Just because an atom has 8 electrons in it's outer shell that doesnt mean it'll be neutral. Anyway, the larger molecules require more than 8 to get a full outer shell. As an atom in it's normal state, oxygen is 'neutral' because the protons balance out the electrons. When two or more atoms combine they give, recieve or share electrons and then it's the electrostatic forces (postitive attracted to negative etc.) which hold these atoms together. The reason im confused is that nobody's ever explained (,or at least not to me,) why an electron is positive. How does positive attract negative? I suppose some physicist has worked out a theory using maths, but nothing is proven.