Detailed Discussions On Lithium

Q: Why does lithium show anomalous behaviour?

Lithium differs from the other alkali metals because its atom and ion are exceptionally small and its polarising power is high. The small ion distorts large anions, making its compounds partly covalent and unstable to heat, holds water strongly so its salts are hydrated, and forms very stable lattices with small anions such as oxide and nitride.

Q: Why does lithium resemble magnesium?

The lithium ion, 76 pm, and the magnesium ion, 72 pm, are almost the same size, and their atomic radii, 152 and 160 pm, are also close. Similar size gives similar polarising power and lattice energies, so both metals form nitrides, give only the normal oxide, have unstable carbonates and form covalent, hydrated chlorides.

Q: Why is lithium the only alkali metal that forms a nitride?

Forming the nitride ion needs a lot of energy to break the nitrogen triple bond and add three electrons. Only a very small cation gives a lattice energy large enough to repay this. The lithium ion is small enough, like the magnesium ion, while sodium and the larger alkali ions are not, so they do not react with nitrogen.

Q: Why is LiCl soluble in organic solvents?

The small lithium ion polarises the larger chloride ion and pulls electron density towards itself, so the lithium chlorine bond has partial covalent character. Covalent substances dissolve in organic solvents, so lithium chloride dissolves in ethanol, acetone, ethyl acetate and even pyridine, while sodium chloride, which is fully ionic, does not.

Q: Why is the hydrated lithium ion the least mobile alkali metal ion?

The bare lithium ion is the smallest, so it attracts water most strongly and carries the thickest shell of water molecules. In solution the whole hydrated ion must move, and with a hydrated radius of about 382 pm it is the largest of the group. It therefore moves slowest and has the lowest ionic conductance.

Q: What are the main uses of lithium?

Lithium is used in rechargeable lithium ion batteries because it is the lightest metal with the most negative electrode potential. Its alloys with lead make white metal bearings, with aluminium make aircraft parts and with magnesium make armour plates. It is also used in thermonuclear reactions and to prepare lithium aluminium hydride.

Q: How is the lithium diagonal relationship tested in JEE Advanced?

JEE Advanced usually asks multiple correct or matching questions: which properties lithium shares with magnesium, which statements about lithium are wrong, the products of heating lithium nitrate or carbonate, and the moles of ammonia from lithium or magnesium nitride. The distractors are the normal sodium behaviours, so learn both columns side by side.

Q: Which lithium equations should I know for JEE Advanced?

For JEE Advanced learn five: lithium with nitrogen giving lithium nitride, lithium nitride with water giving lithium hydroxide and ammonia, heating lithium carbonate to lithium oxide and carbon dioxide, heating lithium nitrate to lithium oxide, nitrogen dioxide and oxygen, and lithium hydride with aluminium chloride giving lithium aluminium hydride.

ChemistryThe s-Block ElementsFor JEE aspirants

Lithium is the first alkali metal, and it breaks almost every Group 1 rule. Its atom and ion are exceptionally small and its polarising power is high, so its compounds are more covalent, more hydrated and less stable to heat than those of sodium or potassium. In these ways lithium resembles magnesium, the diagonal neighbour in Group 2. This page lists every point of difference and similarity, explains each from size and polarisation, and covers lithium's reactions and uses. JEE Advanced tests lithium's anomalies mostly through statement and matching questions.

On this page1Why lithium differs2Diagonal relationship3Differences from Na, K4Hydration and mobility5Covalent character6Similarities with Mg7Reactions8Uses9Solved examples

