What would happen if one element in the periodic table was misplaced?

Then the whole table would be messed uo

The table would be messed because the element would not be in the right group. For example, if Potassium was misplaced and switched with helium - it would be misguiding for people looking at the periodic table. Potassium (an alkali metal) and helium (a noble gas) do not have the same characteristics. Potassium is highly reactive while helium is stable. So if elements were misplaced, they’d be in the wrong group, and will cause people to wrongly identify the misplaced element’s characteristics.

1. o2(g)as a product

Yes it would be a physical change.