Group 1: Alkali Metals — Overview
The s-block elements include Groups 1 and 2. Group 1 metals (Li, Na, K, Rb, Cs, Fr) are called alkali metals because they form strongly alkaline hydroxides in water. Group 2 metals (Be, Mg, Ca, Sr, Ba, Ra) are called alkaline earth metals.
General electronic configuration of Group 1: [Noble gas] ns¹.
These metals are highly reactive because they can easily lose their outermost s electron to form stable +1 cations (Group 1) or +2 cations (Group 2). They are strong reducing agents.
Trends in Properties of Alkali Metals
Click an element to see its flame colour, wavelength, and key notes.
Li
Na
K
Rb
Cs
Ca
Sr
Ba
Click an element above to see its flame colour.
Compare properties across Group 1 and Group 2 elements. Click any property header to see the trend explanation.
Li
Lithium
Na
Sodium
K
Potassium
Rb
Rubidium
Cs
Caesium
Atomic radius
152 pm (smallest)
186 pm
227 pm
248 pm
265 pm (largest G1)
IE₁
520 kJ/mol (highest in G1)
496 kJ/mol
419 kJ/mol
403 kJ/mol
376 kJ/mol (lowest G1)
Reaction with H₂O
Burns in water, less vigorous than Na
Vigorous, floats, catches fire
Very vigorous, lilac flame
Very vigorous
Explosive
Nature of oxide
Li₂O (normal oxide)
Na₂O (normal); Na₂O₂ in excess O₂
K₂O (normal); KO₂ (superoxide)
Rb₂O; RbO₂ (superoxide)
Cs₂O; CsO₂ (superoxide)
Hydroxide solubility
LiOH: slightly soluble
NaOH: highly soluble
KOH: highly soluble
RbOH: highly soluble
CsOH: highly soluble
Flame test colour
Crimson red
Golden yellow
Lilac/violet
Red-violet
Blue-violet
Click any property row to see the trend explanation.
| Property | Trend Down the Group | Explanation |
|---|---|---|
| Atomic radius | Increases Li → Cs | More electron shells added |
| Ionisation enthalpy (IE₁) | Decreases Li → Cs | Valence electron farther from nucleus, more shielded |
| Electronegativity | Decreases Li → Cs | Larger atomic radius; weaker nuclear pull |
| Melting/boiling point | Decreases Li → Cs | Metallic bond weakens (larger atomic radius → weaker overlap) |
| Density | Generally increases Li → Cs (Li, Na less dense than water) | Li (0.53), Na (0.97), K (0.86), Rb (1.53), Cs (1.93) g/cm³ |
| Reactivity | Increases Li → Cs | IE decreases; easier to lose electron |
| Hydration energy | Decreases Li → Cs | Smaller ions are more hydrated; Li⁺ most hydrated |
| Ionic radius | Increases Li⁺ → Cs⁺ | More shells; Li⁺ is actually the smallest alkali metal ion |
Flame colours: Li (crimson red), Na (golden yellow), K (violet/lilac), Rb (red-violet), Cs (blue). These are used in flame tests to identify alkali metals.
Reactions of Alkali Metals
Reaction with Water
All alkali metals react with water to produce metal hydroxide and H₂:
Reactivity increases down the group: Li (slow, bubbles), Na (vigorous, melts), K (burns with violet flame), Rb and Cs (explosive).
Reaction with Oxygen
- Li + O₂ → Li₂O (lithium oxide, normal oxide)
- 2Na + O₂ → Na₂O₂ (sodium peroxide) — predominant product on burning in excess O₂
- K + O₂ → KO₂ (potassium superoxide)
- Rb and Cs also form superoxides (RbO₂, CsO₂).
On reaction with water: Na₂O₂ + H₂O → NaOH + H₂O₂; KO₂ + H₂O → KOH + H₂O₂ + O₂.
Reaction with Hydrogen
Ionic hydrides react with water to liberate H₂: NaH + H₂O → NaOH + H₂.
Reaction with Halogens
Anomalous Properties of Lithium and Diagonal Relationship
Lithium has the smallest ionic radius and highest charge density in Group 1. This makes its behaviour quite different from the rest of the group.
