What is the number of valence electrons in tin?

The quick answer here is that because tin, #"Sn"#, is a main-group element, the number of valance electrons will be given by its group number.

Tin is located in group #1color(red)(4)# of the periodic table, which means that it has #color(red)(4)# electrons in its outermost shell, i.e. #color(red)(4)# valence electrons.

Now, you can prove that this is the case by constructing tin's electron configuration.

Tin has an atomic number of #50#, so right from the start you know that its electron configuration must account for a total of #50# electrons. You will have

#"Sn: " 1s^2 2s^2 2p^6 3s^2 3p^6 3d^10 4s^2 4p^6 4d^10 color(blue)(5)s^color(red)(2) color(blue)(5)p^color(red)(2)#

The fifth energy level, #n = color(blue)(5)#, is tin's outermost energy shell. As you can see, tin has a total of #color(red)(4)# electrons on this level

• #color(red)(2)# electrons in the #5s# subshell
• #color(red)(2)# electrons in the #5p# subshell

  Once again, you can conclude that tin has #color(red)(4)# valence electrons.