Chemical Formulae & Oxidation Numbers
- Oxidation numbers are a useful tool for naming compounds as some elements can exist with more than one oxidation number
- For compound with two elements it is straight forward to name the compound
- For example
- PCl3 is phosphorus(III) chloride or phosphorus trichloride
- PCl5 is phosphorus(V) chloride or phosphorus pentachloride
- OF2 is oxygen difluoride
- O2F2 is dioxygen difluoride
- In order to name a more complete compound we use Roman numerals for the element that has a variable oxidation number
- K2CrO4 potassium chromate(VI)
Worked example
Can you name these transition metal compounds?
- Cu2O
- MnSO4
- Na2CrO4
- KMnO4
- Na2Cr2O7
Answer:
Answer 1: copper(I) oxide:
The ox. no. of 1 O atom is -2 and Cu2O has overall no charge so the ox. no. of Cu is +1
Answer 2: manganese(II) sulfate:
The charge on the sulfate ion is -2, so the charge on Mn and ox. no. is +2
Answer 3: sodium chromate(VI):
The ox. no. of 2 Na atoms is +2 so CrO4 has an overall -2 charge, so the ox. no. of Cr is +6
Answer 4: potassium manganate(VII):
The ox. no. of a K atom is +1 so MnO4 has overall -1 charge, so the ox. no. of Mn is +7
Answer 5: sodium dichromate(VI):
The ox. no. of 2 Na atoms is +2 so Cr2O7 has an overall -2 charge, so the ox. no. of Cr is +6. To distinguish it from CrO4 we use the prefix di in front of the anion