How Lithium Differs from Other Alkali Metals
- Lithium forms a normal oxide (Li₂O) on burning, not a peroxide or superoxide.
- Lithium reacts directly with N₂ to form lithium nitride: 6Li + N₂ → 2Li₃N.
- Lithium salts are often hydrated (due to high hydration energy of small Li⁺).
- LiF and Li₂CO₃ are less soluble than the corresponding Na, K salts (due to high lattice energy of small Li⁺).
- Lithium hydroxide (LiOH) decomposes on heating: 2LiOH → Li₂O + H₂O. Other alkali metal hydroxides are stable to heat.
- Li₂CO₃ decomposes on heating: Li₂CO₃ → Li₂O + CO₂. Other alkali carbonates (Na₂CO₃, K₂CO₃) do not decompose on normal heating.
- LiNO₃ on heating gives Li₂O + NO₂ + O₂. NaNO₃, KNO₃ give metal nitrite and O₂ (not the oxide).
Diagonal Relationship: Li and Mg
Li (Group 1, Period 2) resembles Mg (Group 2, Period 3) more than it resembles Na:
- Both form normal oxides (not peroxides) when burned.
- Both react with N₂ to form nitrides (Li₃N and Mg₃N₂).
- Salts of both have low solubility in water for the same anion (e.g., LiF ≈ MgF₂ in solubility).
- Both Li and Mg are converted to the element directly from their carbides (not by electrolysis of fused halide for Li in some methods).
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Group 2: Alkaline Earth Metals — Overview
Group 2 elements (Be, Mg, Ca, Sr, Ba, Ra) have the configuration [Noble gas] ns². They lose 2 electrons to form M²⁺ ions. They are harder, denser, and have higher melting points than the corresponding Group 1 metals because M²⁺ has a higher charge and smaller size than M⁺, giving stronger metallic bonding.
Flame colours: Ca (brick red), Sr (crimson red), Ba (apple green), Mg (no characteristic flame in normal flame test — burns with brilliant white light).
Trends in Properties of Alkaline Earth Metals
| Property | Trend Down Group 2 |
|---|---|
| Atomic radius | Increases Be → Ba |
| Ionisation enthalpy (IE₁ + IE₂) | Decreases Be → Ba (Ba most reactive) |
| Melting/boiling point | Generally decreases, but Mg and Ca have higher mp than Na and K |
| Reactivity with water | Increases Be → Ba; Be does not react, Mg reacts with hot water, Ca-Ba react with cold water |
| Basic strength of hydroxides | Increases Be(OH)₂ (amphoteric) → Ba(OH)₂ (strongly basic) |
| Solubility of hydroxides | Increases: Be(OH)₂ almost insoluble → Ba(OH)₂ soluble (barium hydroxide is a strong base used in analysis) |
| Solubility of sulphates | Decreases: MgSO₄ soluble → BaSO₄ almost insoluble (used as test for Ba²⁺/SO₄²⁻) |
Reactions of Alkaline Earth Metals
Reaction with Water
- Be: does not react with water (thin oxide layer prevents reaction).
- Mg: reacts slowly with cold water, readily with hot water or steam: Mg + 2H₂O → Mg(OH)₂ + H₂.
- Ca, Sr, Ba: react vigorously with cold water: Ca + 2H₂O → Ca(OH)₂ + H₂.
Reaction with Oxygen
All form ionic oxides MO (except Be and Mg which also form covalent compounds). Ca, Sr, Ba form peroxides at high pressure of O₂.
Reaction with Acids
All Group 2 metals dissolve in dilute acids (except Be which dissolves in concentrated HCl and HNO₃):
Anomalous Properties of Beryllium
- Be does not react with water or steam (other Group 2 metals do).
- Be is amphoteric: dissolves in both acids and alkalis. Be + 2NaOH → Na₂BeO₂ + H₂ (forms beryllate).
- BeCl₂ is covalent, acts as a Lewis acid (has an empty p orbital).
- BeO is amphoteric; other Group 2 oxides are basic.
- Be does not form a peroxide (unlike Ca, Sr, Ba).
