Consider the following statements about the lanthanoids.
(A) The hardness of the lanthanoid metals increases with increasing atomic number, samarium being steel hard.
(B) In their chemical behaviour, the earlier members of the lanthanoid series are similar to aluminium, while the later members behave more like calcium.
(C) The carbides Ln3C, Ln2C3 and LnC2 are formed when the lanthanoid metals are heated with carbon.
(D) The hydroxides of the lanthanoids are just hydrated oxides and not definite compounds.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B) and (D) only
The lanthanoids are soft, silvery white metals. Along the series their hardness and chemical behaviour change gradually, and on reaction they form carbides, oxides and basic hydroxides.
(A) The hardness of the lanthanoid metals increases with increasing atomic number, samarium being steel hard. — Correct statement. The metals are soft at the start of the series and grow harder with increasing atomic number; samarium is as hard as steel.
(B) In their chemical behaviour, the earlier members of the lanthanoid series are similar to aluminium, while the later members behave more like calcium. — Incorrect statement. It is the other way round: the earlier members are quite reactive, like calcium, and with increasing atomic number they behave more like aluminium.
(C) The carbides Ln3C, Ln2C3 and LnC2 are formed when the lanthanoid metals are heated with carbon. — Correct statement. On heating with carbon the metals give these three types of carbide.
(D) The hydroxides of the lanthanoids are just hydrated oxides and not definite compounds. — Incorrect statement. The hydroxides M(OH)3 are definite compounds, not just hydrated oxides. They are basic, like the alkaline earth metal hydroxides.
NCERT (2026-27 edition):
4.5.4 General Characteristics – “All the lanthanoids are silvery white soft metals and tarnish rapidly in air. The hardness increases with increasing atomic number, samarium being steel hard.”
“In their chemical behaviour, in general, the earlier members of the series are quite reactive similar to calcium but, with increasing atomic number, they behave more like aluminium.” … “The carbides, Ln3C, Ln2C3 and LnC2 are formed when the metals are heated with carbon. They liberate hydrogen from dilute acids and burn in halogens to form halides. They form oxides M2O3 and hydroxides M(OH)3. The hydroxides are definite compounds, not just hydrated oxides. They are basic like alkaline earth metal oxides and hydroxides.”