Group I properties
| Melting Point | As the atom gets larger, the metallic bonding between the positive ions and the sea of delocalized electrons becomes weaker because the outer electrons are further from the nucleus. Less energy is needed to break these bonds. | Decreases |
|---|---|---|
| Density | The mass of the atoms increases more rapidly than their volume as you go down the group, resulting in a higher mass per unit volume. | Increases |
| Reactivity | The single outer electron is further from the nucleus and shielded by more inner shells. This makes it easier to lose the electron, which is required for chemical reactions. | Increases |
| Softness | Due to the weaker metallic bonding (as explained in Melting Point), the layers of atoms can slide over each other more easily. Lithium is hard enough to cut with a knife, but caesium is so soft it can be shaped by hand. | Becomes Softer |
Flame Test Colors: Each alkali metal produces a characteristic flame color when heated. This is a key physical property used for identification:
- Lithium (Li): Crimson red
- Sodium (Na): Yellow (or orange-yellow)
- Potassium (K): Lilac (or pale violet)
2M_{(s)} + 2H_2O_{(l)} \rightarrow 2MOH_{(aq)} + H_{2(g)}
Where M represents any Group I element (e.g., Li, Na, or K).
Specific Example for Lithium:
2Li_{(s)} + 2H_2O_{(l)} \rightarrow 2LiOH_{(aq)} + H_{2(g)}
Observations:
- The solid floats (density is less than water).
- It fizzes or effervesces rapidly due to hydrogen gas bubbles.
- The solid moves around on the surface.
- The solid melts into a ball/sphere (due to heat of reaction and low melting point).
- The solid eventually disappears.
Comparison with Potassium:
Potassium is below sodium in the group, so it is more reactive. When potassium reacts with water:
- It fizzes very rapidly.
- It melts instantly into a ball.
- The hydrogen gas ignites immediately, producing a lilac flame.
Correct Understanding: While melting point decreases, density increases. This is because the increase in atomic mass outweighs the increase in atomic volume. Always check the specific trend for each property.
Correct Understanding: For potassium, you must mention the lilac flame. This distinguishes it from lithium (crimson) and sodium (yellow/orange). If you only say 'bubbles form', you miss the key identifying feature of potassium.
Why examiners accept this: Examiners look for logical extrapolation of trends. You must explicitly state that the prediction is based on the position in the group (e.g., 'Since Rb is below K...').
Correct Usage Example:
'Rubidium is below Potassium in Group I. Therefore, its melting point will be lower than 63°C (the MP of K), and its reactivity will be higher than potassium.'
Why examiners accept this: Examiners require specific physical changes, not just chemical outcomes. Words like 'dissolves' are incorrect because the metal turns into a hydroxide solution; the metal itself disappears by reacting, not dissolving.
Correct Usage Example:
'Use effervescence or bubbles form to describe gas production. Use melts into a ball to describe the phase change. Do not say 'dissolves'.'
(Reasoning: MP decreases down the group, so K's MP is lower than Li's. Density increases down the group, so K's density is higher than Li's.)
Any two from:
- Sodium floats on the surface.
- Bubbles/fizzing/effervescence are observed.
- The solid melts into a ball/sphere.
- The solid moves around on the surface.
- The solid disappears/dissolves (in the sense of reacting away).
A) Melting point increases down the group.
B) Density decreases down the group.
C) Reactivity increases down the group.
D) They become harder down the group.