Diagonal relationship: Be and Al. Both are amphoteric (dissolve in alkalis). Both form hydrated salts. Both chlorides are covalent and Lewis acids. BeF₂ and AlF₃ have high melting points for a different reason (more ionic character due to F).
Important Compounds of s-Block Elements
Sodium Hydroxide (NaOH) — Caustic Soda
Prepared by electrolysis of brine (chlor-alkali process):
Uses: soap making, paper, textiles, petroleum refining, manufacturing of compounds.
Sodium Carbonate (Na₂CO₃) — Washing Soda
Washing soda is Na₂CO₃·10H₂O. It is prepared by the Solvay process (removed from NEET 2027 syllabus as an industrial process, but its properties are still relevant). Na₂CO₃ is used as a cleansing agent, water softening, and in glass and paper industries.
Na₂CO₃ is a salt of strong base and weak acid, so its aqueous solution is alkaline.
Sodium Bicarbonate (NaHCO₃) — Baking Soda
Prepared by passing CO₂ through a solution of NH₃ in brine:
Uses: baking, mild antacid, fire extinguisher (dry powder types), effervescent salts.
Decomposes on heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂.
Calcium Oxide (CaO) — Quicklime
Slaking of lime: CaO + H₂O → Ca(OH)₂ + heat (exothermic).
Calcium Hydroxide (Ca(OH)₂) — Slaked Lime
Sparingly soluble. Used in whitewashing walls, treating acidic soils, water treatment. Lime water turns milky with CO₂: Ca(OH)₂ + CO₂ → CaCO₃↓ + H₂O. Excess CO₂: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (milkiness disappears).
Plaster of Paris (CaSO₄·½H₂O)
Setting: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (gypsum). Slight expansion on setting. Used in surgery for bone fractures, dentistry, construction.
Magnesium Sulphate (MgSO₄·7H₂O) — Epsom Salt
Used as a laxative and in leather tanning.
Biological Importance
| Element | Biological Role |
|---|---|
| Na⁺ | Controls blood pressure, fluid balance; main extracellular cation |
| K⁺ | Nerve impulse transmission, enzyme activation; main intracellular cation |
| Mg²⁺ | Component of chlorophyll; activates enzymes in photosynthesis |
| Ca²⁺ | Bone and teeth structure (as Ca₃(PO₄)₂), blood clotting, muscle contraction, nerve function |
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Worked NEET Problems
NEET-style problem · Reactions with Oxygen
Question
Which of the following products forms when sodium burns in excess oxygen?
Solution
When sodium burns in excess oxygen, it forms sodium peroxide (Na₂O₂), not the simple oxide (Na₂O):
Lithium forms the normal oxide (Li₂O). Potassium, Rb, Cs form superoxides (KO₂ etc). This is a classic NEET question.
NEET-style problem · Anomalous Properties of Li
Question
Lithium differs from other Group 1 metals in reacting with N₂. What product forms and why is this important?
Solution
6Li + N₂ → 2Li₃N (lithium nitride). Lithium is the only alkali metal that reacts directly with atmospheric N₂ at room temperature. This is because Li has the smallest size and highest charge density in Group 1, which allows strong covalent interaction with N₂. This diagonal relationship with Mg (which also reacts with N₂ to form Mg₃N₂) is why Li is sometimes called the "honorary Group 2 metal."
NEET-style problem · Plaster of Paris
Question
What is the chemical formula of Plaster of Paris? Write the reaction for its setting.
Solution
Plaster of Paris: CaSO₄·½H₂O (calcium sulphate hemihydrate).
Setting reaction:
The reaction is slightly exothermic and involves a slight expansion in volume, which is why it gives a sharp impression of the mould.
