Here are a few doubts that I had while reading for an upcoming exam.While answering please keep in mind that I'm in 9th grade so simple answers would be better- 1)Why is the electronic configuration of Chromium - [Ar] 3d5 4s1 instead of [Ar] 3d4 4s2 ? Similarly why does Coppers electronic configuration not follow the pattern ? 2)Why do substances combine ? I was told they combine to become stable but what does 'stable' mean ? 3)My textbook says that substances combine so that they can attain octet (8 electrons in outer shell ) or duet (2 electons in outer shell ) configuration.I think that I read somewhere that an element cannot have a valency of more than 4 but Nickel which has an electronic configuration of 2,8,16,2 will have to gain 6 electrons to achieve octet configuration and 6 is more than 4.How is this possible ? 4)I also read that elements try to attain the nearest noble gas's electronic configuration.But the noble gas closest to Fe (2,8,14,2) is Argon (2,8,8).So Fe should lose 8 electrons.But the valency of Fe is 2 or 3.Why ? 5)Why do elements have variable valencies ? 6)Elements combine to become stable.That means that compounds should be stable.So in the reaction - 2 H2 + O2 → 2 H2O why do the compounds H2 and O2 combine if they are already stable ? 7)I think someone told me that ionic compounds dissociate in solution.For example NaCl.Why does this happen ?