Question

Is the number of valence electrons the same for all elements within a group on the periodic table?

Answer 1

This is not always true.

Explanation:

For main group elements, the number of valence electrons is the same in every element in the same group.

[Example]
Group 1 elements have 1 valence electron in the `s` orbital.
H `1s^1`
Li `[He]2s^1`
Na `[Ne] 3s^1`
etc…

Group 16 elements have 6 valence electrons in `s` and `p` orbitals.
O `[He]2s^2 2p^4`
S `[Ne] 3s^2 3p^4`
Se `[Ar] 3d^10 4s^2 4p^4`
…

However, some transition metals don't follow this rule.
For example, let's see the electronic structure of Group 8.
Fe `[Ar]3d^6 color(red)(4s^2)`
Ru `[Kr]4d^7 color(blue) (5s^1)`
Os `[Xe] 4f^14 5d^6 color(green) (6s^2)`

The electronic sturcture of iron, ruthenium and osmium are cited from WebElements (https://www.webelements.com/).