Questions: The d- and f-Block Elements

Q1 What is a transition element? Why is zinc not counted as one although it lies in the d block?

Answer (English): A transition element is one whose atom or common ion has a partly filled d subshell. Zinc is 3d10 4s2 in the atom and Zn2+ is 3d10, so in both the d subshell is complete. With no partly filled d subshell there can be no d-d transition and no variable oxidation state arising from d electrons, so zinc behaves as a main group metal and is excluded from the transition elements. Copper, in contrast, has 3d10 4s1 as the atom but Cu2+ is 3d9, so copper counts as a transition element.

Hinglish: Transition element woh hai jiske atom ya common ion mein d subshell partially filled ho. Zn atom 3d10 4s2 aur Zn2+ 3d10 hai, matlab d subshell full hai, isliye zinc transition element nahi hai. Cu2+ mein 3d9 hota hai isliye copper transition element hai.

Q2 Explain the anomalous electronic configurations of chromium and copper and write the configurations of their +2 ions.

Answer (English): The expected configuration of chromium would be 3d4 4s2, but the actual configuration is 3d5 4s1 because a half filled d subshell with five parallel electrons is more stable. Copper would be expected to be 3d9 4s2, but it is 3d10 4s1 because a completely filled d subshell is more stable. In their ions the 4s electron is lost first, so Cr2+ is 3d4 and Cu2+ is 3d9. Both ions therefore have partly filled d subshells and both form coloured salts, with Cr2+ giving blue solutions and Cu2+ giving blue solutions in water.

Hinglish: Cr ka expected 3d4 4s2 tha lekin actual 3d5 4s1 hai kyunki half filled d subshell stable hoti hai. Cu ka expected 3d9 4s2 tha lekin actual 3d10 4s1 hai kyunki fully filled d stable hoti hai. Ions mein pehle 4s electron nikalta hai, isliye Cr2+ 3d4 aur Cu2+ 3d9 hote hain.

Q3 Why does the atomic radius of the transition metals fall across a series and then rise slightly at the end? Why are their melting points high?

Answer (English): Across a series the nuclear charge increases by one at each step while the extra electron enters the (n-1)d subshell, which shields the outer electrons poorly. The effective nuclear charge felt by the outer electrons therefore rises and the radius falls. Near the end of the series the added d electrons shield each other more and the radius rises slightly. Melting points are high because both the ns and the (n-1)d electrons take part in metallic bonding, so the metallic bond is strong. The melting point is highest near the middle of the series, where the largest number of unpaired d electrons is available for bonding.

Hinglish: Series mein nuclear charge badhta hai aur d electrons poor shielding karte hain, isliye radius girti hai; end mein thoda badh jaati hai. Melting points high hote hain kyunki ns aur (n-1)d dono electrons metallic bonding mein participate karte hain. Maximum melting point series ke middle mein aata hai.

Q4 Calculate the spin only magnetic moments of Fe3+ and Cu2+. Which is more paramagnetic?

Answer (English): Fe3+ is 3d5, so it has five unpaired electrons. The magnetic moment is the square root of 5 times 7, which is the square root of 35, equal to 5.92 BM. Cu2+ is 3d9, which can be written as d10 with one hole, so it has one unpaired electron and its moment is the square root of 3, equal to 1.73 BM. Fe3+ is far more paramagnetic because its moment is roughly three times larger, and the calculation uses the spin only formula because the orbital contribution is nearly quenched in these ionic complexes.

Hinglish: Fe3+ 3d5 hai, isliye n = 5 aur mu = sqrt(35) = 5.92 BM. Cu2+ 3d9 hai, isliye ek unpaired electron aur mu = sqrt(3) = 1.73 BM. Fe3+ zyada paramagnetic hai kyunki uske unpaired electrons zyada hain.

Q5 Why is [Ti(H2O)6]3+ violet while Zn2+ salts are colourless? How does permanganate get its colour?

Answer (English): Ti3+ is a d1 ion, so the single d electron can be excited from the lower t2g level to the higher eg level by absorbing light in the visible region. The light not absorbed is seen, and for the hydrated titanium(III) ion the transmitted colour is violet. Zn2+ is d10, so every d orbital is full and no d-d transition is possible within the d subshell; the ion is colourless. Permanganate is also not a simple d-d case: manganese is in +7 with a d0 configuration, and the deep purple colour comes from charge transfer from the oxygen ligands to the empty metal orbitals, which absorbs in the visible region.

Hinglish: Ti3+ d1 hai, isliye ek electron t2g se eg mein jaata hai aur violet colour dikhta hai. Zn2+ d10 hai, saare d orbitals full hain, isliye d-d transition nahi hota aur colourless hai. KMnO4 ka rang d-d transition se nahi, ligand se metal charge transfer se aata hai.

Q6 Write the balanced ionic equations for the reaction of acidified potassium permanganate with iron(II) ions and with oxalate ions.

