Anomalous Behaviour of Lithium: Physical and Chemical Properties

The deviation of an element’s features and behaviour from its regular or normal behaviour is called anomalous behaviour. Lithium in the S-block elements in the standard periodic table, have unusual properties compared to other elements in the same group for various reasons. 

The group’s first element often has different features than the other elements, and it resembles the second element of the neighbouring group on the right, i.e. the diagonally opposite element. This form of resemblance is known as a diagonal relationship. For example, Lithium resembles magnesium. In this article, we will explore this peculiar property of the first element of group \(1\) that is Lithium and the causes for such behaviour.

What is Anomalous Behaviour?

Anomaly is defined as behaviour that deviates from the normal or original order. In terms of properties, it differs from the other members of its group. Anomalous elements have distinct properties and produce unique compounds.

The irregular behaviour of periodic table elements is caused by the three factors listed below-

i. Small size- Atoms and ions are much smaller in size than other elements in the group.
ii. High electronegativity- The first element has the highest electronegativity.
iii. Lack of d–orbitals- Higher members of the group belonging to higher periods possess d–orbitals and may use them for bond formation.

What is Lithium?

Lithium is a chemical element with the atomic number \(3\) and the symbol ‘\({\rm{Li}}\)’. It is a silvery-white alkali metal that is soft. It is the lightest metal and the lightest solid element under standard conditions. Lithium, like all alkali metals, is highly reactive and combustible and must be stored in a vacuum, inert environment, or inert liquid-like pure kerosene or mineral oil. It has a metallic lustre when cut, but moist air quickly corrodes it to a dull silvery grey, then black tarnish. It is never found naturally but only in (usually ionic) compounds such as pegmatite minerals.

Anomalous Behaviour of Lithium

Lithium, the first member of group \(1\). Although it exhibits most of the characteristic properties of the group, yet it differs from the other elements of its own group in several properties and thus shows an anomalous behaviour. The anomalous behaviour of Lithium is due to the following factors:

i. The lithium atoms and lithium ions are of very small size. Due to its small size, lithium-ion has a high positive charge density, resulting in high polarising power. As a result, its compounds develop a partial covalent character.
ii. In comparison to other alkali metals, Lithium has higher ionization energy and a less electropositive character. This explains Lithium’s low reactivity.
iii. Lithium has no d-orbitals in its valence shell.

Difference Between Lithium and other Alkali Metals

Some important properties in which Lithium differs from other alkali metals are as follows:

