After 2 years of chemistry, i have some nagging problems which glare at me everytime. When atoms combine to form compounds, they combine to form compounds with zero valence right? But in that case why are there compounds such as NO3^-? or SO4^2- or OH^-? If such compounds exist, then why can't there be compounds like such as PS^2-, or BBr^3-, etc with any combinations like these? And since the bonds are all essentially metallic, covalent and ionic, then ultimately couldn't i have any combination of elements, example KMgFeBOHSe, after all, it would just be a matter of placing the right number of atoms at the right places, so it all adds up. Also, my teacher says that sulphuric acid is a stronger acid then hydrochloric acid, because it has more hydrogen atoms in one molecule(roughly like that). But H2SO4 has a lot of bonds, so for it to dissociate in water, it needs a lot of energy right? But for HCl, there is only 1 bond, so it doesn't need a lot of energy to dissociate. So shouldn't my teacher's statement be true only at high temperatures?