Answer:
Electrons are always marked negative Hence electronic energy is also always taken as negative. It is because when an electron is at infinite distance from nucleus , there the kinetic energy of that electron can not be calculated. So, it is taken to be zero . ... And thus the energy of electron becomes less negative
O2.
Because it is only made up of one type of element it is an element. Howevern there is more than one oxygen atom because oxygen cannot exist as a single atom, therefore making it a molecule as well.