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?

Cu

The correct answer is Option (4) → Cu

Among Fe, Co, Ni and Cu, Copper (Cu) will have the highest density.

Reasoning:

Density depends on two factors:

1. Atomic Mass (Mass of the atom)
2. Atomic Radius / Atomic Volume (Size of the atom)

In simple terms:

• Higher mass $\rightarrow$ higher density
• Larger size $\rightarrow$ lower density

So density is decided by the combined effect of mass and size.

Now compare the four elements:

Observations:

• Cu has the highest atomic mass among all four.
• Yes, Cu also has a slightly larger radius (128 pm), which normally reduces density.
• But the increase in mass is much more significant than the increase in size.

So even though Copper is a bit larger, it is much heavier, and that heaviness dominates the effect of size.

Hence, Copper ends up having the highest density.