Lithium

Lithium (Li) is the lightest and least reactive of the Group I alkali metals studied at IGCSE. It is the baseline of the reactivity trend: understand why lithium is slowest and the whole Li -> Na -> K pattern follows.

Where lithium appears in 0620

• Group I trends: lithium is the “least reactive” end of the comparison, used to explain why reactivity increases down the group.
• Qualitative analysis: the red flame test for lithium compounds.
• Reactivity series: as an alkali metal, lithium is still very high and cannot be extracted with carbon.

Position in the periodic table

• Symbol: Li
• Atomic number: 3
• Group 1 (I), Period 2
• Electron configuration: 2, 1
• Forms Li+ ions by losing 1 electron

Physical properties

Property	Detail
Appearance	Silver-white, soft
Melting point	181 C (highest of the Group I metals studied)
Density	0.53 g/cm3 (lightest metal; floats on water and oil)
Hardness	Soft, can be cut with a knife

Lithium follows the Group I pattern of a soft, low-density metal, though within the group it has the highest melting point and lowest density. It is the only metal with a density less than half that of water.

Reaction with water

2Li(s) + 2H2O(l) -> 2LiOH(aq) + H2(g)

Observations:

• Lithium floats on the water
• Steady fizzing/effervescence (hydrogen gas released)
• Moves gently on the surface (it does not melt into a ball)
• Gradually gets smaller and dissolves
• The solution becomes alkaline (lithium hydroxide, LiOH), turning universal indicator blue/purple

This is the gentlest of the three Group I reactions. Compare: sodium melts into a ball and darts about; potassium ignites with a lilac flame and may spit or explode.

Why lithium is least reactive in Group I

1. Lithium has only 2 electron shells (2, 1)
2. Its outer electron is closest to the nucleus
3. There is minimal shielding from inner electrons
4. The attraction between the nucleus and outer electron is strongest
5. Most energy is therefore needed to remove that outer electron
6. So lithium loses its electron — and reacts — most slowly

Flame test

Lithium gives a red (crimson) flame. See flame tests.

Alkali metal	Flame colour
Lithium	Red
Sodium	Yellow
Potassium	Lilac

Key compounds

Compound	Formula	Notes
Lithium hydroxide	LiOH	Alkali, formed in the water reaction
Lithium chloride	LiCl	White solid, soluble
Lithium oxide	Li2O	White solid, basic oxide

Key facts

• Symbol: Li — proton number: 3
• Position: Group I, Period 2
• Electron configuration: 2, 1 (forms Li+)
• Key property: the lightest metal (0.53 g/cm3); least reactive alkali metal studied
• Flame colour: red/crimson
• Water product: lithium hydroxide, an alkali (2Li + 2H2O -> 2LiOH + H2)

Storage and modern use

Like all alkali metals, lithium is stored under oil to keep out air and moisture, though its lower reactivity makes it safer to handle than sodium or potassium. Beyond the syllabus, lithium’s low density and readiness to form Li+ make it central to the rechargeable batteries in phones, laptops and electric vehicles.

Common exam mistakes

• Thinking a smaller atom means more reactive. In Group I the reverse is true: lithium’s small size holds the outer electron tightly, making it the least reactive.
• Confusing the flame colours. Lithium is red; sodium is yellow; potassium is lilac. Do not swap lithium and sodium.
• Writing the hydroxide wrongly. It is LiOH (Li+ with OH-), and the balanced equation is 2Li + 2H2O -> 2LiOH + H2.
• Describing a violent reaction. Lithium fizzes gently and does not melt into a ball — that vigorous behaviour belongs to sodium and potassium.

Worked exam questions

Describe what you would observe when lithium is added to water. (3 marks)

Mark scheme

• Lithium floats on the water [1]
• Steady effervescence / fizzing / bubbles of gas [1]
• Lithium gradually gets smaller and disappears / dissolves [1]

Examiner note: keep it to observations. “Moves gently” is accepted; “darts about” or “melts into a ball” describes sodium and would be marked wrong.

Lithium reacts less vigorously with water than potassium. Explain why in terms of electronic structure. (3 marks)

Mark scheme

• Lithium has fewer electron shells than potassium (2 vs 4) [1]
• Its outer electron is closer to the nucleus / less shielded [1]
• That electron is held more strongly and harder to lose, so lithium reacts less vigorously [1]

Examiner note: compare the two metals explicitly. A description of lithium alone, with no reference to shells or shielding, will not reach full marks.