[Fourier]

I am confused on how to determine the number of valence electrons an atom has when you are given the electron configuration.  Is there a quick, easy method?  I think my method is incorrect.

Take arsenic for example.  The electron configuration is 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p³.
The number of electrons in the 4th shell is 5.  Therefore, I assumed there were 5 valence electrons.   This agrees with my periodic table showing the element listed in group 5A.

Then I tried to determine the number of valence electrons in nickel by looking at its electron configuration - 1s²2s²2p⁶3s²3p⁶4s²3d⁸.

The outermost shell is 3.  However, there are not 16 valence electrons.  Apparently, we only count the electrons in the 3d subshell.  Why do we only count the electrons in the 3d subshell unlike arsenic, where we counted the both the 4s and 4p subshells?

Thanks

[vhpk]

I am confused on how to determine the number of valence electrons an atom has when you are given the electron configuration.  Is there a quick, easy method?  I think my method is incorrect.

Take arsenic for example.  The electron configuration is 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p³.
The number of electrons in the 4th shell is 5.  Therefore, I assumed there were 5 valence electrons.   This agrees with my periodic table showing the element listed in group 5A.

Then I tried to determine the number of valence electrons in nickel by looking at its electron configuration - 1s²2s²2p⁶3s²3p⁶4s²3d⁸.

The outermost shell is 3.  However, there are not 16 valence electrons.  Apparently, we only count the electrons in the 3d subshell.  Why do we only count the electrons in the 3d subshell unlike arsenic, where we counted the both the 4s and 4p subshells?

Thanks

To determine the valence shell electron you must first determine whether the element is in group A or group B. To do this you must write the energy level initially if the highest belong to d or f it is group B -element, if the highest belong to s or p it is group A - element.
With group A - element, the valence electron is equal to the amount of the electron in the outer shell
With group B - element, the valence electron is the sum of the amount of the outer layer's electron and the electron of the energy level that's next to it.
Ex: 1s₂2s₂2p₃ -> group A - element, the amount of valence electron = 2 + 3 = 5
1s₂2s₂2p₆3s₂3d₁4s₂ - > group B -element, the amount of valence electron = 2 + 1 = 3

[Bioionic]

See if this link helps any.
http://environmentalchemistry.com/yogi/periodic/Ni.html

[Borek]

1s²2s²2p⁶3s²3p⁶4s²3d⁸.

The outermost shell is 3.

No, it is 4.