Sodium

Sodium (Na) is the most commonly examined Group I (alkali metal) in 0620. It is the “middle” alkali metal of the Li -> Na -> K trend, so it appears whenever the syllabus tests reactivity increasing down a group.

Where sodium appears in 0620

• Group I trends: sodium is the benchmark between gentle lithium and violent potassium.
• Electrolysis: molten NaCl (to extract sodium) and the electrolysis of brine (to make chlorine, hydrogen and NaOH).
• Qualitative analysis: the yellow flame test, and NaOH as the reagent for identifying metal cations.
• Reactivity series: sodium is far above carbon, so it cannot be extracted by reduction with carbon.

Position in the periodic table

• Symbol: Na (from Latin natrium)
• Atomic number: 11
• Group 1 (I), Period 3
• Electron configuration: 2, 8, 1
• Forms Na+ ions by losing 1 electron

Physical properties

Property	Detail
Appearance	Soft, silver when freshly cut; tarnishes rapidly
Melting point	98 C (low for a metal)
Density	0.97 g/cm3 (floats on water)
Hardness	Soft enough to cut with a knife

These properties are typical of alkali metals: low melting point, low density, soft.

Reaction with water

2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g)

Observations:

• Sodium floats on the water (low density)
• Melts into a shiny ball (the reaction is exothermic)
• Moves rapidly across the surface (pushed by hydrogen gas)
• Fizzing/effervescence (hydrogen released)
• May ignite with a yellow flame if the piece is large
• The solution becomes alkaline (NaOH formed), turning universal indicator purple

Sodium reacts more vigorously than lithium but less vigorously than potassium. Reactivity increases down Group I because the outer electron is further from the nucleus, more shielded, and therefore lost more easily.

Flame test

Sodium gives a persistent yellow/orange flame. See flame tests.

Key compounds

Compound	Formula	Notes
Sodium hydroxide	NaOH	Strong alkali, used to test for metal cations
Sodium chloride	NaCl	Common salt; electrolysis of brine
Sodium carbonate	Na2CO3	Washing soda, glass making
Sodium hydrogen carbonate	NaHCO3	Baking soda, antacid

Sodium hydroxide

Sodium hydroxide (NaOH) is a strong alkali used throughout qualitative analysis to identify metal cations by the colour of the hydroxide precipitate. See tests for ions.

Sodium chloride

Sodium chloride (NaCl) is obtained by evaporating sea water or mining rock salt. Electrolysis of brine (concentrated NaCl solution) produces chlorine, hydrogen and sodium hydroxide.

Storage and extraction

Sodium is stored under oil to keep air and water away; it tarnishes within seconds when exposed. Because it is far above carbon in the reactivity series, carbon cannot reduce its compounds, so the metal is obtained by electrolysis of molten sodium chloride.

Key facts

• Symbol: Na — proton number: 11
• Position: Group I, Period 3
• Electron configuration: 2, 8, 1 (forms Na+)
• Key property: soft, low density (0.97 g/cm3), floats on water
• Flame colour: yellow/orange
• Main use in the lab: NaOH is the cation-test reagent; brine electrolysis is a major industrial process

Common exam mistakes

• Leaving the water equation unbalanced. It is 2Na + 2H2O -> 2NaOH + H2, not Na + H2O -> NaOH + H2.
• Writing sodium oxide as the water product. With water the product is sodium hydroxide (NaOH); Na2O forms only when sodium reacts with oxygen in air.
• Saying sodium sinks. Its density is below 1 g/cm3, so it floats and darts across the surface.
• Forgetting the cobalt-glass point. The strong yellow flame can hide other flame colours, so a blue cobalt glass is used to view them.

Worked exam questions

Describe what you would see when a small piece of sodium is added to water. (4 marks)

Mark scheme

• Sodium floats on the water [1]
• It melts into a ball / moves rapidly across the surface [1]
• Fizzing / effervescence / bubbles of gas [1]
• Sodium gets smaller and eventually disappears / dissolves [1]

Examiner note: these are observations only. Do not include “hydrogen is produced” as a seen point — you observe bubbles, you infer the gas.

Explain why potassium reacts more vigorously with water than sodium. (3 marks)

Mark scheme

• Potassium is below sodium in Group I / has more electron shells [1]
• Its outer electron is further from the nucleus / more shielded from the nuclear charge [1]
• The outer electron is lost more easily, so the reaction is faster [1]

Examiner note: the argument must be about losing the single outer electron. Answers about “gaining electrons” describe non-metals and score nothing here.