Lesson focus: Group 1 metals share a reaction pattern, while their reactivityHow easily a substance takes part in chemical reactions. changes predictably down the group.
Describe Group 1 reactions with water and use atomic structure to explain the increase in reactivity down the group.
Build on earlier learning
Groups and periodsThe previous lesson linked groups to shared outer-electron patterns. Group 1 provides the first detailed test of that relationship and of a trend down a group.
Practical
Group 1 metals in water
- Watch a teacher demonstration of lithium, sodium and potassium on water.
- Compare how fast each moves, fizzes and whether it ignites.
- Test the water afterwards with universal indicator.
Record: Observations for each metal ranked by reactivity, and the pH of the solution.
Safety: Demonstration only, behind a safety screen.
You will be successful when you can…
- Recall the outer-electron pattern of Group 1 atoms.
- Describe the products and observations when Group 1 metals react with water.
- Describe trends in reactivity and melting point down the group.
- Explain increasing reactivity using distance, shielding and nuclear attraction.
Words that carry the science
- alkali metal
- An element in Group 1 of the periodic table.
- reactivity
- How readily a substance undergoes a chemical reaction.
- alkaline
- Having a pH greater than 7 because hydroxide ions are present in solution.
- shielding
- The reduction in attraction between the nucleus and an outer electron caused by inner electron shells.
- trend
- A general direction of change across a set of ordered data.
- extrapolation
- A prediction beyond the measured data based on an established pattern.
Use shared reactions and changing reactivity to understand Group 1
01One outer electron produces a family pattern
Every Group 1 atom has one electron in its outer shell. In reactions, that electron is lost to form a positive ion with a charge of +1. This common change gives the elements similar chemical reactions.
Group 1 metals are soft compared with many other metals and must be stored so that air and moisture cannot reach them. Their hazardous reactions belong in controlled teacher demonstrations or simulations, not unsupervised practical work.
02Water reveals a clear reactivity trend
Lithium, sodium and potassium all react with cold water to form a metal hydroxideA compound made of a metal joined with hydroxide (oxygen and hydrogen), formed when some metals react with water. and hydrogenThe lightest gas; it burns with a squeaky pop when lit.: metal + water → metal hydroxide + hydrogen. The solution becomes alkalineHaving a pH above 7, like soap or toothpaste. because hydroxide ions are produced.
Lithium fizzes steadily. Sodium reacts faster and often melts into a moving ball because heat is released. Potassium reacts still more vigorously and the hydrogen may ignite, often with a lilac flame. These are comparative observations, not instructions for carrying out the reaction.
03Outer electrons are lost more easily down the group
Down Group 1, atoms have more occupied shells. The outer electron is farther from the positive nucleus and is shielded by more inner electrons. The attractive force on it is therefore weaker.
Because the outer electron is lost more easily, reactivity increases down Group 1. Melting point shows a different trendThe general direction that results move in, such as going up over time. and generally decreases from lithium to caesium; one trend should never be assumed from another.
Group 1 metals with water
The alkali metals react with water to make a metal hydroxide (an alkali) and hydrogen gas. They get more reactive down the group: lithium fizzes, sodium whizzes around, potassium bursts into flame.
Universal indicator: green (neutral) turns purple as the alkali forms. Small circles: hydrogen bubbles.
Controls
Speeds in the animation are illustrative. Rubidium and caesium react explosively, so they are never used in school.
Predicting rubidium from evidence
Rubidium is below potassium in Group 1. Predict its reaction with water and explain your prediction.
- Place rubidium below potassium in the ordered Group 1 sequence.
- Use the observed trend: reactivityHow easily a substance takes part in chemical reactions. increases from lithium to sodium to potassium.
- Predict a more vigorous reaction with faster hydrogen production and rapid formation of rubidium hydroxide.
- Explain that rubidium has an additional occupied shell, giving greater distance and shieldingWhen inner electrons block some of the pull of the nucleus on the outer electrons., so its outer electron is lost more easily.
How to make the reasoning visible: Rubidium is predicted to react more vigorously than potassium. This is an evidence-based extrapolationExtending a graph line beyond your results to predict values you did not measure. and must not be tested in an ordinary classroom activity.
Interrogate the Group 1 pattern
The first three reaction descriptions are observations from controlled demonstrations. Rubidium is included to test whether a prediction is distinguished from an observation.
| Metal | Melting point / °C | Reaction with cold water | Evidence status |
|---|---|---|---|
| Lithium | 180.5 | Floats and fizzes steadily | Observed pattern |
| Sodium | 97.8 | Melts into a ball and fizzes rapidly | Observed pattern |
| Potassium | 63.5 | Very vigorous; hydrogen may ignite with a lilac flame | Observed pattern |
| Rubidium | 39.3 | Predicted to be more vigorous than potassium | Prediction from trend |
Describe two different trends shown by the table and support each with data.
Why must the rubidium reaction statement be labelled as a prediction rather than an observation in this data set?
Check your understanding
Answer from memory, then check against the suggested answer.
How many outer-shell electrons does a Group 1 atom have?
What two products form when a Group 1 metal reacts with water?
How does reactivity change down Group 1?
A Group 1 metal reacts with water to form a solution with pH 13 and a gas. Explain both results.
Sodium has a lower melting point but is more reactive than lithium. Explain why this does not contradict the Group 1 pattern.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Potassium reacts more vigorously with water than lithium.
Explain why potassium reacts more vigorously than lithium.
(4)Name the two products formed when either metal reacts with water.
(1)Total for this question: 5 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., build a causal chain: shells → distance and shielding → attraction → electron loss → reactivity. Then answer the products part explicitly.