Key Formulas - Quick Reference

★ Must learnCause of every anomaly: very small $L i^{+}$ (76 pm) and high polarising power (charge/radius).
★ Must learnDiagonal pair: Li-Mg by similar size (Li 152, Mg 160 pm; $L i^{+}$ 76, $M g^{2 +}$ 72 pm).
★ Must learnNitride: $6Li + N_{2} \to 2L i_{3} N$ ; $L i_{3} N + 3 H_{2} O \to 3LiOH + N H_{3}$ (Mg: $M g_{3} N_{2} + 6 H_{2} O \to 3Mg (OH)_{2} + 2N H_{3}$ ).
★ Must learnHeat: $L i_{2} C O_{3} \to L i_{2} O + C O_{2}$ ; $4LiN O_{3} \to 2L i_{2} O + 4N O_{2} + O_{2}$ (others give nitrite); no solid $LiHC O_{3}$ .
Burning: Li gives mainly $L i_{2} O$ and $L i_{3} N$ ; never a superoxide. Li gives no ethynide with ethyne.
★ Must learnHydration: LiCl $\cdot$ 2H $_{2}$ O; mobility $L i^{+} < N a^{+} < K^{+}$ (Li $^{+}$ most hydrated).
Covalent character: LiCl soluble in ethanol, acetone, pyridine; LiI more soluble than KI in ethanol; LiF, $L i_{2} O$ poorly soluble in water.
Lithium aluminium hydride: $4LiH + AlC l_{3} \to LiAl H_{4} + 3LiCl$ (in dry ether).

1. Why Lithium Is Different

The anomalous behaviour of lithium has two causes: (i) the exceptionally small size of its atom and ion, and (ii) its high polarising power, the ability of a cation to distort the electron cloud of a nearby anion, which grows with charge/radius. A small, polarising cation pulls electron density back from an anion and gives the bond partial covalent character. This explains why lithium compounds dissolve in organic solvents, why lithium salts of large anions break down on heating, and why $L i^{+}$ binds water so strongly.

Diagonal relationship: the similarity between the first element of a group and the second element of the next group (Li and Mg, Be and Al, B and Si). Moving right across a period makes an ion smaller and more highly charged; moving down a group makes it larger. Along a diagonal the two changes cancel, so the ions end up with similar size or similar charge/radius ratio.

Figure 1: The diagonal relationship. Li resembles Mg because

L i^{+}

(76 pm) and

M g^{2 +}

(72 pm) are almost the same size; Be and Al pair up through similar charge/radius ratio.

Figure 2: In size lithium sits next to magnesium, not next to sodium (NCERT: Li 152, Mg 160 pm;

L i^{+}

76,

M g^{2 +}

72 pm), so its compounds behave like those of magnesium.

Key idea

Lithium is anomalous because $L i^{+}$ is tiny and strongly polarising; it resembles Mg because

M g^{2 +}

is almost exactly the same size.

2. Points of Difference from the Other Alkali Metals

Figure 3: Lithium's differences from its own group. In every row lithium gives the oxide, the nitride or a hydrate, because its tiny ion polarises large anions and holds water.

Harder, higher melting. Lithium is much harder than the other alkali metals, and its m.p. (454 K) and b.p. (1615 K) are the highest in the group, because its small atoms give the strongest metallic bonding.
Least reactive, yet the strongest reducing agent. Lithium reacts most slowly with water, but its $E^{\circ}$ ( $- 3.04$ V) is the most negative of all the alkali metals, because $L i^{+}$ has the largest hydration enthalpy.
Oxide and nitride on burning. In air lithium forms mainly the monoxide $L i_{2} O$ and the nitride $L i_{3} N$ , unlike the other alkali metals.
Hydrated, deliquescent chloride. LiCl is deliquescent and crystallises as LiCl $\cdot$ 2H $_{2}$ O; the other alkali metal chlorides form no hydrates.
No solid hydrogencarbonate. $LiHC O_{3}$ is not obtained in the solid form, while all the other alkali metals form solid hydrogencarbonates.
No ethynide. Unlike the other alkali metals, lithium forms no ethynide with ethyne (sodium gives $2Na + 2HC \equiv CH \to 2HC \equiv CNa + H_{2}$ ).
Nitrate gives the oxide. $4LiN O_{3} \to 2L i_{2} O + 4N O_{2} + O_{2}$ , whereas $2NaN O_{3} \to 2NaN O_{2} + O_{2}$ .
Poorly soluble fluoride and oxide. LiF and $L i_{2} O$ are much less soluble in water than the corresponding compounds of the other alkali metals (very high lattice enthalpy of small ions).

