xSkill: notes, lectures and practice for CBSE and CSE

1.1 Configurations and terms The d block fills the (n-1)d orbitals after the ns orbital, and the f block fills (n-1)f. The first row of the d block runs from scandium to zinc. Two elements have anomalous configurations because a half filled or completely filled d subshell is more stable: chromium is 3d5 4s1 and copper is 3d10 4s1. A transition element is defined by a partly filled d subshell in the atom or in one of its common ions, which is why zinc is not counted as a transition element. The common oxidation state of the lanthanoids is +3, with +2 and +4 appearing where a stable empty, half filled or completely filled f subshell results. Cerium forms Ce4+, europium and ytterbium form M2+, and these exceptions are explained by the stability of the 4f0, 4f7 and 4f14 arrangements.

sample : 3 d^{5} 4 s^{1} for Cr, 3 d^{10} 4 s^{1} for Cu

Anomalous configurations of chromium and copper

μ = n (n + 2) BM

Spin only magnetic moment from n unpaired electrons

E_{cell}^{0} = E_{cathode}^{0} - E_{anode}^{0}

Feasibility from the standard potentials of transition metal couples

1.2 Key permanganate and dichromate equations The chemistry of manganese and chromium in the +7 and +6 states is tested through balanced ionic equations. Acidified permanganate is a strong oxidiser and is reduced to Mn2+ in acidic medium, to MnO2 in neutral medium and to MnO4(2-) in strongly alkaline medium. Acidified dichromate is reduced to Cr3+. Both are written below in their acidic forms, which are the ones asked in the board paper.

Mn O_{4}^{-} + 8 H^{+} + 5 e^{-} \to M n^{2 +} + 4 H_{2} O

Reduction of permanganate in acidic medium

C r_{2} O_{7}^{2 -} + 14 H^{+} + 6 e^{-} \to 2C r^{3 +} + 7 H_{2} O

Reduction of dichromate in acidic medium

2Mn O_{4}^{-} + 5 C_{2} O_{4}^{2 -} + 16 H^{+} \to 2M n^{2 +} + 10C O_{2} + 8 H_{2} O

Permanganate with oxalate, a standard titration

1.3 Trends and constants Across a transition series the atomic radius passes through a small minimum because nuclear charge rises while the added d electrons shield poorly. Melting points are high because both ns and (n-1)d electrons take part in metallic bonding, and they peak near the middle of the series. The lanthanoid contraction is the steady fall in atomic and ionic radius from lanthanum to lutetium, caused by poor shielding by 4f electrons; it makes the second and third transition series almost identical in radius. Ionisation enthalpy rises slowly across a series, and the standard potentials of the M2+/M couples do not follow a smooth order because the total energy includes ionisation, sublimation and hydration terms.

Exam

In a reason question about colour, tie the answer to d-d transitions or charge transfer. A d0 or d10 ion has no d-d transition and is colourless, unless a charge transfer band gives colour, as in permanganate.

Write the configuration with the anomalous cases first.
Count unpaired electrons before using the magnetic moment formula.
Check whether the medium is acidic, neutral or alkaline before writing a permanganate half reaction.
For lanthanoid questions, name the stability of empty, half filled and filled f subshells.

1.4 Units and checks Magnetic moments are quoted in Bohr magnetons, enthalpies of atomisation in kilojoules per mole and standard potentials in volts. In a configuration question, write the noble gas core first and then place the electrons, so the outer shell count stays clear. In a colour question, state the d count before naming a transition; in a permanganate question, state the medium before balancing the half reaction.