Properties and Chemical Reactivity — Revision Notes

⚡ 30-Second Revision

Electronic Configuration — $[Noble Gas]ns^1$
Ionization Enthalpy — Lowest in period, decreases down group ($Li > Na > K > Rb > Cs$)
Atomic/Ionic Radii — Increases down group
Density — Increases down group (Exception: $K < Na$)
Melting/Boiling Points — Decreases down group
Reactivity — Increases down group
Reducing Power (Aqueous) — $Li > Cs > Rb > K > Na$ (due to high hydration enthalpy of $Li^+$)
Flame Colors — Li (crimson), Na (golden yellow), K (lilac), Rb (red-violet), Cs (sky blue)
Reaction with $O_2$ — $Li \rightarrow Li_2O$ (oxide), $Na \rightarrow Na_2O_2$ (peroxide), $K, Rb, Cs \rightarrow MO_2$ (superoxide)
Reaction with $H_2O$ — $2M + 2H_2O \rightarrow 2MOH + H_2$ (reactivity increases down group)
Reaction with $N_2$ — Only $Li$ forms $Li_3N$ directly
Liquid $NH_3$ — Deep blue solution (ammoniated electrons), strong reducing agent

2-Minute Revision

Alkali metals (Group 1) are highly reactive due to their single valence electron ($ns^1$) and very low ionization enthalpies, which decrease down the group. This makes them strong reducing agents and highly electropositive.

Atomic and ionic radii increase down the group, while melting points and electronegativity decrease. Density generally increases, but Potassium is less dense than Sodium. They exhibit characteristic flame colors: Lithium (crimson), Sodium (golden yellow), Potassium (lilac), Rubidium (red-violet), Caesium (sky blue).

Their reactions with oxygen vary: Lithium forms normal oxide ($Li_2O$), Sodium forms peroxide ($Na_2O_2$), and Potassium, Rubidium, Caesium form superoxides ($MO_2$). They react vigorously with water, forming hydroxides and hydrogen, with reactivity increasing down the group.

Lithium shows anomalous behavior due to its small size, including direct reaction with nitrogen to form $Li_3N$ and forming more covalent compounds. In aqueous solution, Lithium is the strongest reducing agent due to its exceptionally high hydration enthalpy, despite having the highest ionization enthalpy.

5-Minute Revision

Alkali metals are Group 1 elements with an $ns^1$ electronic configuration, making them highly reactive. Their key properties stem from the ease of losing this single valence electron. Ionization enthalpy is lowest in their respective periods and decreases down the group ($Li > Na > K > Rb > Cs$), leading to increasing metallic character and reactivity.

Atomic and ionic radii increase down the group, while melting and boiling points decrease due to weakening metallic bonds. Density generally increases, but Potassium is an important exception, being less dense than Sodium.

They are powerful reducing agents. In aqueous solution, Lithium is the strongest reducing agent ($Li > Cs > Rb > K > Na$) because the high hydration enthalpy of its small $Li^+$ ion compensates for its higher ionization enthalpy.

They react vigorously with air, forming different oxides: $Li$ forms $Li_2O$, $Na$ forms $Na_2O_2$, and $K, Rb, Cs$ form $MO_2$. This trend is due to the stabilization of larger anions by larger cations.

With water, they produce hydroxides and hydrogen gas, with reactivity increasing down the group. They react with hydrogen to form ionic hydrides ($MH$) and with halogens to form ionic halides ($MX$). A unique reaction is their dissolution in liquid ammonia to form deep blue, conducting solutions due to ammoniated electrons ($[e(NH_3)_y]^-$).

Lithium exhibits anomalous behavior due to its small size and high polarizing power, such as forming $Li_3N$ directly with nitrogen, having more covalent compounds, and showing a diagonal relationship with Magnesium.

Remember these trends, exceptions, and specific reactions for NEET.

Prelims Revision Notes

Alkali Metals: Properties & Reactivity (NEET Revision)

1. General Characteristics:

Electronic Configuration: — $[Noble Gas]ns^1$. One valence electron.
Oxidation State: — Always +1.
Metallic Character: — Highly metallic, electropositive. Increases down the group.
Physical State: — Soft, silvery-white solids. Low melting/boiling points. Low densities.

2. Trends in Properties (Li to Cs):

Atomic/Ionic Radii: — Increases.
Ionization Enthalpy ($IE_1$): — Decreases (lowest in period). Order: $Li > Na > K > Rb > Cs$.
Electronegativity: — Decreases.
Melting/Boiling Points: — Decreases (due to weaker metallic bonding).
Density: — Generally increases. Exception: $K < Na$.
Reducing Power (Gaseous): — Increases (Cs > Rb > K > Na > Li).
Reducing Power (Aqueous): — Exception: $Li > Cs > Rb > K > Na$ (due to high hydration enthalpy of $Li^+$).

3. Chemical Reactivity:

Reactivity: — Very high, increases down the group.
Reaction with Air/Oxygen: — Tarnish rapidly. Form different oxides:

* $Li$: Forms normal oxide ($Li_2O$). $4Li + O_2 \rightarrow 2Li_2O$ * $Na$: Forms peroxide ($Na_2O_2$). $2Na + O_2 \rightarrow Na_2O_2$ * $K, Rb, Cs$: Form superoxides ($MO_2$). $M + O_2 \rightarrow MO_2$ (Reason: Stabilization of larger anions by larger cations).

Reaction with Water: — Violent, forms hydroxide and hydrogen gas. Reactivity increases down group.

* $2M + 2H_2O \rightarrow 2MOH + H_2$

Reaction with Hydrogen: — Forms ionic hydrides ($MH$) at $673,\text{K}$.

* $2M + H_2 \rightarrow 2MH$

Reaction with Halogens: — Forms ionic halides ($MX$).

* $2M + X_2 \rightarrow 2MX$

Reaction with Liquid Ammonia: — Dissolve to form deep blue, conducting solutions.

* $M + (x+y)NH_3 \rightarrow [M(NH_3)_x]^+ + [e(NH_3)_y]^-$ (ammoniated electron responsible for color). * At high concentration, solution becomes bronze and diamagnetic (electron clusters).

4. Anomalous Behavior of Lithium:

Reasons: — Smallest size, high polarizing power, high charge density, absence of d-orbitals.
Key Differences:

* Harder, higher MP/BP than other alkali metals. * Forms only $Li_2O$ with oxygen. * Reacts directly with $N_2$ to form $Li_3N$. (Other alkali metals don't). * More covalent compounds, less soluble (e.g., $LiCl$ soluble in organic solvents). * Less reactive with water.

Diagonal Relationship: — Shows similarities with Magnesium (Mg) due to similar charge/radius ratio (e.g., forms nitrides, relatively insoluble carbonates/hydroxides).

5. Flame Coloration:

Mechanism: — Excitation of valence electron by flame heat, followed by de-excitation and emission of characteristic light.
Colors: — Li (crimson red), Na (golden yellow), K (lilac/pale violet), Rb (red-violet), Cs (sky blue).

Vyyuha Quick Recall

To remember the flame colors of alkali metals: Little Naughty Kids Run Crazy.

Lithium: Crimson Red
Natrium (Sodium): Golden Yellow
Kalium (Potassium): Lilac (Pale Violet)
Rubidium: Red-Violet
Caesium: Sky Blue