Exam Trick Big anion + tiny Li $^{+}$ = trouble. Whenever the anion is large and polarisable (

C O_{3}^{2 -}

N O_{3}^{-}

HC O_{3}^{-}

O_{2}^{2 -}

I^{-}

), lithium either decomposes the salt to

L i_{2} O

or makes it covalent. Small anions (

F^{-}

O^{2 -}

N^{3 -}

) give lithium very stable, poorly soluble solids.

2.1 Hydration and ionic mobility

Because $L i^{+}$ is so small, it holds a large shell of water molecules. Lithium salts are therefore commonly hydrated, while those of the other alkali ions are usually anhydrous. The same water shell makes the moving ion in solution large and slow: the hydrated $L i^{+}$ ion is the biggest and least mobile of the group.

Figure 4:

L i^{+}

binds four water molecules tightly in a tetrahedron. This strong hydration is why lithium salts are commonly hydrated and those of the other alkali metals are not (NCERT 10.22).

Figure 5: Hydrated radii after Nightingale.

L i^{+}

drags the most water, so it is the slowest ion (38.7 S cm

^{2}

mol

^{- 1}

). Mobility rises to

K^{+}

;

R b^{+}

and

C s^{+}

are almost equal (the NCERT order

L i^{+} < N a^{+} < K^{+} < R b^{+} < C s^{+}

is the expected answer).

2.2 Covalent character and solubility

LiF is almost insoluble in water, whereas LiCl is soluble not only in water but also in acetone. In LiF both ions are small, so the lattice enthalpy is very high (1036 kJ mol $^{- 1}$ ) and the bond is essentially ionic. The larger $C l^{-}$ ion is polarised by $L i^{+}$ , so LiCl has a lower lattice enthalpy (853 kJ mol $^{- 1}$ ) and partial covalent character, which lets it dissolve in organic solvents. For the same reason LiI is more soluble than KI in ethanol: LiI is largely covalent, while KI is ionic.

Figure 6: LiF has too much lattice enthalpy to dissolve; LiCl has less, and its partial covalent character (Fajans polarisation) also lets it dissolve in acetone (NCERT 10.23).

Key idea

Every difference from sodium comes back to one of three things: polarisation of large anions, hydration of the small ion, or the huge lattice enthalpy of small ions (

L i_{2} O

L i_{3} N

, LiF).

Quick Recall: tap to check

Why is

L i_{2} C O_{3}

decomposed at a lower temperature than

N a_{2} C O_{3}

L i^{+}

polarises the large carbonate ion and the product

L i_{2} O

has a very high lattice enthalpy;

N a^{+}

is too large to do either.

Which alkali metal ion has the largest hydrated radius?

L i^{+}

(about 382 pm against 329 pm for

C s^{+}

Which lithium compound gives ammonia with water?

L i_{3} N + 3 H_{2} O \to 3LiOH + N H_{3}

3. Similarities between Lithium and Magnesium

The similarity between lithium and magnesium is particularly striking and arises from their similar sizes: atomic radii Li 152 pm, Mg 160 pm; ionic radii $L i^{+}$ 76 pm, $M g^{2 +}$ 72 pm. The main points of similarity are:

Figure 7: Lithium and magnesium, row by row. Learn the two nitride equations in full; they are the most examined pair.

Both are harder and lighter than the other elements of their groups.
Both react slowly with water. Their oxides and hydroxides are much less soluble, and the hydroxides decompose on heating ( $2LiOH \to L i_{2} O + H_{2} O$ ; $Mg (OH)_{2} \to MgO + H_{2} O$ ).
Both form nitrides by direct combination with nitrogen: $6Li + N_{2} \to 2L i_{3} N$ , $3Mg + N_{2} \to M g_{3} N_{2}$ . Both nitrides are hydrolysed to ammonia.
The oxides $L i_{2} O$ and MgO do not combine with excess oxygen to give a superoxide.
The carbonates decompose easily on heating to the oxide and $C O_{2}$ ; solid hydrogencarbonates are not formed by either metal.
Both LiCl and $MgC l_{2}$ are soluble in ethanol, are deliquescent, and crystallise from water as hydrates, LiCl $\cdot$ 2H $_{2}$ O and $MgC l_{2} \cdot 6 H_{2} O$ .

