posted by riley .
how can you tell how many valence electrons family 3 (transition metals) have?
im confused about this.
I am unsure how you are using family. Do you mean period 3 (period 3 elements are Na, Mg, Al, Si, P, S, Cl, and Ar. Group 3 (or Group III)(or group 13)consists of B, Al, Ga, In, and Tl.
In the first case, the number of valence electrons consists of the outside electrons and in this case that is the group number; i.e., Na has 1 for it is in group IA, Mg has 2 for it is in group IIA, Al has 3 for it is in group IIIA, etc. If the latter case B, Al, Ga, In and Tl have 3 electrons in their outer shell(s) and they have a valence of 3. (Thalium also exhibits a +1 valence in some of its compounds). If you mean transition metals, they are neither period 3 nor group 3. The first transition series (the 3d series) is in period 4. This scenario can be confusing for the number of valence electrons for they vary. One way to make an educated guess is to look at the electron configuration. They are as follows:
Scandium is [Ar] 3d1 4s2
Titanium is [Ar] 3d2 4s2
Vanadium is [Ar] 3d3 4s2
Chromium is [Ar] 3d5 4s1
You could make a case for Sc as +3 and it is. The last three electrons are the outside ones.
You could make a case for Ti being +2 or +4. Actually it has common valences of +3 and 4. For V, one could make the case for +2 and +5 whereas it is commonly 2,3,4, and 5. So the number of so-called valence electrons depends upon what anion it is combining with and how many of those d electrons are drawn away. Educated guesses help a little; memorizing does the rest. Experience doesn't hurt anything.