Alkali Metal Reactions with Water

The observations of alkali metals reactions with water become more vigorous down Group 1:

• Lithium: Floats with some fizzing, produces alkaline solution
• Sodium: Floats, melts, and moves vigorously across surface
• Potassium: Floats, splits, explodes with lilac flame
• Rubidium: Sinks, sparks vigorously, produces flame
• Caesium: Very explosive, breaks container, produces flame

Example: Rubidium + water → rubidium hydroxide + hydrogen 2Rb $s$ + 2H₂O $l$ → 2RbOH (aq) + H₂ $g$

Alkali metals also react with oxygen in air to form metal oxides:

4M $s$ + O₂ $g$ → 2M₂O $s$

Definition: Oxidation reaction - A reaction in which a substance combines with oxygen

Group 7 Halogens

The properties of Group 7 elements (halogens) include:

• Non-metallic properties: brittle, dull solids, poor conductors
• Diatomic molecules with weak intermolecular forces
• Seven electrons in outermost shell
• Form covalent bonds and 1- ions (halide ions) in reactions

Trends and properties in Group 7 halogens down the periodic table:

• Colors become darker
• Reactivity and bleaching effect decrease
• Density increases
• Melting and boiling points increase

Vocabulary: Diatomic - Consisting of two atoms

Example: The diatomic halogens are F₂, Cl₂, Br₂, and I₂

Halogen Properties and Reactivity

The physical properties of Group 7 elements vary:

• Fluorine and chlorine: Gases at room temperature
• Bromine: Liquid at room temperature
• Iodine and astatine: Solids at room temperature

Chemical properties of Group 7:

• React with metals to form ionic metal halides
• React with non-metals to form covalent compounds
• React with hydrogen to form hydrogen halides

Highlight: Fluorine is the most reactive halogen due to its small size and strong electron-attracting ability.

The trend in reactivity of Group 7 decreases down the group as it becomes harder for larger atoms to gain electrons.

Example: Chlorine can displace bromine from potassium bromide solution, demonstrating its higher reactivity.

Halogen Displacement Reactions

Halogen displacement reactions occur when a more reactive halogen displaces a less reactive halogen from an aqueous solution. This demonstrates the trend in reactivity down Group 7.

For example:

• Chlorine can displace bromine from potassium bromide
• Chlorine can displace iodine from potassium iodide
• Bromine can displace iodine from potassium iodide

Definition: Displacement reaction - A reaction where a more reactive element takes the place of a less reactive element in a compound

These reactions provide evidence for the decreasing reactivity trend in Group 7 elements.

Group 1 Alkali Metals

The physical properties of alkali metals in Group 1 include being soft, low density, and having relatively low melting and boiling points. They are shiny when freshly cut and good conductors of electricity and heat.

Key chemical properties of Group 1 alkali metals:

• Form alkaline solutions when reacting with water
• Have one electron in outermost shell
• Increase in reactivity down the group
• Form 1+ ions when reacting
• Number of electron shells increases down group

Example: The general equation for alkali metals reacting with water is: 2M $s$ + 2H₂O $l$ → 2MOH (aq) + H₂ $g$

Highlight: Alkali metals must be stored in oil to prevent reaction with oxygen and water vapor in air.

Vocabulary: Alkalinity - The property of being alkaline or basic (pH greater than 7)