L i_{3} N + 3 H_{2} O 3LiOH + N H_{3}

M g_{3} N_{2} + 6 H_{2} O 3Mg (OH)_{2} + 2N H_{3}

JEE Advanced

Why only Li (in Group 1) and Mg form nitrides. Making $N^{3 -}$ is very costly: the $N \equiv N$ bond (946 kJ mol $^{- 1}$ ) must break, and adding three electrons to N is strongly endothermic overall. The cost is repaid only by the lattice enthalpy, which is huge only when the cation is small (or highly charged). $L i^{+}$ and $M g^{2 +}$ qualify; $N a^{+}$ and the larger ions do not. The same reasoning explains why $L i_{2} O$ , not a peroxide, is lithium's main oxide, and why LiH is the most stable alkali metal hydride.

Consequences examiners like: $L i_{3} N$ (ruby red) and $M g_{3} N_{2}$ (yellow-green) both give $N H_{3}$ with water; a mixture of Mg ash from burning in air therefore smells of ammonia when moistened. Per mole of nitride, $L i_{3} N$ gives 1 mol $N H_{3}$ and $M g_{3} N_{2}$ gives 2 mol.

Exam Trick Lithium is magnesium in a Group 1 coat. If a statement is true for Mg (nitride, oxide only, carbonate breaks down, no solid bicarbonate, chloride in ethanol, hydrated chloride), it is true for Li and false for Na and K.

Lithium vs sodium: differsOxide

L i_{2} O

(not peroxide); forms

L i_{3} N

; nitrate → oxide; no solid

LiHC O_{3}

; hydrated, covalent chloride; slowest with water.

Lithium vs magnesium: alikeBoth form nitrides; both carbonates → oxide; neither has a solid hydrogencarbonate; both chlorides dissolve in ethanol and are deliquescent.

Key idea

Li and Mg share size, so they share chemistry: nitrides, simple oxides, unstable carbonates, covalent and hydrated chlorides.

Quick Recall: tap to check

Name two compounds that both Li and Mg form by direct combination but Na does not.

The nitrides,

L i_{3} N

and

M g_{3} N_{2}

(and neither forms a superoxide).

Why are solid

LiHC O_{3}

and

Mg (HC O_{3})_{2}

not known?

The small cations polarise the hydrogencarbonate ion so strongly that it breaks down; the salts exist only in solution.

4. Reactions and Uses of Lithium

Lithium reacts with the same reagents as the other alkali metals, but it is the least reactive of the group. Two reactions are special: it combines with nitrogen even at room temperature, and its hydride is the starting point for the powerful reducing agent lithium aluminium hydride.

Figure 8: Lithium's own reactions. Two are unique in Group 1: it reacts with

N_{2}

even at room temperature, and its hydride LiH is used to make the reducing agent

LiAl H_{4}

4LiH + AlC l_{3} dry ether LiAl H_{4} + 3LiCl

Lithium is stored in paraffin wax or petroleum jelly rather than kerosene, because it is so light (0.53 g cm $^{- 3}$ ) that it floats on kerosene.

4.1 Uses

Alloys: with lead to make 'white metal' bearings for motor engines, with aluminium for aircraft parts, and with magnesium for armour plates.
Electrochemical cells: lithium and lithium-ion batteries, because lithium is the lightest metal and has the most negative $E^{\circ}$ .
Thermonuclear reactions: lithium is used in thermonuclear reactions (a source of tritium).
Reagents: $LiAl H_{4}$ and organolithium compounds in organic synthesis.

Figure 9: In a lithium-ion cell only

L i^{+}

moves between the electrodes. Lithium's low mass and very negative

E^{\circ}

give the highest energy per gram of any cell metal.

4.2 Spotting a lithium anomaly

Figure 10: Three checks find every lithium anomaly: a large anion (polarisation), water (hydration), or

N_{2}

O_{2}

(lattice energy of small ions).

Figure 11: Lithium on one page. Every branch goes back to the first one: a tiny ion with a high polarising power.

5. Solved Examples

Solved Example 1

Which statement about lithium is incorrect?
(A) It combines directly with nitrogen.
(B) Its nitrate on heating gives

LiN O_{2}

and

O_{2}

.
(C) LiCl is soluble in ethanol.
(D)

L i_{2} C O_{3}

decomposes on heating.

