chem令我困難的問題

first you should remember the electronic configuration of the first 20 element

for example, the electronic configuration of calcium is 2,8,8,2 and that of chlorine is 2,8,7
calcium is metal while chlorine is non-metal. So it will form ionic bond, which means calcium will lose it outermost electron(it will lose 2 electrons from its outermost shell)
and sulphur will gain 1 electron to attain the octet structure.

after losing electrons, calcium become positive ion because it has 20 positively -charged protons but 18 negatively-charged electron, the charge cannot be balanced so the calcium ion contain a negative charge.
For chlorine, after gaining electrons, it will carry 1 negative charge.

calcim tend to lose 2 electrons, while chlorine tend to gain 1 electron.
So we need 2 chlorine to react with 1 calcium to form the compound

and I also drew some electronic diagram for your reference.........（畫工差請見諒）
http://www.pikipimp.com/pp/pimped_photo/s/image/14/451/412/e1.JPG?ts=1193496264953

you can also draw it in this way..it is also correct.
http://www.pikipimp.com/pp/pimped_photo/s/image/14/451/412/e2.JPG?ts=1193496372656

if you have further questions to ask you are welcome to add my msn
[email protected]