Transition metal ions

The elements in the middle ‘d’ block of the periodic table are collectively known as transition elements. Since these elements are all metals, they are also called transition metals. Important features of transition elements are: they have partially-filled ‘d’ orbitals they have several stable oxidation states they tend to form compounds and ions which are coloured.

Copper has 29 electrons. Its electron configuration is [Ar] 3d 10 4s 1. (We would expect it to be 3d 9 4s 2, but there is greater stability in having a completely filled 3rd energy level.)

The most common oxidation state of copper is Cu 2+, which has the configuration [Ar] 3d 9. Most copper(II) salts are soluble in water and form a blue solution due to the formation of the [Cu(H 2 O) 6 ] 2+ complex. Cu 2+ also forms a deep blue complex with ammonia: [Cu(NH 3 ) 4 ] 2+.

Copper(II) oxide, CuO, is a black solid which is not soluble in water.

Copper also forms a few compounds with the +1 oxidation state. None of these compounds are soluble in water. Copper(I) iodide, CuI, is white. It is formed (along with iodine) when copper(II) sulfate solution is poured into potassium iodide solution. Cu + : [Ar] 3d 10

Copper(I) oxide, Cu 2 O, is brick red. It is formed when Benedict or Fehling’s solution reacts with glucose or an aldehyde. Cu 2 O Cu 2+ (aq)

Iron has 26 electrons. Fe: [Ar] 3d 6 4s 2 It has two common ions: Fe 2+ : [Ar] 3d 6 Fe 3+ : [Ar] 3d 5 Soluble Fe 2+ solutions are pale green. Insoluble Fe(OH) 2 is an olive-green gel.

Soluble Fe 3+ solutions are usually pale orange. Insoluble Fe(OH) 3 is an orange precipitate.

Fe 3+ also forms a blood red complex with the thiocyanate ion (SCN – ): [FeSCN] 2+.

Manganese has 25 electrons: [Ar] 3d 5 4s 2. It has many stable oxidation states: + 2: Mn 2+ which is pale pink in high concentrations, but appears colourless in dilute solutions. +4: MnO 2, an insoluble compound which is black in a reagent jar and appears dark brown when it forms in a test tube. +6: MnO 4 2–, a dark green solution forming in alkaline conditions. +7: MnO 4 –, the familiar dark purple species.

MnO 4 – MnO 4 2– Mn 2+ (pale pink) MnO 2

Chromium has 24 electrons: [Ar] 3d 5 4s 1. Its most important oxidation states are +6 and +3. Chromium is in the +6 oxidation state in both the orange dichromate ion, Cr 2 O 7 2–, and the yellow chromate ion, CrO 4 2–. Cr 2 O 7 2– CrO 4 2–

Addition of acid to yellow chromate solution will turn it into the orange dichromate: 2CrO 4 2– + 2H + Cr 2 O 7 2– + H 2 O Adding base to the dichromate turns it yellow.

Dichromate is reduced……to blue-green Cr 3+.

Vanadium has 23 electrons: [Ar] 3d 3 4s 2. Although we seldom use vanadium compounds in the classroom, like other transition elements it exists in a range of oxidation states which have different colours. II III IV V V 2+ V 3+ VO 2+ VO 2 + purplegreenblueyellow

Zinc has 30 electrons: [Ar] 3d 10 4s 2. Although it is a d-block element, its d-orbitals are full. Zinc only forms one stable ion, Zn 2+, and its compounds are white. If soluble, they form colourless solutions. Zinc oxide is a white, insoluble powder. It has the unusual property of changing colour, to bright yellow, when heated strongly. No chemical reaction occurs: when the yellow powder cools it changes back to white again.