Question

Which has least covalent radius?
A. $Mn$
B. $Cu$
C. $Zn$
D. $Ni$

Answer 1

There are three major factors that contribute towards the effect in the covalent radius of an atom. They are the number of shells in an atom, the effective nuclear charge on an atom and the number of unpaired electrons in that atom. They decide the trend in the atomic radii both across the period and down the group.

Complete solution:
- The periodic table is made in such a form that it can show the variations of several properties of the atoms both along the period and down the group. So, there is also a trend for the atomic radius both across the period and down the group.
-There are 7 periods and 18 groups in periodic tables where atoms are arranged in the increasing order of their atomic number because the atomic number gives us the number of electrons of the atom. Electrons only decide the chemical properties of an atom as they are loosely held by the atom.
-Covalent radius of an atom denotes half of the internuclear distance between the atoms of the monatomic species. This is why it is different from ionic and atomic radius. It depends on the bond order of the compound formed by the covalent bonding.
-Along the period, the atomic number increases and so does the number of electrons without altering the number of shells of atoms. So effective nuclear charge increases thereby increasing the force of attraction of the nucleus towards the electrons. This leads to decrease in the atomic radii across a period.
-Down the group, the number of shells increases but the valence electrons remain the same. Atomic size increases down the group. This increases the distance between the nucleus of an atom to the valence electrons. So the atomic radii increase down the group due to the number of shells.
-The elements given in the question are d-block elements and they show the effect in their radius due to the partially-filled and full-filled d-orbitals also. The atomic radius is more for the elements with these configurations and so they show irregularities in the trend.
-As the unpaired electrons increase, the attraction increases due to metal-metal bonding and so the size decreases due to decrease in the bond length. This contradicts the normal behavior of the size of the atom.
-So, the size decreases initially, then remains constant and later on increases. The size for the given period of the atom is maximum for $Sc$ and it decreases till $Mn$. Then it remains almost constant till $Ni$ and then again increases uptil $Zn$.

So, $Ni$ has the least covalent radius among the following given elements and the correct option is, (D).

Note: The size of the covalent radius for the mentioned elements in the picometer can be stated as 125 for $Ni$, 128 for $Cu$, 137 for both $Zn$ and $Mn$. Zinc is the further last element in the d-block for this period among the given ones, but it has more radius than Cu due to the unpaired electron difference.