Read the passage carefully and answer the questions.
Nearly all the transition elements display typical metallic properties such as high tensile strength, ductility, malleability, high thermal and electrical conductivity and metallic lustre. The transition metals (with the exception of Zn, Cd and Hg) are very hard and have low volatility. Their melting and boiling points are high. They have high enthalpies of atomization. Ions of the same charge in a given series show progressive decrease in radius with increasing atomic number. However, the variation within a series is quite small. The filling of 4f before 5d orbital results in a regular decrease in atomic radii called Lanthanoid contraction which essentially compensates for the expected increase in atomic size with increasing atomic number. The net result of the lanthanoid contraction is that the second and the third d series exhibit similar radii (e.g., Zr 160 pm, Hf 159 pm) and have very similar physical and chemical properties. There is an increase in ionisation enthalpy along each series of the transition elements from left to right. The first ionisation enthalpy, in general, increases, but the magnitude of the increase in the second and third ionisation enthalpies for the successive elements, is much higher along a series. One of the notable features of a transition elements is the great variety of oxidation states these may show in their compounds.
Metallic radii of some transition elements are as follows:
Fe: 126 pm; Co: 125 pm; Ni: 125 pm; Cu: 128 pm
Which of these elements will have highest density?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Cu
Density is mass per unit volume. For metals with a similar packing of atoms, the density depends on the mass of one atom and on the volume it occupies, which grows as the cube of the metallic radius:
\( \displaystyle d \propto \frac{M}{r^{3}} \)
Taking the atomic masses Fe 55.85, Co 58.93, Ni 58.69 and Cu 63.55 with the given radii:
\( \displaystyle \ce{Fe}: \frac{55.85}{126^{3}} = 2.79 \times 10^{-5} \)
\( \displaystyle \ce{Co}: \frac{58.93}{125^{3}} = 3.02 \times 10^{-5} \)
\( \displaystyle \ce{Ni}: \frac{58.69}{125^{3}} = 3.00 \times 10^{-5} \)
\( \displaystyle \ce{Cu}: \frac{63.55}{128^{3}} = 3.03 \times 10^{-5} \)
Copper has the largest radius, but its atomic mass is larger by a bigger factor, so it has the highest density. The measured densities agree: Fe 7.87, Co 8.90, Ni 8.91 and Cu 8.96 g cm−3.