[socomplicated]

Calcium chloride exists as dihydrate (CaCl2x2h20). it has 2 valence while Potassium chloride has 1 valence.

as i know valence means whether the atoms form one, two, three, or four bond, but i can't figure out how these combinations get these valences, since they are salts.

thanks!

[jdurg]

Well, even though these bonds are ionic, the valency still deals with how many pairs of electrons the atom can share.  All of the alkali metals are considered to have "one" valence electron since they have their S shells half filled.  Therefore, they are willing to bond with something that will give them a full shell.  When they bond with the chlorine atom, they are able to donate their electron to chlorine thus given both atoms a noble gas configuration.  With Calcium, there are two S-orbital electrons which can be "donated" so now two chlorine atoms can take an electron and wind up giving all three atoms the noble gas electron configuraiton.  This is why Calcium, Magnesium, Strontium and Barium form halides which have two halogen atoms in there.  

[socomplicated]

hi djurg!

thanks for your reply, but this is not the answer i am looking for. what i wonder is why CaCl2 has a total valence of 2, since Ca2+ alone gives a valence of 2 and Cl- alone has a valence of 1. how do i know that CaCl2 has a valence of 2 and not 1?

hope for replies!
thanks again!

[Mitch]

The center atom is the one used to determine valency.

[socomplicated]

The center atom is the one used to determine valency.

hi Mitch!

chemistry is not my strong side, so it is hard for me to figure out which atom in a molecule is the central atom or not. does we have another "trick" we can use to decide the total valence of a molecule? for me it seems like the highest valence of an atom in a molecule decides the total valence of that molecule....does this make sense?

thanks!

[Mitch]

The atom with the higher valency means it has more neighbors and most likely means it would be in the center of the molecule.