Solution:

Answer: (B). Lithium nitrate gives the oxide: $4LiN O_{3} \to 2L i_{2} O + 4N O_{2} + O_{2}$ . Only the nitrates of Na to Cs give the nitrite.

Solved Example 2

Which properties are shown by both lithium and magnesium? (One or more options.)
(A) Direct formation of a nitride
(B) Formation of a solid hydrogencarbonate
(C) Chloride soluble in ethanol
(D) Carbonate decomposes to the oxide on heating

Solution:

Answer: (A), (C), (D). Neither lithium nor magnesium forms a solid hydrogencarbonate, so (B) is wrong.

Solved Example 3

3.48 g of lithium nitride is treated with excess water. What mass and what volume (at STP, 22.7 L mol

^{- 1}

) of ammonia is formed? (Li = 6.94, N = 14.01)

Solution:

$L i_{3} N + 3 H_{2} O \to 3LiOH + N H_{3}$ . $M (L i_{3} N) = 3 (6.94) + 14.01 = 34.83$ g mol $^{- 1}$ .

$n = \frac{3.48}{34.83} = 0.100$ mol, so $N H_{3}$ = 0.100 mol = 1.70 g = $0.100 \times 22.7$ = 2.27 L.

Solved Example 4

How many of the following are true for lithium but not for sodium? (Integer answer.) (i) forms a nitride on burning in air; (ii) chloride crystallises as a hydrate; (iii) reacts with water to give hydrogen; (iv) forms a superoxide in excess oxygen; (v) nitrate gives the oxide on heating; (vi) carbonate decomposes on heating.

Solution:

(i), (ii), (v) and (vi) are lithium-only. (iii) is true for both; (iv) is true for neither (sodium gives a peroxide).

Answer: 4.

Solved Example 5

Arrange

L i^{+}

N a^{+}

and

K^{+}

in increasing order of (a) ionic radius, (b) hydrated radius, (c) ionic mobility in water.

Solution:

(a) $L i^{+} < N a^{+} < K^{+}$ (76, 102, 138 pm).

(b) $K^{+} < N a^{+} < L i^{+}$ : the smallest ion is the most hydrated.

Solved Example 6

The diagonal relationship between Li and Mg is mainly due to
(A) similar electronic configuration
(B) similar ionic size
(C) same number of valence electrons
(D) same oxidation state

Solution:

Answer: (B). $L i^{+}$ (76 pm) and $M g^{2 +}$ (72 pm) are almost the same size. Their configurations, valence electrons and oxidation states all differ.

Practice Questions

In what ways does lithium show similarities to magnesium in its chemical behaviour? (NCERT 10.7)Answer: Both form nitrides directly, give only the normal oxide, have carbonates that decompose to the oxide, form no solid hydrogencarbonates, have hydroxides that decompose on heating, and have deliquescent chlorides soluble in ethanol.
Why is $L i_{2} C O_{3}$ decomposed at a lower temperature whereas $N a_{2} C O_{3}$ at a higher temperature? (NCERT 10.14)Answer: $L i^{+}$ polarises $C O_{3}^{2 -}$ and forms the very stable $L i_{2} O$ ; the larger $N a^{+}$ cannot, so $N a_{2} C O_{3}$ is stable.
Why are lithium salts commonly hydrated and those of the other alkali ions usually anhydrous? (NCERT 10.22)Answer: $L i^{+}$ is the smallest ion with the highest hydration enthalpy ( $- 506$ kJ mol $^{- 1}$ ), so it keeps water in its crystals.
Why is LiF almost insoluble in water whereas LiCl is soluble not only in water but also in acetone? (NCERT 10.23)Answer: LiF has a very high lattice enthalpy (small ions); LiCl has a lower lattice enthalpy and partial covalent character from polarisation of Cl-, so it dissolves in water and acetone.
Comment: lithium is the only alkali metal to form a nitride directly. (NCERT 10.26b)Answer: Only the small $L i^{+}$ gives a lattice enthalpy with $N^{3 -}$ large enough to pay for breaking $N \equiv N$ and forming $N^{3 -}$ .
Explain: LiI is more soluble than KI in ethanol. (NCERT 10.29 iii)Answer: $L i^{+}$ polarises the large $I^{-}$ , so LiI is largely covalent and dissolves in the organic solvent; KI is ionic.
Which one of the following alkali metals gives hydrated salts? (a) Li (b) Na (c) K (d) Cs (NCERT 10.31)Answer: (a) Li.

