點解oxygen atom like to form O2- ion?

沒錯，佢的second e.a. 是positive，但不要忘記，oxgen不是無lala form oxide的。佢是要和其他的metal form ionic bond的。雖然拿兩位電子是要energy，但這個energy可以provided by lattice energy when oxide ion form ionic bond with other metal ions。由於charge大左(charge是2-)，lattice enthalpy就大左，所以整個rection來說，(e.g. sodium+oxygen去到sodium oxide)佢的enthapy change= 1st ionization enthalpy+1st and second electron affinty-lattice enthalpy會是負數，即是個reaction依然是spontaneous的。
所以一句講哂，form左oxide ion之後，charge大左，可以放多d lattice energy出來，所以o是想form2- ion的。
(不可以form3-，因為個e.a.太正，因為electron added會變成是再上一個shell，吸力會好小，而且repulsion亦大。balance之後，個lattice energy compensate唔到個e.a.)

oxygen atom 有8粒proton, 6粒eletron 喺outermost shell, 要多兩粒electron 先至達到 stable octet structure.
總共10粒 electron, 8粒 proton,
negative charge 大過 positive charge, 咁米 2- ion

It doesn have a reason it is just the way it is. And there are six more element like O. They are Carbon, Nitrogen, Fluorine, Phosphorus, Sulfur, Cchlorine, Bromine, and Iodine.And when they from a compounds with other element their name are change to Oxide, Carbide, Nitride, Fluoride, Phosphide, Sulfide, Chloride, Chloride, Bromide, Iodide. But it is just when they are at the end of the compounds