General Properties: Reasoning Questions

CHEExam Practice & Reasoning

This subtopic is reason-based: explain the characteristic bulk properties of transition metals. Recurring themes are strong metallic bonding (high m.p./b.p.), interstitial compounds, alloy formation, and ionisation-enthalpy anomalies for $Mn$ and $Zn$ .

High melting and boiling points: transition metals have strong metallic bonding from a large number of unpaired $d$ and $s$ electrons available for delocalised (and some covalent) bonding.
Interstitial compounds form when small atoms ( $H$ , $C$ , $N$ , $B$ ) occupy holes in the metal lattice; they are very hard, have high melting points, retain metallic conductivity, and are often non-stoichiometric.
$Mn$ has an anomalously high third ionisation enthalpy because removing a third electron breaks its stable half-filled $3d^5$ core; $Zn$ has a high first ionisation enthalpy because of its stable fully filled $3d^{10}4s^2$ configuration.
Alloys form readily because transition metals have similar atomic radii, so atoms of one metal can replace another in the lattice (substitutional alloys); alloys are typically harder, stronger and more corrosion-resistant.
Across the $3d$ series atomic radii change only slightly because added $d$ electrons screen the increasing nuclear charge fairly well, nearly cancelling its contracting effect.
Most transition metals are good conductors and malleable, reflecting the mobile delocalised electrons of strong metallic bonding.
Melting points rise to a maximum near the middle of the $3d$ series (around $Cr$ , where the unpaired $d$ electrons available for bonding are greatest) then fall; $Mn$ dips because its stable half-filled $3d^5$ set lowers metallic bonding, and $Zn$ , $Cd$ , $Hg$ are low-melting because their $d^{10}$ shells take no part in bonding.
Enthalpy of atomisation is high for transition metals and roughly tracks the number of unpaired $d$ electrons available for metallic bonding.
Transition metals are typically dense, hard, high-tensile-strength metals, again a consequence of strong $d$ -orbital metallic bonding.
They show catalytic activity, form coloured ions, are mostly paramagnetic, and readily form complexes — a cluster of properties tied to partly filled $d$ -orbitals.
Ionisation enthalpies increase only gradually across the $3d$ series because the rising nuclear charge is partly offset by the shielding of added $d$ electrons.

Where the marks go

Explaining high melting points by ionic bonding; transition metals are metals — the cause is strong metallic (and partial covalent) $d$ -orbital bonding.
Saying interstitial compounds are stoichiometric and soft; they are typically non-stoichiometric and very hard while keeping metallic conductivity.
Crediting $Mn$ 's anomaly to its first ionisation enthalpy; it is the third ionisation enthalpy that is high (it breaks the $3d^5$ half-filled set).
Claiming transition-metal atomic radii fall sharply across a period like $s$ -block metals; they change only slightly due to $d$ -electron shielding.
Assuming alloys are chemical compounds; substitutional alloys are solid solutions where atoms of similar size replace one another in the lattice.

How the board asks it

Give reasonsstrong metallic bonding from unpaired d and s electrons
Give reasons: Transition metals have high melting and boiling points.
Give reasonsionisation-enthalpy anomaly from stable half-filled $3d^5$
Account for the fact that the third ionisation enthalpy of $Mn$ ( $Z = 25$ ) is unexpectedly high.
Give reasons $d^{10}$ configuration takes no part in metallic bonding
Give reasons: $Zn$ has a much lower melting point than the other elements of the $3d$ transition series.
Define / statesmall atoms occupying lattice holes; non-stoichiometric
What are interstitial compounds? State two of their characteristic properties.
Give reasonssimilar atomic radii allow substitutional replacement
Give reasons: Transition metals readily form alloys with one another.
Give reasonsadded $d$ electrons screen the increasing nuclear charge
Why do the atomic radii of the $3d$ transition elements change only slightly across the series?

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Written for Sublevo. Question text quoted anywhere in these notes is the Council’s and carries its year and paper; the board’s own diagrams are not reproduced.