Group I properties Revision Notes

Lithium (Li), sodium (Na), and potassium (K) are alkali metals located in Group I of the periodic table. These metals share common physical properties: they are soft enough to be cut easily with a knife, have a shiny metallic appearance when freshly cut, and are good conductors of electricity.

The softness of these metals increases down the group. Lithium is the hardest among the three, sodium is softer, and potassium is the softest. This softness is related to the metallic bonding strength, which decreases as the atomic size increases down the group.

• All are soft metals, easily cut with a knife.
• Freshly cut surfaces are shiny but tarnish quickly in air.
• Softness increases from lithium to potassium.

The melting points of alkali metals decrease as you move down the group. Lithium has the highest melting point among the three, followed by sodium, then potassium with the lowest melting point.

This trend occurs because the metallic bonding becomes weaker down the group. As the atomic radius increases, the single outer electron is further from the nucleus and less strongly attracted, reducing the strength of the metallic bond and thus lowering the melting point.

• Melting point decreases: Li > Na > K.
• Weaker metallic bonding down the group causes lower melting points.
• Potassium melts at a temperature low enough to be near room temperature (63.5 °C).

Density is defined as mass per unit volume (density = mass/volume). As you move down Group I from lithium to potassium, the density increases. This is because the atomic mass increases more rapidly than the atomic volume.

Lithium has the lowest density, sodium is denser, and potassium is the densest among the three. However, potassium’s density is still relatively low compared to many other metals, which is why these alkali metals can float on water.

• Density increases: Li < Na < K.
• Atomic mass increases faster than atomic volume down the group.
• All three metals have low densities and can float on water.

Reactivity of alkali metals increases down the group. Potassium is more reactive than sodium, which is more reactive than lithium.

This is because the outer electron is more easily lost as the atomic radius increases and the attraction between the nucleus and the outer electron decreases. This makes it easier for the atom to form a positive ion (M⁺) and react with other substances such as water.

The reaction with water produces a metal hydroxide and hydrogen gas. The general equation is: M (s) + H₂O (l) → MOH (aq) + ½ H₂ (g) (where M is the alkali metal). For example, sodium reacts with water as follows: 2Na (s) + 2H₂O (l) → 2NaOH (aq) + H₂ (g)

• Reactivity increases: Li < Na < K.
• Outer electron is lost more easily down the group.
• Reacts vigorously with water to form hydroxides and hydrogen gas.
• Example: 2Na + 2H₂O → 2NaOH + H₂

Given the trend in reactivity, predict which metal reacts most vigorously with water: lithium, sodium, or potassium.

Since reactivity increases down Group I, potassium reacts most vigorously, followed by sodium, then lithium.

This is confirmed by observations: potassium reacts rapidly with water, often igniting the hydrogen gas produced; sodium reacts less vigorously but still produces hydrogen gas and a strong alkali solution; lithium reacts slowly and less vigorously.

• Potassium is most reactive with water among Li, Na, K.
• Observation: potassium may ignite hydrogen gas during reaction.
• Sodium reacts moderately; lithium reacts slowly.

Group I elements are soft metals with low densities compared to other metals. Lithium, sodium, and potassium are less dense than water and can float on it. As you move down the group from lithium to francium, the softness increases, making the metals easier to cut.

Melting and boiling points decrease down the group. For example, lithium melts at 180.5°C, sodium at 97.8°C, and potassium at 63.5°C. This trend continues with rubidium and caesium having even lower melting points. The decrease is due to weaker metallic bonding as atomic size increases, reducing the attraction between positive ions and delocalised electrons.

• Softness increases down the group.
• Density generally increases down the group but remains low compared to many metals.
• Melting and boiling points decrease down the group.

Group I elements are highly reactive metals that readily lose their single outer electron to form +1 ions. Reactivity increases down the group because the outer electron is further from the nucleus and more shielded by inner electrons, making it easier to lose.

For example, lithium reacts slowly with cold water, sodium reacts more vigorously, and potassium reacts even more rapidly, often producing enough heat to ignite the hydrogen gas released.

• Reactivity increases down the group due to decreasing ionisation energy.
• All Group I metals react with water to form hydroxides and hydrogen gas.
• Reactions become more vigorous down the group.

When Group I metals react with cold water, they form metal hydroxides and hydrogen gas. The general equation is:

2M(s) + 2H₂O(l) → 2MOH(aq) + H₂(g) (where M = Group I metal)

For example, sodium reacts as: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g) Observations include fizzing (hydrogen gas), the metal moving on the water surface, and the solution becoming alkaline due to the hydroxide formed.

• Fizzing due to hydrogen gas release.
• Metal floats and moves on water surface.
• Solution turns alkaline (pH > 7).

Group I metals react with oxygen to form oxides, peroxides, or superoxides depending on the metal and conditions. Lithium forms lithium oxide (Li₂O), sodium forms sodium peroxide (Na₂O₂), and potassium forms potassium superoxide (KO₂).

Balanced equations: 4Li(s) + O₂(g) → 2Li₂O(s) 2Na(s) + O₂(g) → Na₂O₂(s) K(s) + O₂(g) → KO₂(s)

These oxides are ionic compounds and react with water to form alkaline solutions.

• Different oxides form depending on the metal.
• Oxides react with water to form hydroxides.
• Oxides are ionic solids.

Based on the trends observed in lithium, sodium, and potassium, predictions for rubidium, caesium, and francium include:

Increased softness and lower melting points than potassium.

Higher densities than potassium due to larger atomic mass and closer packing despite softness increase (e.g., caesium density ~1.9 g/cm³). Greater reactivity, reacting more vigorously with water and oxygen, often explosively. Formation of superoxides more readily than peroxides or oxides. These predictions rely on the periodic trend of increasing atomic radius and decreasing ionisation energy down the group.

• Softness and reactivity increase down the group.
• Melting points continue to decrease.
• Oxide types shift towards superoxides.
• Density generally increases but remains low compared to many metals.