Group I properties

1. Overview

Group I elements, known as the Alkali Metals, are a group of highly reactive metals located in the first column of the Periodic Table. Understanding their properties is essential because they demonstrate clear periodic trends and show how the atomic structure of an element influences its chemical behavior and physical characteristics.

Key Definitions

• Alkali Metals: The name given to the elements in Group I of the Periodic Table (Lithium, Sodium, Potassium, Rubidium, Caesium, and Francium).
• Tarnish: The process where a shiny metal surface reacts with oxygen in the air to form a dull metal oxide layer.
• Density: The mass per unit volume of a substance; many Group I metals are unique because they are less dense than water.
• Periodicity: The recurring trends in physical and chemical properties of elements as you move across or down the Periodic Table.

Core Content

Physical Properties of Lithium, Sodium, and Potassium

The first three elements of Group I share several distinct physical properties that differentiate them from transition metals:

• Softness: They are extremely soft and can be easily cut with a knife.
• Appearance: They are shiny and silvery when freshly cut, but they tarnish rapidly when exposed to air.
• Low Density: Lithium, sodium, and potassium are all less dense than water (they will float).

Trends Down Group I

As you move down the group from Lithium (Li) to Francium (Fr), the following trends are observed:

1. Decreasing Melting Point

• The melting points decrease as the atoms get larger.
• Example: Lithium melts at 180°C, while Potassium melts at 63°C.

2. Increasing Density

• The atoms increase in mass significantly as you go down the group, leading to a general increase in density.
• Note: Potassium is a slight anomaly as it is slightly less dense than sodium, but the overall trend for the group is an increase.

3. Increasing Reactivity

• Reactivity increases down the group because the outer shell electron is further from the nucleus, making it easier to lose.
• Lithium: Fizzes steadily and disappears slowly.
• Sodium: Melts into a silvery ball, fizzes rapidly, and moves across the surface.
• Potassium: Reacts violently, burns with a lilac flame, and may explode.

Predicting Properties of Other Elements

Given the trends above, we can predict the properties of elements further down the group like Rubidium (Rb) and Caesium (Cs):

• Predicting Reactivity: Rubidium and Caesium will be more reactive than Potassium. They react explosively with water.
• Predicting Melting Point: They will have lower melting points than Potassium (Caesium melts at approximately 28°C).
• Predicting Density: They will be denser than Potassium.

Extended Content (Extended Only)

There are no specific additional learning objectives for the Supplement curriculum for this topic; the Core objectives cover the requirements for all students.

Key Equations

General Reaction with Water

All Group I metals react with water to produce a metal hydroxide (an alkali) and hydrogen gas.

General Word Equation:
Metal + Water → Metal Hydroxide + Hydrogen

General Symbol Equation:
$2\text{M}(s) + 2\text{H}_2\text{O}(l) \to 2\text{MOH}(aq) + \text{H}_2(g)$

Specific Examples

Lithium:

• Word: Lithium + Water → Lithium hydroxide + Hydrogen
• Symbol: $2\text{Li}(s) + 2\text{H}_2\text{O}(l) \to 2\text{LiOH}(aq) + \text{H}_2(g)$

Sodium:

• Word: Sodium + Water → Sodium hydroxide + Hydrogen
• Symbol: $2\text{Na}(s) + 2\text{H}_2\text{O}(l) \to 2\text{NaOH}(aq) + \text{H}_2(g)$

Potassium:

• Word: Potassium + Water → Potassium hydroxide + Hydrogen
• Symbol: $2\text{K}(s) + 2\text{H}_2\text{O}(l) \to 2\text{KOH}(aq) + \text{H}_2(g)$

Common Mistakes to Avoid

• ❌ Wrong: Thinking Group I metals have high melting points like iron or copper.
• ✓ Right: Group I metals have relatively low melting points that decrease down the group.
• ❌ Wrong: Writing the formula for hydrogen gas as just $H$.
• ✓ Right: Hydrogen is diatomic; it must always be written as $\text{H}_2$ in equations.
• ❌ Wrong: Forgetting state symbols in chemical equations.
• ✓ Right: Always include $(s)$, $(l)$, $(g)$, and $(aq)$ to show the state of matter.
• ❌ Wrong: Stating that reactivity decreases down the group.
• ✓ Right: Reactivity increases down Group I as it becomes easier to lose the outer electron.

