Transition elements (d-block)
Read the following passage and answer the questions given:
The d-block occupies the large middle section of the periodic table flanked between s- and p-blocks in the modern periodic table. These elements have a general electronic configuration (n-1)d1-10 ns2 (except Pd). These elements possess a partially filled penultimate d-subshell in their ground state. Most of the transition element compounds are colored in solid or in solution state which arise due to excitation of electrons from lower energy d-orbitals to higher energy d-orbitals. The transition metal ions which have completely filled d-orbitals are colorless due to lack of excitation within d-orbitals. Most of the transition metals and their compounds exhibit paramagnetic behavior (attracted by a magnetic field) which is attributed to an incomplete d-subshell and the presence of unpaired electrons. An element or ion having no unpaired electron shows diamagnetic behavior (repelled by a magnetic field). The magnetic moment of transition elements arises due to spin and orbital motion in ions or molecules. The magnetic moment is determined by the number of unpaired electrons (n) and is calculated by using the spin-only formula μ= [n (n+2)]1/2 Bohr magneton (BM). The magnetic moment increases with the increasing number of unpaired electrons. Thus, the observed magnetic moment gives a useful indication about the number of unpaired electrons present in the transition metal or its ions. Transition elements exhibit certain important characteristic properties such as variable oxidation state, complex formation, colored ion formation, catalytic activity and metallic properties like high tensile strength, ductility, malleability etc. Transition metals are hard and have high melting points (with exceptions of Zn, Cd, Hg).
The electronic configuration of a transition element X in +1 oxidation state is [Ar] 3d5. The atomic number of the element X is:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 24
When a transition metal atom forms a positive ion, the 4s electrons are removed before the 3d electrons, because once the 3d orbitals are occupied they are lower in energy than 4s.
The ion \( \ce{X^{+}} \) has the configuration [Ar] 3d5:
\( \displaystyle \text{Electrons in }\ce{X+}=18+5=23 \)
\( \displaystyle \text{Electrons in X}=23+1=24 \)
So X has atomic number 24, which is chromium. Its ground state configuration is [Ar] 3d54s1, not 3d44s2: a half-filled 3d5 set has extra stability (exchange energy). Removing the single 4s electron gives \( \ce{Cr^{+}} \) = [Ar] 3d5.
Note: It seems that there is a mistake in the original NTA paper. The passage gives the general electronic configuration of the d-block elements as (n−1)d1–10 ns2 (except Pd). The correct general configuration is (n−1)d1–10 ns1–2, because some elements have only one ns electron, such as Cr (3d54s1) and Cu (3d104s1), and Pd (4d105s0) has none.