Group 1
What you'll learn
- Why Group 1 elements are called the alkali metals.
- How lithium, sodium and potassium react with oxygen, chlorine and water.
- Why reactivity increases as you go down Group 1.
- How to predict properties of Group 1 elements from trends.
Starting point: groups, shells and outer electrons
The periodic table is arranged so that elements with similar chemical properties are in the same vertical columns.
Group
A group is a vertical column in the periodic table. Elements in the same group have the same number of electrons in their outer shell, so they often react in similar ways.
An electron shell is an energy level around the nucleus where electrons are found. The outer shell is the shell furthest from the nucleus.
Group 1 elements all have one electron in their outer shell. This is the key reason they behave similarly.
The alkali metals
The elements in Group 1 are known as the alkali metals. The first three are:
- lithium, Li
- sodium, Na
- potassium, K
Further down the group are rubidium, caesium and francium, but GCSE questions usually focus on lithium, sodium and potassium, then ask you to predict what happens for the others.
Alkali metals
The alkali metals are the Group 1 metals. They are called “alkali” metals because they react with water to form alkaline solutions of metal hydroxides.
Hydrogen is not an alkali metal
Hydrogen is sometimes shown above Group 1 because it has one outer electron, but it is a non-metal gas. In GCSE Chemistry, “Group 1” in this topic means the alkali metals such as lithium, sodium and potassium.
Why Group 1 metals have similar reactions
Group 1 atoms have one outer-shell electron. In chemical reactions, they tend to lose this electron.
When an atom loses an electron, it becomes a positive ion. Group 1 metals form ions with a 1+ charge:
- lithium forms Li+
- sodium forms Na+
- potassium forms K+
One outer electron controls the chemistry
Group 1 metals have similar chemical properties because each atom has one electron in its outer shell. They react by losing this electron to form 1+ ions.
This helps explain the formulae of their compounds. For example:
- chlorine forms chloride ions, Cl−, so sodium chloride is NaCl
- oxygen forms oxide ions, O2−, so sodium oxide is Na2O
- hydroxide ions are OH−, so sodium hydroxide is NaOH
Working out the formula of lithium oxide
Lithium forms Li+ ions. Oxygen forms O2− ions. What is the formula of lithium oxide?
- Compare the charges: each lithium ion has a charge of 1+, but the oxide ion has a charge of 2−.
- Balance the total charge by using two lithium ions for every one oxide ion: two 1+ charges balance one 2− charge.
- Write the formula with the metal first: lithium oxide is Li2O.
General physical properties of Group 1 metals
Group 1 elements are metals, so they conduct electricity and are shiny when freshly cut. However, compared with many other metals, they are unusual because they are:
- soft — they can be cut with a knife
- low density — lithium, sodium and potassium float on water
- very reactive — they must be stored under oil to keep air and water away
When freshly cut, they are shiny, but they quickly become dull. This is called tarnishing, and it happens because they react with oxygen and water vapour in the air.
Why they are stored in oil
Oil keeps oxygen and water away from the metal surface. This slows down unwanted reactions before the metal is used.
Reacting with oxygen
Group 1 metals react with oxygen to form metal oxides. Oxygen gas exists as O2 molecules, so we write it as O2(g).
Metal oxide
A metal oxide is a compound made from a metal and oxygen. For Group 1 metals, the oxide contains metal ions and oxide ions, O2−.
The reactions become more vigorous as you go down the group:
- lithium reacts slowly and forms lithium oxide
- sodium reacts more quickly and forms sodium oxide
- potassium reacts even more vigorously and forms potassium oxide
The balanced equations are:
4Li(s) + O2(g) → 2Li2O(s)
4Na(s) + O2(g) → 2Na2O(s)
4K(s) + O2(g) → 2K2O(s)
In each case, the product is a solid metal oxide.
Keep to the GCSE pattern
In more advanced chemistry, sodium and potassium can form other oxygen compounds under certain conditions. For AQA GCSE, use the pattern “Group 1 metal + oxygen → metal oxide” unless the question gives you extra information.
Reacting with chlorine
Group 1 metals react with chlorine to form metal chlorides. Chlorine gas exists as Cl2 molecules, so we write it as Cl2(g).
Metal chloride
A metal chloride is a compound made from a metal and chlorine. Group 1 metal chlorides contain metal ions and chloride ions, Cl−.