Common Mistakes to Avoid

Watch out

Explaining the diagonal relationship of Li and Mg by 'similar electronic configuration'. The reason is similar ionic size.
Writing $LiN O_{3}$ → $LiN O_{2}$ + $O_{2}$ . Lithium nitrate gives $L i_{2} O$ , $N O_{2}$ and $O_{2}$ .
Giving $M g_{3} N_{2}$ + water only 1 mol of $N H_{3}$ : it gives 2 mol, while $L i_{3} N$ gives 1 mol.
Calling lithium the most reactive alkali metal because it is the strongest reducing agent. It is the least reactive.
Saying $L i^{+}$ is the most mobile ion because it is the smallest. Its hydrated ion is the largest and slowest.
Writing LiCl as an ionic salt insoluble in organic solvents. It dissolves in ethanol, acetone and pyridine.
Assuming lithium forms a superoxide or peroxide as its main product in excess oxygen, like potassium.
Storing lithium in kerosene: it floats; it is kept in paraffin wax or petroleum jelly.

Frequently Asked Questions

Why does lithium show anomalous behaviour?

Lithium differs from the other alkali metals because its atom and ion are exceptionally small and its polarising power is high. The small ion distorts large anions, making its compounds partly covalent and unstable to heat, holds water strongly so its salts are hydrated, and forms very stable lattices with small anions such as oxide and nitride.

Why does lithium resemble magnesium?

The lithium ion, 76 pm, and the magnesium ion, 72 pm, are almost the same size, and their atomic radii, 152 and 160 pm, are also close. Similar size gives similar polarising power and lattice energies, so both metals form nitrides, give only the normal oxide, have unstable carbonates and form covalent, hydrated chlorides.

Why is lithium the only alkali metal that forms a nitride?

Forming the nitride ion needs a lot of energy to break the nitrogen triple bond and add three electrons. Only a very small cation gives a lattice energy large enough to repay this. The lithium ion is small enough, like the magnesium ion, while sodium and the larger alkali ions are not, so they do not react with nitrogen.

Why is LiCl soluble in organic solvents?

The small lithium ion polarises the larger chloride ion and pulls electron density towards itself, so the lithium chlorine bond has partial covalent character. Covalent substances dissolve in organic solvents, so lithium chloride dissolves in ethanol, acetone, ethyl acetate and even pyridine, while sodium chloride, which is fully ionic, does not.

Why is the hydrated lithium ion the least mobile alkali metal ion?

The bare lithium ion is the smallest, so it attracts water most strongly and carries the thickest shell of water molecules. In solution the whole hydrated ion must move, and with a hydrated radius of about 382 pm it is the largest of the group. It therefore moves slowest and has the lowest ionic conductance.

What are the main uses of lithium?

Lithium is used in rechargeable lithium ion batteries because it is the lightest metal with the most negative electrode potential. Its alloys with lead make white metal bearings, with aluminium make aircraft parts and with magnesium make armour plates. It is also used in thermonuclear reactions and to prepare lithium aluminium hydride.

How is the lithium diagonal relationship tested in JEE Advanced?

JEE Advanced usually asks multiple correct or matching questions: which properties lithium shares with magnesium, which statements about lithium are wrong, the products of heating lithium nitrate or carbonate, and the moles of ammonia from lithium or magnesium nitride. The distractors are the normal sodium behaviours, so learn both columns side by side.

Which lithium equations should I know for JEE Advanced?

For JEE Advanced learn five: lithium with nitrogen giving lithium nitride, lithium nitride with water giving lithium hydroxide and ammonia, heating lithium carbonate to lithium oxide and carbon dioxide, heating lithium nitrate to lithium oxide, nitrogen dioxide and oxygen, and lithium hydride with aluminium chloride giving lithium aluminium hydride.

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