Exam Tips

• Command Words: If asked to "Describe" the trend in reactivity, state that it increases down the group. If asked to "Explain" the trend (often in paper 4), mention that the outer electron is further from the nucleus and shielded by more shells, so it is attracted less strongly.
• Observations: When describing the reaction with water, always mention "effervescence" or "fizzing" (due to $\text{H}_2$ gas) and that the resulting solution turns universal indicator purple/blue (due to the alkaline hydroxide).
• Identification: Remember the specific flame color for Potassium reacting with water is lilac. This is a frequent "hidden" question in identity tests.
• Predictions: You may be given a table of data for Li, Na, and K and asked to estimate a value for Rb. Look at the difference between the numbers and apply the same logic to predict the new value.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [5 marks]

Question:

Rubidium (Rb) is an alkali metal in Group I, located below potassium (K) in the periodic table.

(a) State two physical properties that rubidium has in common with other Group I metals. [2]

(b) Predict one physical property of rubidium based on its position in Group I, compared to potassium. [1]

(c) Rubidium reacts with water. Complete the word equation for this reaction. [2]

rubidium + water → __________ + __________

Worked Solution:

(a)

1. All Group I metals are soft.
   The metals can be cut with a knife.

2. All Group I metals are good conductors of electricity.

How to earn full marks:

• Award 1 mark for each correct physical property.
• Accept any two of: soft, good conductors of electricity, good conductors of heat, shiny when freshly cut, low density.

(b)

1. As you go down Group I, the melting point decreases.
   Rubidium is below potassium so it will melt at a lower temperature.

How to earn full marks:

• Award 1 mark for stating a lower melting point than potassium.
• Accept any prediction consistent with Group I trends.

(c)

1. Rubidium reacts with water to form rubidium hydroxide.
   The product is an alkali.

2. Rubidium reacts with water to form hydrogen gas.
   The gas produced is flammable.

How to earn full marks:

• Award 1 mark for rubidium hydroxide.
• Award 1 mark for hydrogen.

Common Pitfall: When asked for physical properties, do not give reactivity: "reacts readily with water" is a chemical property. Choose properties such as softness, low density or conductivity. Also, remember that "low density" is a general property of alkali metals, not just a comparison.

Exam-Style Question 2 — Short Answer [6 marks]

Question:

Sodium is a Group I metal.

(a) State two observations you would make when a small piece of sodium is added to water. [2]

(b) Write a balanced chemical equation for the reaction between sodium and water. Include state symbols. [3]

(c) Explain why sodium is stored under oil. [1]

Worked Solution:

(a)

1. Sodium floats on the surface of the water.
   Sodium is less dense than water.

2. Sodium melts into a ball.
   The reaction is exothermic and the heat melts the sodium.

How to earn full marks:

• Award 1 mark for each correct observation.
• Accept any two of: sodium floats, sodium melts, sodium moves around the surface, sodium fizzes, sodium dissolves, sodium gets smaller, a gas is produced.

(b)

1. The reactants are sodium and water.
   $2Na(s) + 2H_2O(l) \rightarrow$

2. The products are sodium hydroxide and hydrogen.
   $2NaOH(aq) + H_2(g)$

3. The equation must be balanced with correct state symbols.
   $2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$

How to earn full marks:

• Award 1 mark for correct reactants and products.
• Award 1 mark for correct balancing.
• Award 1 mark for correct state symbols.

(c)

1. Sodium is very reactive and reacts with oxygen or water vapour in the air.
   To prevent reaction with air/oxygen/water
   The oil prevents contact with air/oxygen/water.

How to earn full marks:

• Award 1 mark for stating that the oil prevents sodium from reacting with air or water.

Common Pitfall: Don't just say "it's reactive." Explain why sodium is stored under oil – to prevent it from reacting with specific substances in the air. Also, ensure your state symbols are correct and clearly written in the chemical equation.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

A student investigates the reaction of three Group I metals, lithium (Li), sodium (Na) and potassium (K), with water. The student uses the same mass of each metal and the same volume of water in each experiment. The student measures the temperature rise of the water in each experiment.

The results are shown in the table:

Metal	Temperature Rise (°C)
Lithium	15
Sodium	25
Potassium	45

(a) Describe two safety precautions the student should take when carrying out these experiments. [2]

(b) Explain why the temperature rise increases from lithium to potassium. [3]

(c) Predict the temperature rise if the student repeated the experiment with the same mass of rubidium (Rb). Explain your answer. [3]

Worked Solution:

(a)

1. Sodium hydroxide is formed, which is corrosive.
   Wear safety goggles
   To protect eyes from splashes of the alkaline solution.

