8.2 Group I Properties Revision Notes

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.