Summary Cheat Sheet
| Concept | Key Point |
|---|---|
| Group 1 config | [Noble gas] ns¹; form M⁺ ions |
| Group 1 reactivity | Increases Li → Cs; all react with water → MOH + H₂ |
| Products with O₂ | Li → Li₂O; Na → Na₂O₂; K, Rb, Cs → MO₂ (superoxide) |
| Li unique reactions | Reacts with N₂ (gives Li₃N); LiOH/Li₂CO₃ decompose on heating |
| Li diagonal relationship | Li resembles Mg (same charge density, covalent compounds, N₂ reaction) |
| Flame colours | Li: crimson red; Na: golden yellow; K: violet; Ca: brick red; Sr: crimson; Ba: green |
| Group 2 config | [Noble gas] ns²; form M²⁺ ions |
| Be anomalous | Does not react with water; amphoteric (reacts with acids and alkalis); diagonal relationship with Al |
| Sulphate solubility Group 2 | MgSO₄ soluble → BaSO₄ insoluble (decreases down group) |
| Hydroxide solubility Group 2 | Be(OH)₂ almost insoluble → Ba(OH)₂ soluble (increases down group) |
| NaOH preparation | Chlor-alkali process: electrolysis of brine; also gives Cl₂ and H₂ |
| NaHCO₃ (baking soda) | Decomposition on heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ |
| Plaster of Paris | CaSO₄·½H₂O; sets to gypsum (CaSO₄·2H₂O) by taking up water |
| Lime water + CO₂ | Turns milky (CaCO₃); excess CO₂ clears (Ca(HCO₃)₂ forms) |
Frequently asked questions
Why does lithium behave differently from other Group 1 metals?
Lithium is the smallest and has the highest charge density (charge/size ratio) in Group 1. Its small size gives it a high polarising power, so many of its compounds are covalent rather than ionic (e.g., LiCl is more covalent than NaCl). Li forms a nitride (Li₃N) with nitrogen directly, which other alkali metals do not. It also forms only the monoxide (Li₂O) on burning in air, whereas Na forms Na₂O₂ (peroxide) and K forms KO₂ (superoxide). Lithium's properties resemble magnesium more than sodium — this is the diagonal relationship.
Why does beryllium differ from the rest of Group 2?
Beryllium (Be) has the smallest size and highest charge density in Group 2. Its compounds are largely covalent (e.g., BeCl₂). It does not react with water at room temperature or even at 100 °C (other Group 2 metals do). Be does not react with nitrogen. It dissolves in both acids and alkalis (amphoteric), unlike other Group 2 metals which react with acids only. Be follows a diagonal relationship with aluminium (Al). BeO and Al₂O₃ are both amphoteric oxides.
How does reactivity with water change down Group 1 and Group 2?
Group 1: All react with water, and reactivity increases down the group: Li reacts slowly, Na vigorously (melts and moves on water), K burns with violet flame, Rb and Cs react explosively. Group 2: Reactivity also increases down the group, but more gently than Group 1. Be does not react with water even at high temperature (due to oxide layer). Mg reacts only with hot water or steam. Ca, Sr, Ba react with cold water, with increasing vigour down the group.
What is the diagonal relationship?
A diagonal relationship refers to the similarity between an element in one group and the element in the next group of the next period (diagonally adjacent). This happens because moving one period down increases atomic size, while moving one group to the right in the same period decreases it; these two changes roughly cancel out, giving similar charge density and polarising power. Examples: Li resembles Mg; Be resembles Al; B resembles Si.
What is Plaster of Paris and how does it set?
Plaster of Paris is calcium sulphate hemihydrate: CaSO₄·½H₂O. It is made by heating gypsum (CaSO₄·2H₂O) to 120–130 °C: 2CaSO₄·2H₂O → 2CaSO₄·½H₂O + 3H₂O. When Plaster of Paris is mixed with water, it reabsorbs water and sets (hardens) to form gypsum: CaSO₄·½H₂O + 1½H₂O → CaSO₄·2H₂O (gypsum). The slight expansion during setting makes it ideal for moulds and for setting bones.
Why is sodium kept under kerosene oil?
Sodium is a very reactive metal. It reacts vigorously with atmospheric moisture (water) to form NaOH and H₂ gas (a fire hazard), and it also reacts with oxygen to form Na₂O₂. To protect it from both water vapour and oxygen, sodium is stored under kerosene oil. Kerosene is a non-polar liquid that does not react with sodium and provides a non-aqueous, non-reactive barrier.
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