i. Lithium is the hardest of all alkali metals.
ii. Lithium has much higher melting and boiling points than other alkali metals.
iii. Lithium has the lowest reactivity of any alkali metal. When exposed to air, it does not tarnish easily. When other alkali metals are exposed to moist air, they tarnish quickly.
iv. At \(300\;{\rm{K}}\), Lithium slowly reacts with bromine and decomposes water. Other alkali metals react violently with bromine and decompose water rapidly.
v. Because lithium compounds, particularly halides, are partially covalent, they are less soluble in water. They dissolve in organic solvents like pyridine. Other alkali metal halides are ionic andly soluble in water. Lithium fluoride, phosphate, carbonate, and oxide are only marginally soluble in water. Water dissolves the corresponding salts of other alkali metals.
vi. Lithium is the only alkali metal that reacts with Nitrogen to form a nitride, \({\rm{L}}{{\rm{i}}_3}\;{\rm{N}}\).
\(6{\rm{Li}} + {{\rm{N}}_2} \to 2{\rm{L}}{{\rm{i}}_3}\;{\rm{N}}\)
vii. When Lithium reacts with oxygen, it only produces a monoxide, \({\rm{L}}{{\rm{i}}_2}{\rm{O}}\). Peroxides and superoxides are also formed by the other alkali metals.
viii. Other alkali metal hydroxides are strong bases, whereas lithium hydroxide is a weak base.
ix. The hydroxide and carbonate of Lithium are less stable and decompose on heating.
The hydroxide and carbonates of other alkali metals are quite stable and do not decompose on heating.
\(2{\rm{LiOH}} \to {\rm{L}}{{\rm{i}}_2}{\rm{O}} + {{\rm{H}}_2}{\rm{O}}\)
\({\rm{L}}{{\rm{i}}_2}{\rm{C}}{{\rm{O}}_3} \to {\rm{L}}{{\rm{i}}_2}{\rm{O}} + {\rm{C}}{{\rm{O}}_2}\)
x. Lithium nitrate on heating decomposes to give \({\rm{L}}{{\rm{i}}_2}{\rm{O}},{\rm{N}}{{\rm{O}}_2},{{\rm{O}}_2}\). On the other hand, the nitrates of other alkali metals on decomposition give corresponding nitrites along with the liberation of oxygen.
\(4{\rm{LiN}}{{\rm{O}}_3} \to 2{\rm{L}}{{\rm{i}}_2}{\rm{O}} + 4{\rm{N}}{{\rm{O}}_2} + {{\rm{O}}_2}\)
\(2{\rm{NaN}}{{\rm{O}}_3} \to 2{\rm{NaN}}{{\rm{O}}_2} + {{\rm{O}}_2}\)
xi. Lithium chloride forms a dihydrate, \({\rm{LiCl}}.2{{\rm{H}}_2}{\rm{O}}\); other alkali metal chlorides, on the other hand, do not form hydrates.

Resemblance of Lithium with Magnesium (Diagonal Relationship)

The resemblance between the diagonally opposite elements of the second and the third periods can be explained on the basis of-
a. The diagonally opposite elements possess almost similar electronegativity and, therefore, similar electropositive character.
b. The diagonally opposite elements possess almost similar polarizing powers due to the comparable sizes of their cations.

The following points illustrate the anomalous behaviour of \({\rm{ Li }}\) and its resemblance with \({\rm{ Mg }}\).

i. The atomic radius of Lithium and magnesium are almost similar. \({\rm{ Li }}\) has a radius of \(152\,{\rm{pm}}\), and \({\rm{ Mg }}\) has a radius of \(160\,{\rm{pm}}\).
ii. Both \({\rm{ Li }}\) and \({\rm{ Mg }}\) have almost the same electronegativities.
iii. Both \({\rm{ Li }}\) and \({\rm{ Mg }}\) have covalent characters.
iv. They are harder and lighter than the other elements in their groups.
The melting points of \({\rm{ Li }}\) and \({\rm{ Mg }}\) are higher than the other members of their respective groups. The hydroxides \({\rm{LiOH}}\) and \({\rm{Mg}}{({\rm{OH}})_2}\) are weak bases and decompose on heating.
\(2{\rm{LiOH}} \to {\rm{L}}{{\rm{i}}_2}{\rm{O}} + {{\rm{H}}_2}{\rm{O}},\quad {\rm{Mg}}{({\rm{OH}})_2} \to {\rm{MgO}} + {{\rm{H}}_2}{\rm{O}}\)
v. Chlorides \({\rm{LiCl}}\) and \({\rm{MgC}}{{\rm{l}}_2}\) are deliquescent and crystallize to form an aqueous hydrate. Also, these chlorides are soluble in ethanol and pyridine.
vi. Both decompose water slowly to liberate hydrogen.
Both elements form only monoxide when heated in presence of oxygen. The monoxides are soluble in water and do not combine with excess oxygen to form superoxide.
\(4{\rm{Li}} + {{\rm{O}}_2} \to 2{\rm{L}}{{\rm{i}}_2}{\rm{O}},\quad 2{\rm{Mg}} + {{\rm{O}}_2} \to 2{\rm{MgO}}\)

