keep in mind that between 4-7, there could be #1 in front of the main name. for example with #4: 3,4-dimethyl-1- hexene. this honestly depends on the professor how he/she likes it. It is not necessary because if the number is not specified, it is assumed is #1
Oxygen is a diatomic element hence exists as O2 for majority of its existence in our atmosphere. Although small portion does exist in form of O3 which protects earth from sun's harmful ray, the majority portion of oxygen has O2 which is the oxygen we breathe.
