C2.3 Group 1 – The Alkali Metals

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: properties, safe handling and reactions of the alkali metals, and the patterns going down Group 1. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe the physical properties of the alkali metals and how they are stored and handled safely
• Describe and write equations for the reactions of Group 1 metals with water, oxygen and chlorine
• Describe the trends in reactivity and melting point going down Group 1
• Predict the properties of other Group 1 elements such as rubidium, caesium and francium

Key words

• Alkali metals: The Group 1 elements: lithium, sodium, potassium, rubidium, caesium and francium.
• Alkali: A soluble base. Its solution has a pH above 7 and contains hydroxide ions, OH−.
• Hydroxide: A compound containing OH− ions, e.g. sodium hydroxide, NaOH.
• Tarnish: When a shiny metal surface goes dull because it reacts with oxygen in the air.
• Reactivity: How quickly and vigorously an element reacts.

Properties, storage and handling

The alkali metals all have one electron in their outer shell, so they have similar reactions. Compared with most other metals they are unusual:

• they are soft enough to cut with a knife
• they have low densities: lithium, sodium and potassium are less dense than water (below 1 g/cm³), so they float
• they have low melting points
• they are shiny when freshly cut, but quickly tarnish in air
• like all metals, they conduct electricity and heat

They react with oxygen and water vapour in the air, so they are stored under oil. In the lab, use small pieces, pick them up with tongs or forceps (never fingers), wear safety glasses and use a safety screen when adding them to water.

Reaction with water

Alkali metals react with water to make a metal hydroxide and hydrogen. The hydroxide dissolves to form an alkaline solution, which is how the group gets its name.

metal + water → metal hydroxide + hydrogen

2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g)

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)

Key fact: Test for the gas: hold a lighted splint at the mouth of the tube. Hydrogen burns with a squeaky pop.

Why every Group 1 metal forms a 1+ ion

An alkali metal atom has one electron in its outer shell. In every reaction it loses this electron to reach the stable electronic structure of the noble gas in the period above. It is left with one more proton than electrons, so it forms an ion with a 1+ charge (Li⁺, Na⁺, K⁺). Because the metal always transfers electrons to a non-metal, alkali metal compounds are always ionic.

Reactions with oxygen and the halogens

Freshly cut alkali metals tarnish as an oxide layer forms. When heated, they burn in oxygen with coloured flames (lithium red, sodium yellow-orange, potassium lilac) to form white metal oxides.

4Li(s) + O₂(g) → 2Li₂O(s)

4Na(s) + O₂(g) → 2Na₂O(s)

Heated alkali metals react vigorously with chlorine gas to form white solid metal chlorides. These are ionic compounds containing 1+ metal ions, such as Na⁺, and Cl− ions. They dissolve in water to give colourless solutions.

2Na(s) + Cl₂(g) → 2NaCl(s)

2K(s) + Cl₂(g) → 2KCl(s)

They react in the same way with fluorine, bromine and iodine. Every product is a white ionic solid that dissolves to give a colourless solution. Because each metal forms a 1+ ion, the formulae follow one pattern:

• lithium; Oxide (O²−) Li₂O; Hydroxide (OH−) LiOH; Chloride (Cl−) LiCl; Iodide (I−) LiI
• sodium; Oxide (O²−) Na₂O; Hydroxide (OH−) NaOH; Chloride (Cl−) NaCl; Iodide (I−) NaI
• potassium; Oxide (O²−) K₂O; Hydroxide (OH−) KOH; Chloride (Cl−) KCl; Iodide (I−) KI
• any Group 1 metal, M; Oxide (O²−) M₂O; Hydroxide (OH−) MOH; Chloride (Cl−) MCl; Iodide (I−) MI

Worked example: conservation of mass

A student heats 0.46 g of sodium in a gas jar of chlorine. It reacts completely with 0.71 g of chlorine. What mass of sodium chloride forms?
Atoms are not created or destroyed, so the mass of product = total mass of reactants = 0.46 + 0.71

Answer: 1.17 g of sodium chloride

Trends going down Group 1

• Reactivity; Trend increases: reactions with water, oxygen and chlorine get more vigorous
• Melting and boiling points; Trend decrease
• Size of atom; Trend increases (one more shell each period)

Worked example: predicting francium

Francium is at the bottom of Group 1. Use the trends to predict its properties.
Melting point: the values fall from 181 °C to 28 °C, so francium’s should be below 28 °C.
Reactivity: it should react even more violently than caesium with water: 2Fr + 2H₂O → 2FrOH + H₂.
Ion: like all Group 1 metals it forms a 1+ ion, Fr⁺, so its chloride is FrCl.

Answer: a very reactive metal with a melting point below 28 °C that forms Fr⁺ ions

Link: The reasons for these trends (in terms of electrons and shells) are covered in section C2.5.

Common mistakes

• Saying sodium + water makes sodium oxide. It makes sodium hydroxide and hydrogen.
• Saying the gas produced is oxygen. It is hydrogen (squeaky pop).
• Writing NaOH(s) in water equations. The hydroxide is dissolved: NaOH(aq).
• Saying melting points increase down the group. They decrease; it is reactivity that increases.
• Giving an alkali metal a 2+ charge, e.g. writing NaO or KCl₂. Group 1 ions are always 1+: Na₂O, KCl.
• Writing Na₂ or Cl in equations. Sodium is written Na; chlorine is diatomic, Cl₂.

Revision questions

Try these, then check your answers below each one.

Question 1. What are the products when sodium reacts with water? (Choose from: sodium hydroxide and hydrogen, sodium oxide and hydrogen, sodium hydroxide and oxygen, sodium oxide and water.)

Answer: sodium hydroxide and hydrogen

Working: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g). Sodium hydroxide dissolves to give an alkaline solution; the gas is hydrogen.

Question 2. Use the bar chart to predict the melting point of caesium.

Answer: 28 °C

Working: The melting points fall down the group: 181, 98, 63, 39 °C, with smaller drops each time. So caesium should melt a little below 39 °C – about 25–35 °C. The real value is 28 °C.

Question 3. True or false? The alkali metals are stored under water to keep them safe.

Answer: False

Working: False. They react with water (and with oxygen), so they are stored under oil.

Related lessons

• Previous lesson: C2.2 Electronic Structures and The Periodic Table
• Next lesson: C2.4 Group 7 – The Halogens
• In this chapter: C2.1 Development of The Periodic Table
• In this chapter: C2.5 Explaining Trends

Continue in the full course

This lesson is section C2.3 (AGCSC1903) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.