2. The reaction is exothermic and can produce hydrogen gas, which is flammable and may explode.
   Use a safety screen
   To protect from any possible explosions.

How to earn full marks:

• Award 1 mark for each correct safety precaution with justification.
• Accept: Wear gloves (to protect skin from alkali), Carry out in a fume cupboard (to contain hydrogen gas).

(b)

1. Reactivity increases down Group I.

2. The outer electron is further from the nucleus.
   There is less attraction between the nucleus and the outer electron.

3. It is easier to lose the outer electron.
   Less energy is required to remove the electron to form an ion.

How to earn full marks:

• Award 1 mark for stating that reactivity increases down the group.
• Award 1 mark for stating that the outer electron is further from the nucleus.
• Award 1 mark for stating that it is easier to lose the outer electron.

(c)

1. Rubidium is below potassium in Group I.
   The temperature rise will be greater than 45°C.

2. Reactivity increases down Group I.
   Because rubidium is more reactive than potassium.

3. The outer electron is further from the nucleus and more easily lost.
   Because there is less attraction between the nucleus and the outer electron.

How to earn full marks:

• Award 1 mark for predicting a temperature rise greater than 45°C.
• Award 1 mark for stating that rubidium is more reactive than potassium.
• Award 1 mark for linking reactivity to electron loss/distance from the nucleus.

Common Pitfall: When discussing safety precautions, always provide a justification for why that precaution is necessary. For example, don't just say "wear goggles"; say "wear goggles to protect your eyes from splashes of corrosive alkali." Also, remember to link the increased reactivity down the group to the ease of losing the outer electron due to decreasing attraction from the nucleus.

Exam-Style Question 4 — Extended Response [9 marks]

Question:

Francium (Fr) is a radioactive alkali metal located at the bottom of Group I. It is extremely rare and difficult to study. However, its properties can be predicted based on the trends in Group I.

(a) Predict three physical properties of francium. [3]

(b) Francium reacts with water to form francium hydroxide and hydrogen gas. Write a balanced chemical equation for this reaction. Include state symbols. [3]

(c) Suggest one reason why it is difficult to determine the accurate melting point of francium experimentally. [1]

(d) When rubidium reacts with chlorine it forms an ionic compound. Draw a 'dot-and-cross' diagram to show the arrangement of electrons in rubidium chloride. Only show the outer shell electrons. [2]

Worked Solution:

(a)

1. Melting point decreases down Group I.
   Francium is at the bottom of the group, so it will have a very low melting point.

2. Density increases down Group I.
   Francium is at the bottom of the group, so it will have a very high density.

3. All Group I metals are good conductors of electricity.
   Francium is a Group I metal, so it will also be a good conductor of electricity.

How to earn full marks:

• Award 1 mark for each correct physical property prediction.
• Acceptable properties: soft, good conductor of electricity, good conductor of heat, shiny when freshly cut, low melting point, high density. Do not give "very reactive": reactivity is a chemical property.

(b)

1. The reactants are francium and water.
   $2Fr(s) + 2H_2O(l) \rightarrow$

2. The products are francium hydroxide and hydrogen.
   $2FrOH(aq) + H_2(g)$

3. The equation must be balanced with correct state symbols.
   $2Fr(s) + 2H_2O(l) \rightarrow 2FrOH(aq) + H_2(g)$

How to earn full marks:

• Award 1 mark for correct reactants and products.
• Award 1 mark for correct balancing.
• Award 1 mark for correct state symbols.

(c)

1. Francium is radioactive and decays quickly.
   The sample may decay before the melting point can be accurately measured.

How to earn full marks:

• Award 1 mark for stating that francium is radioactive and decays quickly.

(d)

1. Correct ions and charges
   Rubidium loses one electron to form a +1 ion. Chlorine gains one electron to form a -1 ion.

2. Correct arrangement of electrons
   Rubidium has no outer shell electrons shown. Chlorine has 8 outer shell electrons shown.

How to earn full marks:

• Award 1 mark for correct ions and charges, including brackets and charges.
• Award 1 mark for correct arrangement of electrons, showing 8 electrons on chloride ion and none on rubidium ion.

Common Pitfall: When predicting properties for elements like francium, make sure your predictions are based on clear trends within Group I. For dot-and-cross diagrams, remember to include the charges on the ions and the square brackets around them. Also, double-check that the number of electrons in the outer shell of each ion is correct after the electron transfer.