Again, the reactions get more vigorous down the group:
- lithium forms lithium chloride
- sodium forms sodium chloride
- potassium forms potassium chloride
The balanced equations are:
2Li(s) + Cl2(g) → 2LiCl(s)
2Na(s) + Cl2(g) → 2NaCl(s)
2K(s) + Cl2(g) → 2KCl(s)
The products are white solid salts.
Spot the pattern
For Group 1 chlorides, the formula is usually metal symbol + Cl: LiCl, NaCl, KCl. That is because Group 1 ions are 1+ and chloride ions are 1−.
Reacting with water
This is the reaction you are most likely to be asked about.
Group 1 metals react with water to form a metal hydroxide and hydrogen gas.
The general word equation is:
Group 1 metal + water → metal hydroxide + hydrogen
The general symbol equation is:
2M(s) + 2H2O(l) → 2MOH(aq) + H2(g)
Here, M represents a Group 1 metal.
Alkaline solution
An alkaline solution has a pH greater than 7. Group 1 metals form alkaline solutions because their hydroxides dissolve in water to produce hydroxide ions, OH−.
Observations for the first three alkali metals
Lithium reacts gently:
- floats on water
- fizzes as hydrogen gas is produced
- moves slowly on the surface
- gradually disappears
- forms lithium hydroxide solution
2Li(s) + 2H2O(l) → 2LiOH(aq) + H2(g)
Sodium reacts more vigorously:
- floats on water
- fizzes more strongly
- moves quickly around the surface
- often melts into a small ball because the reaction releases heat
- forms sodium hydroxide solution
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Potassium reacts very vigorously:
- floats on water
- fizzes rapidly
- moves very quickly
- may produce enough heat to ignite the hydrogen gas
- often burns with a lilac flame
- forms potassium hydroxide solution
2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
Forgetting hydrogen gas
The products of a Group 1 metal reacting with water are always a metal hydroxide and hydrogen gas. Do not write “metal oxide” for the water reaction.
Why reactivity increases down Group 1
As you go down Group 1, the atoms get larger because they have more electron shells.
The outer electron is:
- further from the positive nucleus
- shielded by more inner electron shells
- less strongly attracted to the nucleus
So it is easier for the atom to lose its outer electron. That means the metal is more reactive.
Shielding
Shielding is the reduced attraction between the nucleus and the outer electron because inner electron shells get in the way.
The trend in reactivity
Reactivity increases down Group 1 because the outer electron is further from the nucleus and more shielded, so it is lost more easily.
Comparing sodium and potassium reactivity
Explain why potassium is more reactive than sodium.
- Compare their electron arrangements: sodium is 2,8,1, while potassium is 2,8,8,1.
- Potassium has one more occupied electron shell than sodium, so its outer electron is further from the nucleus and more shielded.
- The attraction between the potassium nucleus and its outer electron is weaker, so potassium loses its outer electron more easily and reacts more vigorously.
Predicting properties from trends
A trend is a pattern in data or properties. In Group 1, the most important trend is that reactivity increases down the group.
So if you are asked to predict what rubidium or caesium does, you should apply the same reaction pattern, but make the reaction more vigorous than potassium.
For example, rubidium would be expected to:
- have one electron in its outer shell
- form Rb+ ions
- react with chlorine to form rubidium chloride, RbCl
- react with water to form rubidium hydroxide and hydrogen
- react more vigorously than potassium
2Rb(s) + 2H2O(l) → 2RbOH(aq) + H2(g)
Predicting the reaction of rubidium with water
Predict what would happen when rubidium is added to water.
- Locate rubidium in Group 1 below potassium, so it should be more reactive than potassium.
- Apply the Group 1 water reaction pattern: metal + water → metal hydroxide + hydrogen.
- Predict the products using rubidium’s 1+ ion: rubidium hydroxide, RbOH(aq), and hydrogen, H2(g).
- Describe the observations as more vigorous than potassium: very rapid fizzing, fast movement, lots of heat, and likely ignition.
In the exam
- Link Group 1 reactions to the one outer-shell electron: losing it forms a 1+ ion.
- For water reactions, always give both products: metal hydroxide and hydrogen gas.
- When explaining the trend down the group, use the chain: more shells → more shielding → weaker attraction → electron lost more easily → more reactive.
Check yourself
- Why do lithium, sodium and potassium have similar chemical properties?
- What are the products when sodium reacts with water?
- Why is potassium more reactive than sodium?