Carbonates of Li and Mg decompose on heating to give oxide and carbon dioxide.
\({\rm{L}}{{\rm{i}}_2}{\rm{C}}{{\rm{O}}_3} \to {\rm{L}}{{\rm{i}}_2}{\rm{O}} + {\rm{C}}{{\rm{O}}_2},\quad {\rm{MgC}}{{\rm{O}}_3} \to {\rm{MgO}} + {\rm{C}}{{\rm{O}}_2}\)

Their oxides and hydroxides are much less soluble, and their hydroxides decompose on heating.
\(2{\rm{LiOH}} \to {\rm{L}}{{\rm{i}}_2}{\rm{O}} + {{\rm{H}}_2}{\rm{O}},\quad {\rm{Mg}}{({\rm{OH}})_2} \to {\rm{MgO}} + {{\rm{H}}_2}{\rm{O}}\)

On heating nitrates of Li and Mg, they decompose and produce a mixture of nitrogen oxide and oxygen.
\(4{\rm{LiN}}{{\rm{O}}_3} \to 2{\rm{L}}{{\rm{i}}_2}{\rm{O}} + 4{\rm{N}}{{\rm{O}}_2} + {{\rm{O}}_2},\quad 2{\rm{Mg}}{\left( {{\rm{N}}{{\rm{O}}_3}} \right)_2} \to 2{\rm{MgO}} + 4{\rm{N}}{{\rm{O}}_2} + {{\rm{O}}_2}\)

Uses of Lithium

The applications of Lithium are as follows:

i. Lithium is used for making alloys; e.g., its alloy with lead is called white metal and is used to make bearings for motor engines and sheets for cables.
Its alloy with aluminium is used for making aircraft parts as it has high tensile strength and elasticity.
Its alloy with magnesium \(({\rm{14}}\% \,{\rm{Li}})\), being extremely tough and corrosion-resistant, is used in making armour plates.
ii. It is used in the thermonuclear reaction and also used in an electrochemical cell as batteries.
iii. It is also used for the synthesis of compounds that are very useful for us- like waterproof glass, treating rheumatism, organic synthesis, etc.

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Summary

In this article, we studied that the first element of the respective groups shows different properties than the rest of the elements due to their smaller size, high electronegativity, and absence of vacant d-orbitals. Now we know the points of differences between alkali metals and Lithium, also the similarity of Lithium with that of magnesium due to the diagonal relationship.

FAQs

Q.1. What is the reason for the anomalous behaviour of Lithium?
Ans: The anomalous behaviour of Lithium is due to the following factors:
i. The lithium atoms and lithium ions are of very small size and consequently have high polarizing power.
ii. Lithium possesses higher ionization energy and less electropositive character as compared to other alkali metals.
iii. Lithium does not possess d-orbitals in its valence shell.

Q.2. What are the four properties of Lithium?
Ans: The important properties of this metal are as follows-
i. Its high specific heat, which is also known as calorific capacity.
ii. It has a large temperature range in the liquid state and high thermal conductivity.
iii. It has a low viscosity.
iv. It has a very low density.

Q.3. What are the three interesting facts about Lithium?
Ans: The three interesting facts about Lithium are as follows-
i. It is the lightest metal.
ii. It has the lowest density of any metal.
iii. It burns with a bright red colour.

Q.4. The similarity between Lithium and magnesium is due to which reason?
Ans: The similarity between Lithium and magnesium is due to the diagonal relationship.

Q.5. Write four points of difference between the properties of Lithium and other elements of the alkali metal?
Ans: The difference between lithium and alkali metals are as follows-
i. Lithium is the hardest of all alkali metals.
ii. Lithium has much higher melting and boiling points than other alkali metals.
iii. Lithium has the lowest reactivity of any alkali metal. When exposed to air, it does not tarnish easily. When other alkali metals are exposed to moist air, they tarnish quickly.
iv. At \(300\;{\rm{K}}\), Lithium slowly reacts with bromine and decomposes water. Other alkali metals react violently with bromine and rapidly decompose water vigorously.