4.4 - Elements in Group 1

Members and Uses

• Group 1 contains lithium, Li; sodium, Na; potassium, K; rubidium, Rb; caesium, Cs; and francium, Fr. They are called alkali metals.
• Lithium is used in batteries, sodium in sodium-vapour lamps and potassium compounds in fertilisers.

Physical Properties and Trends

• Group 1 elements are soft, low-density metals with grey, shiny surfaces at room temperature; they float on water.
• Their melting and boiling points are low compared with many other metals.
• Down the group, atomic radius increases while melting and boiling points decrease.
• Increasing atomic size weakens attraction between metal atoms, so less heat is needed to overcome it.

Element	Atomic radius (nm)	Melting / boiling point (${}^{\circ }\mathrm{C}$)
Li	0.133	$186/1342$
Na	0.186	$98/880$
K	0.203	$64/760$

Chemical Properties

• Each atom has one valence electron, donates it during a reaction and forms a $+1$ ion:

• With water, an alkaline metal hydroxide solution and hydrogen gas form:

• In oxygen, a white metal oxide forms; the oxide produces an alkaline solution in water:

• In chlorine, a white metal chloride forms:

Experiment 4.1: Reactions with Water, Oxygen and Chlorine

Aim: Study the chemical properties of Group 1 elements.

Problem statement: What are the chemical properties of Group 1 elements when they react with water, oxygen and chlorine?

Materials: Lithium, sodium, potassium, distilled water, filter paper, red litmus paper, oxygen gas and chlorine gas.

Apparatus: Forceps, white tile, basin, knife, combustion spoon, lidded gas jars, $10\mathrm{c}{\mathrm{m}}^3$ measuring cylinder and Bunsen burner.

Water reaction — teacher demonstration

• Hypothesis: Reactivity with water increases down Group 1.
• Manipulated variable: type of alkali metal; responding variable: reactivity with water; fixed variable: metal size.

1. Cut a small piece of lithium with a knife and forceps; dry it on filter paper.
2. Place it slowly into a basin of water.
3. After the reaction, test the solution with red litmus paper and record observations.
4. Repeat with sodium and potassium.

Oxygen reaction

1. Cut and dry a small piece of lithium, place it on a combustion spoon and heat until it burns.
2. Insert it immediately into a gas jar of oxygen.
3. After the reaction, add $10.0\mathrm{c}{\mathrm{m}}^3$ water, shake and test with red litmus paper.
4. Record observations and repeat with sodium and potassium.

Chlorine reaction

1. Cut and dry a small piece of lithium, place it on a combustion spoon and heat until it burns.
2. Insert it immediately into a gas jar of chlorine and record observations.
3. Repeat with sodium and potassium.

Safety: Use only small quantities and take care when placing alkali metals in water.

Conclusion: Lithium, sodium and potassium have similar chemical properties, but their reactivity increases in the order $\mathrm{Li}$ < $\mathrm{Na}$ < $\mathrm{K}$.

Reactivity Trend and Other Group 1 Elements

• Reactivity increases down Group 1 because more occupied shells increase atomic size.
• The valence electron is farther from the nucleus and experiences weaker nuclear attraction, so it is donated more easily.
• Rubidium, caesium and francium are soft, shiny metals with low melting and boiling points. They are very reactive with water and oxygen.
• Rubidium and caesium burn readily and usually occur combined with other elements, making chemical isolation difficult.
• Francium is an unstable radioactive isotope with a short half-life.