How do chemists know that an oxygen molecule have unpaired electrons?

In a heat engine of 1000 j of heat enters the system and the piston does 500 j of work what is the final internal energy of the system if the inital energy was 2000 j we have to do all of these down here 1)write the equation 2)list out your know variables 3)plug the numbers into the equations 4)solve 5)write your solution statemtn that includes inital energuy and final energuy added

+= + + balance the equation on coefficients, ex. 1,1,1

Of element x has 22 protons, how many electrons does it have