How do I determine the number of valence electrons in an atom?

Answer 1

Valence electrons are electrons outside of a complete electron shell. for example in the case of Li (Lithium). The valence Electron would be the electron in the 2s location, don't worry if you don't understand that part of it. Basically its the electron or electrons outside of the last filled shell. Or to put it another way the electrons that could be lost in order to have all electron shells filled completely. To determine the number of them you need to know how many electrons can be in any given shell. For example the s sub-shell can take 2 electrons while the p can take up to six and the d can take up to 10. so if you were looking for the valence electrons in Ag (no charge) you would take and count the number of D shell electrons that Ag has and those are your valence electrons. In this case if you count in the D shell section of the periodic table starting at Y (Yttrium) and going over to Ag you count a total of 9 valence electrons (basically Ag's place in it's group) So if it were Ag- (with a single negative charge) it would have one extra electron, therefore filling the shell and leaving no unpaired valence electrons. In the case of Ag+ (missing one electron) there would be 8 valence electrons. Hope you understand it a little bit better.

Answer 2

USe a periodic table until you memorize the pattern. The first column elements will have one valence electron ( sodium)the second will have 2 (calcium) the third 2 ill have 3 ( boron) oxygen has 6 halogens have 7 ( flourine, iodine) and noble gases have 8 ( xenon neon)