Free Electronic Structure of d-Block Elements MCQs with Answers

1. A transition element is best defined as one that

• A. lies between groups II and III of the periodic table
• B. forms at least one stable ion with a partially filled d subshell
• C. has a full d subshell
• D. is a hard metal with a high melting point

Explanation: The definition rests on the partially filled d subshell in the element or in one of its ions, because that is what produces variable oxidation states, coloured ions, catalytic activity and complex formation. Zinc is excluded on this test, since it is 3d10 both as the atom and as the Zn2+ ion, which is why it lacks colour and variable valency. Position in the table and physical hardness are consequences rather than the definition.

2. The electronic configuration of chromium, atomic number 24, is

• A. [Ar] 3d4 4s2
• B. [Ar] 3d6
• C. [Ar] 3d5 4s1
• D. [Ar] 4s2 4p4

Explanation: One 4s electron is promoted so that both the 3d and 4s subshells are exactly half filled, an arrangement of extra stability arising from symmetry and reduced electron repulsion. Copper behaves in the same way, adopting 3d10 4s1 to complete the d subshell. Writing 3d4 4s2 by mechanically following the filling order is the expected error.

3. When a transition metal forms a positive ion, the electrons are lost first from

• A. the 3d subshell
• B. the 4p subshell
• C. the innermost shell
• D. the 4s subshell

Explanation: Although 4s fills before 3d, once the d subshell is occupied the 4s level becomes the higher in energy, so it is emptied first on ionisation, and the Fe2+ ion is therefore 3d6 rather than 3d4 4s2. This detail is asked frequently because it appears to contradict the filling order. It explains why almost every transition metal shows a plus 2 oxidation state.

4. Transition metal compounds are usually coloured because

• A. electrons move between d orbitals split by the ligands, absorbing part of the visible spectrum
• B. the metals reflect all wavelengths equally
• C. they contain water of crystallisation
• D. their ions are very large

Explanation: Ligands split the five d orbitals into groups of slightly different energy, and the small energy gap corresponds to visible light, so the complex absorbs one part of the spectrum and appears in the complementary colour. Ions with an empty or completely full d subshell, such as Sc3+ and Zn2+, have no such transition available and their compounds are white. Changing the ligand changes the gap, and hence the colour.

5. Which of the following is NOT a characteristic property of transition elements?

• A. Variable oxidation states
• B. Formation of complex ions
• C. Catalytic activity
• D. Low melting points and softness

Explanation: Transition metals are hard, dense and high melting, because both the 3d and 4s electrons contribute to metallic bonding, and tungsten melts at over 3400 degrees Celsius. The other three properties all follow from the partly filled d subshell. The soft, low melting, low density metals are those of group I, which is the contrast being tested here.

6. Transition metals show variable oxidation states because

• A. their atomic radii vary greatly
• B. the 3d and 4s subshells are close in energy, so a variable number of electrons can be involved in bonding
• C. they always lose all their outer electrons
• D. they have unusually high electronegativity

Explanation: Because the energy gap between 4s and 3d is small, successive ionisation energies rise only gradually and several different numbers of electrons can be removed, giving manganese oxidation states from plus 2 to plus 7. In group I, by contrast, the second ionisation energy is enormous because it breaks into a noble gas core, so only plus 1 is ever seen. The plus 2 state is common to nearly all first row transition metals.

7. In the complex ion [Cu(NH3)4]2+, the ammonia molecules act as

• A. ligands, donating a lone pair of electrons to the central metal ion
• B. reducing agents
• C. counter ions balancing the charge
• D. solvent molecules with no bonding role

Explanation: A ligand is a species with at least one lone pair that forms a dative covalent bond to the central metal ion, and here each nitrogen donates its lone pair, giving copper a coordination number of four. Water, chloride and cyanide act in the same way. The deep blue colour of this complex is the standard test for copper two ions.

8. The coordination number of the central metal ion in a complex is

• A. the charge on the complex ion
• B. the number of ligands in the solution
• C. the number of dative bonds formed between the ligands and the metal ion
• D. the oxidation state of the metal

Explanation: Coordination number counts the bonds to the central ion, so it is six in the octahedral hexaaquairon complex and four in the tetrahedral tetrachlorocuprate ion. It is not the same as the oxidation state, which counts the charge the metal would carry if the ligands were removed with their electron pairs. A single bidentate ligand such as ethylenediamine contributes two to the coordination number.

9. Iron is used as a catalyst in the Haber process, and this catalytic ability of transition metals is largely due to

• A. their high density
• B. their ability to change oxidation state and to adsorb reactants onto their surface
• C. their low melting points
• D. the complete absence of d electrons

Explanation: Variable oxidation states allow a transition metal to accept and release electrons during a reaction, providing an alternative pathway of lower activation energy, and the partly filled d orbitals let gases bond weakly to the metal surface so that their bonds are stretched and weakened. Vanadium pentoxide in the Contact process and nickel in hydrogenation work in the same ways. Density and melting point have nothing to do with it.

10. Which ion would be expected to be colourless in aqueous solution?

• A. Fe3+
• B. Cu2+
• C. Zn2+
• D. Ni2+

Explanation: The zinc ion has a completely filled 3d10 configuration, so no d to d electronic transition is possible and no visible light is absorbed. Iron three, copper two and nickel two all have partly filled d subshells and give yellow brown, blue and green solutions respectively. Scandium three, being 3d0, is colourless for the complementary reason.