Answer:
Roman numbering in IUPAC naming system
Explanation:
This is quite an open question. Let's firstly separate the periodic table into two standard groups: group A elements and group B elements (transition metals).
The charge (or the oxidation state) of an element in group A can be identified by the group number. For example, group 1A elements would always have a charge of +1, as they have only one valence electron to lose.
Similar trend applies to group 2A: each element in that group would have a charge of +2, as each atom has 2 valence electrons to lose to become a cation.
You will notice that this is true fro group 3A and group 4A as well. Now, since an octet is the desired state for any species, starting with group 5A, it's easier to gain 3 electrons for species than lose 5 electrons to obtain an octer, meaning we'd expect -3 oxidation state for group 5A elements, -2 oxidation state for group 6A elements and -1 oxidation state for group 7A elements.
Notice that in the majority of cases, this is the standard trend and we'd generally only have one predominant oxidation state.
Considering group B, the transition metals, most of them have several oxidation states. That's why we usually memorize the ones which only have one oxidation state (such as zinc, silver) and in any other case when a transition metal has several oxidation states, they're identified in the name by using Roman numbering system.
Let's look at an example. Assume the problem states we have a salt which is iron chloride. This would be an improper name, as iron has two oxidation states: +2 and +3. That's why we have the rules of IUPAC naming to avoid ambiguity. If we had iron with an oxidation state of +2, we'd call the salt iron(II) chloride. An oxidation state of +3 would indicate iron(III) chloride.
To summarize, the main key of knowing the charge of a transition metal in a compound is to follow the IUPAC naming rules.
