d- and f-Block Elements · Study notes

CHEPosition & General Properties

Electronic Configurations & Position

Transition elements have a general configuration $(n-1)d^{1\text{-}10}\,ns^{1\text{-}2}$ and are defined by a partially filled $d$ -subshell in the element or its common ions. Watch for the half-filled/fully-filled stability exceptions ( $Cr$ , $Cu$ ) and the standard reasoning for why $Zn$ is excluded.

General valence-shell configuration

(n-1)d^{1\text{-}10}\, ns^{1\text{-}2}

n

is the outermost principal quantum number; the

(n-1)d

subshell fills across a transition series.

Ion configuration rule (remove ns before (n-1)d)

Fe\,[Ar]3d^6 4s^2 \xrightarrow{-2e^-} Fe^{2+}\,[Ar]3d^6 \xrightarrow{-1e^-} Fe^{3+}\,[Ar]3d^5

4s

electrons are lost before

3d

when forming cations.

Definition: a transition element has an incompletely filled $d$ -subshell in its ground state or in at least one stable oxidation (ion) state.
Anomalies: $Cr$ $(Z=24)$ is $[Ar]3d^5 4s^1$ and $Cu$ $(Z=29)$ is $[Ar]3d^{10} 4s^1$ , because half-filled ( $d^5$ ) and fully filled ( $d^{10}$ ) sets give extra exchange-energy stability.
For ions, remove $4s$ electrons before $3d$ : $Fe^{2+}$ is $3d^6$ and $Fe^{3+}$ is $3d^5$ ; $Fe^{3+}$ is more stable owing to the half-filled $3d^5$ configuration.
Common ion configurations: $Mn^{2+}$ is $3d^5$ , $Cr^{3+}$ is $3d^3$ , $Cu^{2+}$ is $3d^9$ , $Ti^{3+}$ is $3d^1$ .
$Zn$ is not a transition element: both the atom ( $3d^{10}4s^2$ ) and its only ion $Zn^{2+}$ ( $3d^{10}$ ) have a completely filled $d$ -subshell, so no $d$ -subshell is partially filled.
$4d$ and $5d$ congeners like $Zr$ and $Hf$ have nearly identical atomic radii because the lanthanoid contraction cancels the expected size increase from $Zr$ to $Hf$ .
There are three main transition series — $3d$ (period 4), $4d$ (period 5) and $5d$ (period 6) — placed in Groups 3–12; the $f$ -block (lanthanoids and actinoids) is set apart at the foot of the table as inner-transition elements.
By the same logic $Sc$ shows $d^1$ character in its atom and lower oxidation states, so it is counted as a transition element, whereas $Zn$ , $Cd$ and $Hg$ never expose a partly filled $d$ -subshell and are excluded.
Aufbau order places $4s$ below $3d$ on filling, but once $3d$ is occupied it sinks below $4s$ in energy, which is exactly why ionisation removes $4s$ first.
Half-filled and fully-filled stability also explains why elements like $Mn$ ( $3d^5 4s^2$ ) and $Zn$ ( $3d^{10}4s^2$ ) have anomalously high relevant ionisation enthalpies among their neighbours.
Cu, Ag and Au are still classed as transition metals because they show a partly filled $d$ in a common ion ( $Cu^{2+}$ is $3d^9$ ), unlike $Zn^{2+}$ which is $d^{10}$ .

Where the marks go

Writing $Cr$ as $[Ar]3d^4 4s^2$ and $Cu$ as $[Ar]3d^9 4s^2$ — the correct anomalous forms are $3d^5 4s^1$ and $3d^{10}4s^1$ for extra stability.
Removing a $3d$ electron before $4s$ when forming ions: always strip the $ns$ electrons first, so $Fe^{2+}$ is $3d^6$ (not $3d^4 4s^2$ ).
Calling $Zn$ , $Cd$ or $Hg$ transition metals — their $d$ -subshell is full in every common state, so they are excluded.
Saying $Zr$ and $Hf$ differ greatly in size because $Hf$ is a period lower; the lanthanoid contraction makes their radii almost equal.
Confusing the filling order ( $4s$ before $3d$ ) with the removal order ( $4s$ removed first) — they are deliberately opposite.

How the board asks it

Structure / namingion configuration by removing $ns$ before $(n-1)d$ ; half-filled $3d^5$ stability
Write the electronic configurations of $Fe^{2+}$ and $Fe^{3+}$ , and state with a reason which of the two is more stable.
Give reasonshalf-filled/fully-filled exchange-energy stability
Account for the fact that chromium $(Z=24)$ has the configuration $[Ar]3d^5 4s^1$ rather than the expected $[Ar]3d^4 4s^2$ .
Define / statedefinition of a transition element and general valence-shell configuration
Define a transition element and give the general electronic configuration of its valence shell.
Give reasonscompletely filled $d$ -subshell in $Zn$ and $Zn^{2+}$ versus partly filled $d$ in $Cu^{2+}$
Give reasons: zinc is not regarded as a transition element, whereas copper is.
DistinguishAufbau filling order ( $4s$ before $3d$ ) versus ionisation removal order ( $4s$ removed first)
Distinguish between the order in which the $4s$ and $3d$ orbitals are filled during the Aufbau process and the order in which their electrons are removed on ionisation.

Practise this topic

NextVariable Oxidation States & Stability

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.