Group 1 Elements: Alkali Metals — Revision Notes

⚡ 30-Second Revision

General electronic configuration: $[Noble\,gas]ns^1$
Oxidation state: +1
Atomic/Ionic Radii: Increase down group
Ionization Enthalpy: Decreases down group (very low)
Hydration Enthalpy: Decreases down group ($Li^+ > Na^+ > K^+ > Rb^+ > Cs^+$)
Density: Increases down group (K < Na exception)
Melting/Boiling Points: Decrease down group (weak metallic bonding)
Reactivity: Increases down group
Flame colors: Li - Crimson, Na - Golden Yellow, K - Lilac, Rb - Red-violet, Cs - Blue
Li anomalous properties: Smallest, highest IE, highest hydration enthalpy, forms $Li_3N$, forms $Li_2O$, hydrated salts, strongest reducing agent in aqueous solution, diagonal relationship with Mg.
Reactions with $O_2$: $Li \rightarrow Li_2O$, $Na \rightarrow Na_2O_2$, $K, Rb, Cs \rightarrow MO_2$
Reactions with $H_2O$: $2M + 2H_2O \rightarrow 2MOH + H_2$ (vigor increases down group)
Reactions with $NH_3(l)$: Deep blue solution (ammoniated electrons), paramagnetic, forms amides on standing.

2-Minute Revision

Alkali metals (Group 1: Li, Na, K, Rb, Cs, Fr) are highly reactive s-block elements with a general electronic configuration of $ns^1$. They readily lose this single valence electron to form $M^+$ ions, exhibiting a +1 oxidation state and strong electropositive character.

Key trends include increasing atomic/ionic radii and decreasing ionization enthalpy, hydration enthalpy, and melting/boiling points down the group. Reactivity with air and water increases significantly from Li to Cs.

Lithium forms monoxide ($Li_2O$), Sodium forms peroxide ($Na_2O_2$), while Potassium, Rubidium, and Caesium form superoxides ($MO_2$) with oxygen. They impart characteristic colors to a flame (Li-crimson, Na-golden yellow, K-lilac), a crucial identification test.

Lithium shows anomalous behavior due to its small size and high polarizing power, forming a stable nitride ($Li_3N$) and hydrated salts, and acting as the strongest reducing agent in aqueous solution.

It also exhibits a diagonal relationship with Magnesium.

5-Minute Revision

Group 1 elements, the alkali metals, are characterized by their $ns^1$ valence electron configuration, making them highly electropositive and reactive. Their tendency to lose this electron results in low ionization enthalpies, which decrease down the group (Li to Cs) due to increasing atomic size and shielding effect.

This trend also explains the increase in metallic character and reactivity. Conversely, hydration enthalpy decreases down the group, with $Li^+$ having the highest due to its small size and high charge density, making Lithium the strongest reducing agent in aqueous solution despite its high ionization enthalpy.

They have low melting and boiling points, decreasing down the group due to weakening metallic bonds.

Their reactions with oxygen are varied: Lithium forms normal oxide ($Li_2O$), Sodium forms peroxide ($Na_2O_2$), and heavier alkali metals (K, Rb, Cs) form superoxides ($MO_2$). All react vigorously with water to form hydroxides and hydrogen gas, with reactivity increasing down the group. They also react with hydrogen to form hydrides ($MH$) and dissolve in liquid ammonia to produce deep blue, conductive solutions containing ammoniated electrons.

Lithium exhibits anomalous behavior due to its exceptionally small size and high polarizing power. This leads to unique properties like forming a stable nitride ($Li_3N$) directly with nitrogen, forming hydrated salts (e.

g., $LiCl \cdot 2H_2O$), and having a diagonal relationship with Magnesium, sharing similarities in properties like forming insoluble carbonates and nitrides. Flame tests are vital for identification: Li (crimson red), Na (golden yellow), K (lilac), Rb (red-violet), Cs (blue).

Caesium and Rubidium are used in photoelectric cells due to their very low ionization enthalpies.

Prelims Revision Notes

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Electronic Configuration: — General form is $[Noble\,gas]ns^1$. This single valence electron is easily lost, leading to +1 oxidation state.
2
Atomic & Ionic Radii: — Increase down the group (Li to Cs) due to increasing number of shells.
3
Ionization Enthalpy (IE): — Very low, decreases down the group. $Li > Na > K > Rb > Cs$. This makes them strong reducing agents.
4
Hydration Enthalpy: — Decreases down the group. $Li^+ > Na^+ > K^+ > Rb^+ > Cs^+$. $Li^+$ is most hydrated due to smallest size/highest charge density.
5
Reducing Power: — In gaseous state, decreases down group ($Li < Na < K < Rb < Cs$). In aqueous solution, $Li$ is the strongest reducing agent due to high hydration enthalpy of $Li^+$.
6
Density: — Generally increases down the group, but $K < Na$ is an exception.
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Melting & Boiling Points: — Low, decrease down the group due to weak metallic bonding.
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Flame Coloration:

* Lithium (Li): Crimson Red * Sodium (Na): Golden Yellow * Potassium (K): Lilac (Pale Violet) * Rubidium (Rb): Red-violet * Caesium (Cs): Blue

1
Reactions with Oxygen:

* $Li \rightarrow Li_2O$ (monoxide) * $Na \rightarrow Na_2O_2$ (peroxide) * $K, Rb, Cs \rightarrow MO_2$ (superoxide)

1
Reactions with Water: — $2M + 2H_2O \rightarrow 2MOH + H_2$. Reactivity increases down the group (Li gentle, Cs explosive).
2
Reactions with Hydrogen: — Form ionic hydrides ($MH$) at $673K$.
3
Reactions with Halogens: — Form ionic halides ($MX$). Reactivity increases down the group.
4
Reactions with Liquid Ammonia: — Dissolve to form deep blue, conductive, paramagnetic solutions due to ammoniated electrons ($[e(NH_3)_y]^-$). Decompose to amides ($MNH_2$) and $H_2$.
5
Anomalous Behavior of Lithium:

* Smallest size, highest IE, highest electronegativity among alkali metals. * Forms $Li_3N$ directly with $N_2$. * Forms only $Li_2O$ with $O_2$. * Less reactive with water than Na. * Salts are often hydrated (e.g., $LiCl \cdot 2H_2O$). * Diagonal relationship with Magnesium (Mg): similar hardness, forms nitrides, similar solubility of fluorides and carbonates.

1
Uses: — Li in batteries, Na in street lamps/coolant, K in fertilizers, Cs in photoelectric cells.

Vyyuha Quick Recall

For Flame Colors: Little Crimson Naked Gorillas Kiss Lilacs. (Li-Crimson, Na-Golden, K-Lilac)