Question

how can you tell if oxygen has a positive charge when looking at a molecular formula

Answer 1

like\[K _{2}Cr _{2}O _{7}\]

Answer 2

I think you mean oxidation number instead of charge. Oxygen (O2) as a molecule, is neutral (no charge). Do you know about oxidation numbers?

Answer 3

oh yes that is what I mean and yes I do know a little about them

Answer 4

The oxidation number for oxygen is almost always -2. It rarely has a positive oxidation number. That only happens when the atom it bonds with is more electronegative (like fluorine).

Answer 5

In the example of \[K_2Cr_2O_7\] the oxidation number of oxygen is -2. Because oxygen almost never has an oxidation number other than -2, it is a very good reference to use when finding the oxidation numbers of other species in the molecule.

Answer 6

ok, because my teacher had us answer this question (see attached) and she has the correct answer as being "a", but if oxygen's charge is -2 then the answer should be "e" correct?

Answer 7

sorry, i mean if oxygen's oxidation number is -2

Answer 8

I'm afraid your teacher is correct. To do these type of questions, you need to first look at the charge of the entire molecule, which in this case, is 0. The oxidation numbers of each species (K, Cr and O) should add up to the charge of the entire molecule. So we know that O has the oxidation number of -2 and that there are seven oxygen atoms in the molecule. That gives us -14 (-2*7). Now, can you tell me the oxidation number of potassium in the molecule?

Answer 9

potassium's oxidation number is +1, so since there are 2 its +2

Answer 10

Yup, so by adding up the oxidation numbers of oxygen and potassium, we get -12. But the overall charge of the entire molecule is 0. So to make -12 to become 0, we need to +12. This is also the oxidation number of two chromium atoms. Since we only want to know the oxidation number of one chromium atom, we divide that by 2, which is +6.

Answer 11

oh, ok that makes sense. Thank you!

Answer 12

No problem! :)