The S-Block Elements (Anomalous Properties Of Lithium)

Anomalous Properties Of Lithium

Lithium (Li), the first element of Group 1, exhibits several properties that are different from the rest of the alkali metals. This deviation from the general trend is known as anomalous behavior. These differences are primarily attributed to lithium's smaller atomic and ionic size, and its higher electronegativity compared to other alkali metals.

Points Of Difference Between Lithium And Other Alkali Metals

Lithium behaves much less typically than other alkali metals due to its unique electronic structure and physical characteristics:

1. Atomic and Ionic Size: Lithium has the smallest atomic radius and ionic radius ($Li^+$) among the alkali metals. This leads to a higher charge density.
2. Ionization Enthalpy: Lithium has a higher first ionization enthalpy compared to other alkali metals. This means it requires slightly more energy to remove its valence electron.
3. Electronegativity: Lithium has the highest electronegativity (0.98 on the Pauling scale) among the alkali metals. This makes its compounds, particularly halides, exhibit some covalent character.
4. Hydration Enthalpy: Due to its small size and high charge density, $Li^+$ ion has a very high hydration enthalpy, the highest among alkali metal ions. This strong hydration influences many of its properties, like the solubility of its salts and its unusually high melting point for an alkali metal.
5. Reactivity with Air: Lithium reacts slowly with oxygen in air to form the normal oxide ($Li_2O$), whereas heavier alkali metals form peroxides ($Na_2O_2$) and superoxides ($KO_2$, etc.).
6. Reaction with Water: Lithium reacts moderately with water, whereas sodium, potassium, etc., react much more vigorously.
7. Formation of Nitrides: Lithium is the only alkali metal that combines directly with nitrogen from the air to form lithium nitride ($Li_3N$).
8. Carbonate Stability: Lithium carbonate ($Li_2CO_3$) is sparingly soluble in water and decomposes easily on heating to $Li_2O$ and $CO_2$. Other alkali metal carbonates are highly soluble and thermally stable.
9. Bicarbonate Formation: Lithium does not form a stable hydrogencarbonate ($LiHCO_3$).
10. Solubility of Halides: Lithium halides (except $LiF$) are more soluble in organic solvents than in water, indicating a greater covalent character in their bonding, unlike other alkali metal halides which are predominantly ionic and soluble in water.
11. Melting and Boiling Points: Lithium has higher melting and boiling points compared to other alkali metals. This is attributed to its smaller size and stronger lattice structure in the solid state.

Points Of Similarities Between Lithium And Magnesium

Lithium shows diagonal relationships with Magnesium (Mg), the second element of Group 2. This similarity in properties arises because of the similar charge density ($charge/size$ ratio) of $Li^+$ and $Mg^{2+}$ ions.

• Similar Ionic Size: $Li^+$ (ionic radius $\approx$ 76 pm) and $Mg^{2+}$ (ionic radius $\approx$ 72 pm) have similar ionic radii.
• Similar Charge Density: Both ions have a similar charge density.
• Similar Electronegativity: They have relatively similar electronegativity values compared to other members of their respective groups.

Similarities in Properties:

1. Hardness: Both Lithium and Magnesium are harder than other alkali metals and alkaline earth metals, respectively.
2. Melting and Boiling Points: Both have higher melting and boiling points compared to other elements in their respective groups.
3. Reaction with Air: Both react with air to form nitrides as well as oxides.
• Li: $6Li + N_2 \rightarrow 2Li_3N$
• Mg: $3Mg + N_2 \rightarrow Mg_3N_2$
4. Reactivity with Water: Both react moderately with water.
5. Carbonates: Both lithium and magnesium carbonates ($Li_2CO_3$ and $MgCO_3$) are sparingly soluble in water and decompose on heating to form oxides and $CO_2$.

$Li_2CO_3 \xrightarrow{heat} Li_2O + CO_2$

$MgCO_3 \xrightarrow{heat} MgO + CO_2$

6. Chlorides: Both lithium chloride ($LiCl$) and magnesium chloride ($MgCl_2$) are soluble in water and also soluble in organic solvents like ethanol, indicating covalent character.
7. Hydroxides: Both lithium hydroxide ($LiOH$) and magnesium hydroxide ($Mg(OH)_2$) are weak bases and are sparingly soluble in water.

These similarities highlight the importance of factors like ionic size and charge density in determining the chemical behavior of elements, leading to diagonal relationships in the periodic table.