Question

Which of the following elements has the lowest melting point?
  A.Cr
  B.Fe
  C.Ni
  D.Cu

Answer 1

Hint: All the elements in the question are d – block elements. Elements that have electrons (1 to 10) present in the d-orbital of the penultimate (n-1) energy level and in the outermost or ultimate (n) ‘s’ orbital (1-2) are called d – block elements.

Complete step by step answer:
These elements show typical metallic qualities such as malleability and ductility, high electrical conductivity and thermal conductivity and good tensile strength. The d – block has four series – each of which corresponds to the filling up of 3d, 4d, 5d or 6d orbital.
All the given elements belong to 3d series –
$$\Rightarrow Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn$$
The d – block elements are called transition elements because their properties lie intermediate between the s – block and the f – block.
Covalent bonds are formed by unpaired electrons and the empty or partially filled d-orbitals of the d – block elements. In addition to these, metallic bonds are formed by s-electrons. It is due to these two types of bonds that the d – block elements have high melting and boiling points. This trend goes on till the d – orbital is half – filled. After that, the melting and boiling points begin to fall as the number of paired electrons in the d – orbitals increases.
In the 3d series, $Cr$ has the highest melting and boiling points. $Cu$ has the minimum melting point.

So, the correct answer is option (D).

Note:
$$Mn$$ and $$Tc$$ belong to 3d and 4d transition series respectively. They have exceptionally low melting points as they have stable $$d^5$$ configuration. Here, the electrons are less delocalised & their interactions are comparatively weaker.