more reactive than lithium.
Do not explain reactivity by saying the atom 'wants' to be full. Use distance from the nucleus, shielding and weaker attraction.
Section 4
Reactions with water
Lithium, sodium and potassium all react with cold water to give a metal hydroxide and hydrogen gas:
metal + water → metal hydroxide + hydrogen
2Na + 2H₂O → 2NaOH + H₂
The reaction gets more vigorous down the group:
- Lithium floats and fizzes steadily.
- Sodium melts into a ball because the reaction gives out heat, and fizzes rapidly.
- Potassium melts, moves very quickly and burns with a lilac flame.
If universal indicator is added the solution turns purple, which shows that an alkali (the hydroxide) has formed.
Section 5
Reactions with oxygen and storage
Alkali metals react with oxygen in the air to form white solid metal oxides. The shiny surface of a freshly cut piece goes dull within seconds:
4Na + O₂ → 2Na₂O
They also react with water vapour in the air. To stop this, the metals are stored under oil, which keeps out oxygen and water.
Storage under oil is a common exam question. The answer is: it stops the metal reacting with oxygen and water vapour in the air.
Section 6
Writing balanced equations: worked example
Write the equation for potassium and water.
- Word equation: potassium + water → potassium hydroxide + hydrogen
- Formulae: K + H₂O → KOH + H₂
- Count hydrogen: 2 on the left but 3 on the right. Hydrogen gas is H₂, so double everything else: 2K + 2H₂O → 2KOH + H₂
- Check: K 2 = 2, H 4 = 2 + 2, O 2 = 2. Balanced.
Lithium follows the same pattern: 2Li + 2H₂O → 2LiOH + H₂.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Group 1: the alkali metals
- A school technician in Singapore is preparing a demonstration for a Year 9 class. A small piece of sodium is taken from a bottle of oil, cut with a knife, dried and dropped into a large trough of water containing a few drops of universal indicator. The class will watch from behind a safety screen.Write the word equation for the reaction of sodium with water.2 marks
- A teacher drops equal-sized small pieces of lithium, sodium and potassium into three separate troughs of water. Lithium floats and fizzes steadily until it disappears. Sodium floats, melts into a silvery ball and fizzes rapidly, darting across the surface. Potassium floats, melts, darts about very quickly and burns with a lilac flame.Explain, in terms of electrons, why potassium is more reactive than sodium.2 marks
- A student wants to compare the reactivity of lithium, sodium and potassium. She plans to add a small piece of each metal, one at a time, to a large beaker of water at room temperature and to time how long each piece takes to stop fizzing. Her teacher will cut and dry each piece for her.State a testable hypothesis for her investigation, with a scientific reason, and identify the independent variable.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).