Ch 2 · Group I Properties · O Level Chemistry 5070
Topic 2 of 5 in The Periodic Table — O Level Chemistry 5070 — 48 lessons.
Introduction
The alkali metals of Group I are the most reactive metals in the Periodic Table, and the demonstration of sodium and potassium skating across a trough of water is one that few students forget. Examiners expect you to describe the observations precisely, write the equations and, above all, use the trends to predict what rubidium or caesium would do.
Key ideas — Physical properties and trends
Lithium, sodium and potassium are relatively soft metals that can be cut with a knife. A freshly cut surface is shiny and silvery but tarnishes quickly in air as the metal reacts with oxygen, which is why they are stored under oil. They have low densities (lithium, sodium and potassium all float on water) and low melting points compared with most metals.
🧑🏫 Teacher's explanation
Imagine Group I metals as a group of siblings who all have one 'extra' toy (an electron) that they are desperate to get rid of to become stable. Because they only have one electron to lose, they are very eager to react. As you go down the group, the 'distance' between the nucleus and this extra electron increases, making the grip of the nucleus weaker. This makes it even easier for the electron to be lost, which is why they get more reactive as you go down.
📖 Definitions
🔍 Worked example — Easy
A piece of Lithium reacts with water. Write the balanced chemical equation for this reaction, including state symbols.
🔍 Worked example — Standard
Calculate the number of moles in 4.6 g of Sodium (Na). (A_r of Na = 23).
🔍 Worked example — Exam-style
Predict the properties of Cesium (Cs), which is below Potassium (K) in Group I. State the trend for: (i) Reactivity with water, (ii) Melting point, and (iii) Density.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Write a balanced chemical equation for this reaction. [2] (b) Describe two observations the student would make during this reaction. [2] [4 marks]
Key ideas — Reactions with water
Every alkali metal reacts with cold water to form a metal hydroxide (an alkali, hence the name of the group) and hydrogen:
🧑🏫 Teacher's explanation
Imagine you are dropping a piece of sugar into a glass of water. It dissolves and disappears. Now, imagine that instead of just dissolving, the sugar started boiling and fizzing violently, releasing gas and getting hot. That is what happens with Group I metals. They don't just 'sit' in water; they react chemically with it.
📖 Definitions
🔍 Worked example — Easy
Write a balanced chemical equation for the reaction between Sodium (Na) and water (H_2O). State the products and their states.
🔍 Worked example — Standard
A student reacts 2.3 g of Sodium (Na) with excess water. Calculate the volume of Hydrogen gas produced at room temperature and pressure (RTP). Assume 1 mole of gas occupies 24 dm^3. (A_r: Na = 23, H = 1)
🔍 Worked example — Exam-style
Predict the observation when Potassium (K) is added to water and explain why it is more vigorous than Lithium (Li).
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Why reactivity increases down the group
Reactivity depends on how easily the single outer electron is lost. Down the group each atom has one more shell, so the outer electron is further from the nucleus and shielded by more inner shells. The attraction between the nucleus and the outer electron is weaker, so the electron is lost more easily and the metal reacts faster.
🧑🏫 Teacher's explanation
Imagine the nucleus of an atom is a powerful magnet and the outer electron is a small metal bead. If you hold the magnet very close to the bead, the pull is incredibly strong. But if you place several layers of cardboard between the magnet and the bead, the pull becomes much weaker.
📖 Definitions
🔍 Worked example — Easy
State the trend in reactivity for Group I metals as you move from Lithium to Potassium.
🔍 Worked example — Standard
Explain why Potassium is more reactive than Lithium when reacting with water.
🔍 Worked example — Exam-style
Predict the observation when a small piece of Sodium is added to water and write the balanced equation.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Write a balanced chemical equation for this reaction, including state symbols. [2] (b) Explain why Potassium is more reactive than Sodium. [2] [4 marks]
Key ideas — Predicting the properties of other Group I elements
Because the trends are regular, you can predict the behaviour of rubidium, caesium and francium from the data for the first three metals:
🧑🏫 Teacher's explanation
Think of the Group I elements like a staircase of reactivity. As you walk down the stairs (from Lithium to Francium), each step makes you more energetic and 'unstable'. Because the outer electron is further and further away from the nucleus, the 'grip' the nucleus has on that electron weakens. This makes it much easier for the metal to lose that electron and react.
📖 Definitions
🔍 Worked example — Easy
Predict the state of Rubidium (Rb) at room temperature (25 °C) based on the trend of melting points: Li (-10 °C), Na (98 °C), K (73 °C), Rb (?).
🔍 Worked example — Standard
Write a balanced chemical equation for the reaction between Potassium (K) and water (H_2O).
🔍 Worked example — Exam-style
A sample of Caesium (Cs) has a density of 1.88 g/cm^3. If you have 3.76 g of Caesium, what volume will it occupy?
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Predict the melting point of Rubidium relative to Potassium. [1] (b) Predict the density of Rubidium relative to Potassium. [1] (c) Describe the observation when Rubidium is added to water. [1] [3 marks]
Worked example
Question. The table gives data for three Group I metals.
🔍 Worked example — Easy
Predict the trend in melting points for Group I metals: Lithium (Li), Sodium (Na), and Potassium (K).
🔍 Worked example — Standard
Write a balanced chemical equation for the reaction between Sodium (Na) and water (H_2O).
🔍 Worked example — Exam-style
Rubidium (Rb) is below Potassium (K). Predict the observations when a small piece of Rubidium is added to water.
⚠️ Common errors
Common mistakes
Practice
Answers Soft enough to cut with a knife [B1]; low density (float on water) [B1]; low melting point [B1]. Floats because it is less dense than water [B1]; fizzes because hydrogen is produced [B1]; melts into a ball and moves across the surface because the reaction is exothermic and the gas pushes it [B1]; burns with a lilac flame [B1]; indicator turns purple because potassium hydroxide, an alkali, is formed [B1]. 2Li + 2H_2O arrow 2LiOH + H_2 [M1 formulae, A1 balanced]. Higher density than rubidium [B1]; more reactive, reacting explosively with water [B1]; CsOH [B1].