Answer (English): In acidic medium permanganate is reduced to Mn2+ and gains five electrons, so its half reaction is MnO4- plus 8H+ plus 5 electrons giving Mn2+ plus 4H2O. Iron(II) is oxidised to iron(III) with one electron, so the balanced equation is MnO4- plus 5Fe2+ plus 8H+ giving Mn2+ plus 5Fe3+ plus 4H2O. Oxalate loses two electrons to become carbon dioxide, so two permanganate ions balance five oxalate ions: 2MnO4- plus 5C2O4(2-) plus 16H+ giving 2Mn2+ plus 10CO2 plus 8H2O. In the titration the permanganate colour itself marks the end point.

Hinglish: Acidic medium mein MnO4- 5 electrons lekar Mn2+ ban jaata hai. Fe2+ ke saath equation MnO4- + 5Fe2+ + 8H+ → Mn2+ + 5Fe3+ + 4H2O. Oxalate ke saath 2MnO4- + 5C2O4^2- + 16H+ → 2Mn2+ + 10CO2 + 8H2O. Titration mein purple colour ka disappear hona end point batata hai.

Q7 Explain the effect of pH on a chromate solution and write the reaction of acidified dichromate with iodide.

Answer (English): Chromate and dichromate ions are related by an equilibrium that consumes hydrogen ions: two chromate ions plus two hydrogen ions give dichromate plus water. In acidic solution the equilibrium shifts to the right and the orange dichromate ion dominates, while in basic solution it shifts left and the yellow chromate ion dominates. Acidified dichromate oxidises iodide to iodine while chromium falls from +6 to +3: Cr2O7(2-) plus 14H+ plus 6I- gives 2Cr3+ plus 3I2 plus 7H2O. The solution turns from orange to green because of the Cr3+ formed.

Hinglish: Chromate aur dichromate ke beech equilibrium H+ par depend karta hai: acidic mein orange dichromate banta hai aur basic mein yellow chromate. Acidified dichromate iodide ko iodine mein oxidise karta hai aur khud Cr3+ banta hai, isliye colour orange se green ho jaata hai.

Q8 What is the lanthanoid contraction? Give its cause and one important consequence.

Answer (English): The lanthanoid contraction is the steady decrease in atomic and ionic radius from lanthanum to lutetium. As the atomic number rises, the extra electrons enter the 4f subshell, which shields the outer electrons from the nucleus poorly. The effective nuclear charge therefore increases and pulls the outer electrons closer, so the radius falls. One important consequence is that the second and third transition series have almost the same atomic radii, which makes the pairs zirconium and hafnium, and niobium and tantalum, chemically very similar and hard to separate.

Hinglish: Lanthanoid contraction lanthanum se lutetium tak radius ka steady decrease hai. 4f electrons nucleus ko poorly shield karte hain, isliye effective nuclear charge badhta hai aur radius girti hai. Iska bada result yeh hai ki Zr aur Hf ke radii aur chemistry almost same ho jaati hai, isliye unka separation mushkil hota hai.

Q9 Give four differences between the lanthanoids and the actinoids.

Answer (English): First, the lanthanoids fill the 4f subshell while the actinoids fill the 5f subshell. Second, the common oxidation state of the lanthanoids is +3 with a few exceptions, while the actinoids show a much wider range from +3 to +6 and sometimes +7. Third, all actinoids are radioactive, whereas most lanthanoids are not. Fourth, the actinoid contraction is greater than the lanthanoid contraction because 5f electrons shield the nucleus even less effectively than 4f electrons, so the radii fall faster. The chemistry of the early actinoids resembles that of the transition metals, while the later actinoids behave more like the lanthanoids.

Hinglish: Lanthanoids 4f fill karte hain aur actinoids 5f. Lanthanoids ka common state +3 hai, actinoids +3 se +6 tak jaate hain. Actinoids radioactive hote hain, lanthanoids generally nahi. Actinoid contraction lanthanoid se badi hoti hai kyunki 5f electrons aur bhi poor shielding karte hain.

Q10 Why is Cu+ unstable in aqueous solution? Write the reaction responsible.

Answer (English): In water, Cu+ disproportionates into copper metal and Cu2+: two Cu+ ions give one copper atom and one Cu2+ ion. The reaction is favoured because the hydration energy released by the doubly charged Cu2+ ion is large enough to more than compensate for the second ionisation energy needed to reach the +2 state. As a result, Cu2+ is the stable ion in aqueous solution and Cu+ salts are stable only when the ion is trapped in an insoluble compound or in a complex.

Hinglish: Cu+ aqueous solution mein disproportions karta hai: 2Cu+ → Cu + Cu2+. Cu2+ ki hydration energy itni zyada hoti hai ki second ionisation energy compensate ho jaati hai, isliye +2 state stable hai. Cu+ sirf insoluble compound ya complex mein stable rehta hai.

The chapter quiz has five questions on configurations, magnetic moments, colour and the f block. Attempt it after writing the ten answers above, because each explanation repeats one step from these answers.