The Transition Elements: Quick Revision

A brief note on The Transition Elements. Read it before a test. Try each question at the end before you open its answer.

Position and configuration

• The d block is groups 3 to 12, in four series: 3d, 4d, 5d and 6d.
• A transition element has an incomplete d subshell (1 to 9 electrons) in its atom or in a common ion. Zn, Cd and Hg are d-block elements but not transition elements.
• The 3d series is $(n-1)d^{1\text{ to }10}\,ns^2$, with two exceptions: Cr $3d^5 4s^1$ and Cu $3d^{10} 4s^1$. Palladium is $4d^{10}$ with no $5s$ electron.
• An ion loses its $4s$ electrons first. An $\text{M}^{2+}$ ion has $Z - 20$ electrons in $3d$.

Trends in the first series

• $Z_{eff}$ for a $4s$ electron rises by only $0.15$ per element, because a new $3d$ electron screens by $0.85$. Radius, density and ionisation enthalpy change slowly.
• Radius: falls from Sc to Cr, is almost flat to Cu, and rises at Zn. Mn and Zn are larger than their neighbours.
• Melting point and enthalpy of atomisation are highest near V and Cr, where many unpaired d electrons bond. They are low for Mn ($3d^5$) and Zn ($3d^{10}$).
• The second ionisation enthalpy jumps at Cr ($\text{Cr}^+$ is $3d^5$) and at Cu ($\text{Cu}^+$ is $3d^{10}$). The third is large for Mn and Zn.
• $E^\circ$ for $\text{M}^{2+}/\text{M}$ is negative for all but copper. The sum $\Delta_a H + \Delta_i H_1 + \Delta_i H_2$ is the largest for copper.

Oxidation states

• Many states, in steps of one, because the $4s$ and $3d$ energies are close. The highest state rises from $+3$ for Sc to $+7$ for Mn, then falls.
• Oxygen and fluorine stabilise high states. Oxygen reaches higher than fluorine, because it forms double bonds.
• $3d^5$ ions are specially stable: $\text{Mn}^{2+}$ and $\text{Fe}^{3+}$. Iron(II) is oxidised in air.
• $\text{Cu}^+$ disproportionates in water: $2\text{Cu}^+ \rightarrow \text{Cu}^{2+} + \text{Cu}$.

Magnetism and colour

• Spin-only moment: $\mu = \sqrt{n(n+2)}$ BM, where $n$ is the number of unpaired electrons. Paramagnetic if $n > 0$, diamagnetic if $n = 0$.
• Colour comes from a d-d jump. $d^0$ and $d^{10}$ ions are colourless. An ion shows the complementary colour of the light it absorbs.
• Energy of the absorbed light: $E = 119\,627 / \lambda$ in kJ/mol, with $\lambda$ in nm.

A worked example

Find the configuration, unpaired electrons, moment and colour of $\text{Mn}^{2+}$, with $Z = 25$.

1. Configuration. Manganese is $[\text{Ar}]\,3d^5 4s^2$. Both $4s$ electrons leave, so $\text{Mn}^{2+}$ is $3d^5$.
2. Unpaired electrons. Five boxes, five electrons, so $n = 5$.
3. Moment. $\mu = \sqrt{5 \times 7} = 5.92\ \text{BM}$.
4. Colour. The ion is $d^5$, so a d-d jump is possible, and it is coloured, a very pale pink.

Practice with new numbers

A program to try

The program lists the unpaired electrons, the spin-only moment and the colour for every count of d electrons from 0 to 10.

Mistakes to avoid

• Counting zinc as a transition element. Its d subshell is full.
• Removing $3d$ electrons before $4s$ electrons when you write the configuration of an ion.
• Using all the d electrons in $\mu$. Only the unpaired ones count.
• Giving the colour that is absorbed instead of the colour that is seen.

Check yourself

1. Why does the highest oxidation state rise from scandium to manganese and then fall?

2. Find the spin-only moment of $\text{Co}^{2+}$, which is $3d^7$.

3. Why is $\text{Sc}^{3+}$ colourless?

4. Why is the second ionisation enthalpy of chromium high?