Both lithium and magnesium display several similar properties due to the diagonal relationship; however, the one which is incorrect is-

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Chemistry

Alkali Metals and Properties

Compounds of Alkali Metals

Li and Be Anomalous Behaviour

Question

Both lithium and magnesium display several similar properties due to the diagonal relationship; however, the one which is incorrect is-

both form basic carbonates

nitrates of both Li and Mg yield NO₂ and O₂ on heating

both form nitrides

both form soluble bicarbonates

LiHCO₃ & Mg(HCO₃)₂ both are soluble

6Li +N₂ $\to_{}^{}$ 2Li₃N

3Mg + N₂ $\to_{}^{}$ Mg₃N₂

2LiNO₃ $\overset{Δ}{\to}$ Li₂O + NO₂ + $\frac{1}{2}$ O₂

Mg(NO₃)₂ $\overset{Δ}{\to}$ MgO + 2NO₂ + O₂

Mg forms basic carbonate having formula 4MgCO₃. Mg(OH)₂.6H₂O (white ppt.)

where as Li does not

The diagonal relationship between lithium (Li) and magnesium (Mg) is a phenomenon in the periodic table where elements in the second period show similarities with elements in the third period of the next group. This occurs due to similar ionic sizes and charge/radius ratios.

Let's evaluate each option:

Option 1: Both form basic carbonates
Magnesium forms a basic carbonate, $Mg {(OH)}_{2} \cdot MgC O_{3}$ . However, lithium does not form a basic carbonate; it only forms the normal carbonate, $L i_{2} C O_{3}$ , which is not basic. This property is incorrect for lithium.

Option 2: Nitrates of both Li and Mg yield NO₂ and O₂ on heating
Both lithium nitrate and magnesium nitrate decompose on heating to give nitrogen dioxide and oxygen: $4 LiN O_{3} \to 2 L i_{2} O + 4 N O_{2} + O_{2}$ $2 Mg (N O_{3})_{2} \to 2 MgO + 4 N O_{2} + O_{2}$ This is correct.

Option 3: Both form nitrides
Both lithium and magnesium react directly with nitrogen to form nitrides: $6 Li + N_{2} \to 2 L i_{3} N$ $3 Mg + N_{2} \to M g_{3} N_{2}$ This is correct.

Option 4: Both form soluble bicarbonates
Unlike other alkali metals, lithium bicarbonate is not stable in solid form and exists only in solution. Magnesium bicarbonate is also soluble and exists in solution. Both form soluble bicarbonates, so this is correct.

Therefore, the incorrect property is that both form basic carbonates, as lithium does not form a basic carbonate.

Formulae

Decomposition of nitrates:

Lithium nitrate: $4 LiN O_{3} \to 2 L i_{2} O + 4 N O_{2} + O_{2}$

Magnesium nitrate: $2 Mg (N O_{3})_{2} \to 2 MgO + 4 N O_{2} + O_{2}$

Nitride formation:

Lithium: $6 Li + N_{2} \to 2 L i_{3} N$

Magnesium: $3 Mg + N_{2} \to M g_{3} N_{2}$

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