The Periodic Table of Elements - Elements in Group 1
The Periodic Table of Elements
Group 1 Elements
The elements in Group 18 are lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr).
Group 1 elements are:
- also known as alkali metals (see why under chemical properties)
- shiny when cut, but quickly tarnished upon exposure to air
- soft and can be cut with a knife
- are very reactive (they have to be stored in paraffin oil to prevent contact with oxygen and water in the air)
Group 1 is located at the left end of the periodic table. (refer to the periodic table above)
Reactivity of Group 1 Elements
The reactivity of Group 1 elements increases down the group. This is because:
- Group 1 elements react by donating 1 electron from the outermost shell (valence electron).
- M → M+ + e–
- This makes alkali metals electropositive (they have a tendency to donate electrons).
- The valence electron is held in place by the electrostatic force of attraction with the positively charged nucleus.
- The atomic radius of elements increases down the group (refer to diagram).
- The distance between the nucleus and the valence electron increases.
- This causes the electrostatic force of attraction between the nucleus and the valence electron to weaken.
- Therefore, it gets easier to donate the valence electron going down the group. (The electropositivity of alkali metals increases down the group)
Physical Properties of Group 1 Elements
- Atomic radius increases down the group. This is due to an increase in the number of electron shells down the group.
- Density increases down the group. As the atomic mass increases, the density increases (density = mass/volume). However, alkali metals have relatively low densities – lithium, sodium and potassium float on water.
- Melting and boiling points of the elements decrease down the group.
- Atoms of alkali metals are held together by metallic bonds (refer to diagram).
- The distance between the positive nucleus (M+) and the delocalised electron (blue circles) increases down the group.
- This causes the strength of the electrostatic force of attraction between the positive nucleus and the delocalised electron to decrease down the group.
- As a result, less heat energy is required to break the bonds between atoms.
- Therefore, the melting and boiling points of the elements decrease down the group.
- Since alkali metals are indeed metals, they are good conductors of electricity and heat.
- They are soluble in water (see chemical properties).
Chemical Properties of Group 1 Elements
Alkali metals react with:
- Oxygen
- Water
- Halogens
Reaction of Group 1 Elements with Oxygen
Alkali metals react with oxygen to form metal oxides.
Alkali metal + Oxygen → Metal oxide
4Li (s) + O2 (g) → 2Li2O (s)
Lithium + Oxygen → Lithium oxide
4Na (s) + O2 (g) → 2Na2O (s)
Sodium + Oxygen → Sodium oxide
4K (s) + O2 (g) → 2K2O (s)
Potassium + Oxygen → Potassium oxide
Observations (refer to diagram)
- Lithium burns with a red flame in excess oxygen.
- Sodium burns with a bright yellow flame in excess oxygen.
- Potassium burns with a very bright lilac flame in excess oxygen.
All three reactions produce a white solid.
Metal oxides react with water to form an alkaline solution of metal hydroxide.
Metal oxide + Water → Metal hydroxide
Li2O (s) + H2O (l) → 2LiOH (aq)
Lithium oxide + Water → Lithium hydroxide
Na2O (s) + H2O (l) → 2NaOH (aq)
Sodium oxide + Water → Sodium hydroxide
K2O (s) + H2O (l) → 2KOH (aq)
Potassium oxide + Water → Potassium hydroxide
All three reactions produce colourless solutions that turn red litmus paper blue (alkaline solutions).
Reaction of Group 1 Elements with Water
Alkali metals react with water to form an alkaline solutions of metal hydroxides and hydrogen gas.
Alkali metal + Water → Metal hydroxide + Hydrogen gas
2Li (s) + 2H2O (l) → 2LiOH (aq) + H2 (g)
Lithium + Water → Lithium hydroxide + Hydrogen gas
2Na (s) + 2H2O (l) → 2NaOH (aq) + H2 (g)
Sodium + Water → Sodium hydroxide + Hydrogen gas
2K (s) + 2H2O (l) → 2KOH (aq) + H2 (g)
Potassium + Water → Potassium hydroxide + Hydrogen gas
Observations (refer to diagram)
- Lithium moves slowly on the surface of the water.
- Sodium moves rapidly on the surface of the water while burning with a yellow flame.
- Potassium moves very rapidly on the surface of the water while burning with a lilac flame.
All three reactions produce colourless solutions that turn red litmus paper blue (alkaline solutions).
This is why Group 1 elements are called alkali metals.
TIME TO THINK
Why do lithium, sodium and potassium move on the surface of the water?
- First, remember that all three alkali metals are less dense than water. This enables them to float on the surface of the water.
PHYSICS LINK – think about the conservation of momentum.
- When hydrogen gas is produced, it creates momentum in the direction that it is released.
- By the law of conservation of momentum, the alkali metal must experience a momentum with equal magnitude in the opposite direction!
- This is what causes the alkali metal to move on the surface of the water.
- The more reactive the alkali metal, the higher the rate of hydrogen produced, the faster it moves on the surface of the water.
Reaction of Group 1 Elements with Halogens (Group 17 Elements)
Alkali metals react with halogens to form metal halides.
Alkali metal + Halogen → Metal halide
2Li (s) + Cl2 (g) → 2LiCl (s)
Lithium + Chlorine → Lithium chloride
2Na (s) + Cl2 (g) → 2NaCl (s)
Sodium + Chlorine → Sodium chloride
2K (s) + Cl2 (g) → 2KCl (s)
Potassium + Chlorine → Potassium chloride
This reaction also occurs in the same manner with bromine gas and iodine vapour.
Observations (refer to diagram)
- Lithium burns with a red flame.
- Sodium burns with a bright yellow flame.
- Potassium burns with a very bright lilac flame.
All three reactions produce a white solid.
Practice Questions
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