Valence Electrons

[Melissa Trieu 1K]

For number 85 in chapter 1 it asks for the valence-shell configuration, including the outermost d-electrons. When we count the number of valence electrons, do we usually count those in the d-orbitals even though it is in a lower shell? For example does cobalt have 2 or 9 valence electrons since its electron configuration is [Ar] 3d74s2?

[Niharika Reddy 1D]

Edit: Cobalt has 9 valence electrons. The 3d subshell has a higher energy than the 4s one, and since electrons fill up in an order of increasing energy, the 3d subshell is filled after 4s. However, once the 4s orbital is filled and electrons enter the 3d orbitals, the 3d orbitals are lower in energy than the 4s one (top of page 55 in the course reader), which is why 3d is written before 4s in electron configuration. The valence electrons are considered to be all of those written after the preceding noble gas configuration.

So for Cobalt, the electron configuration is: [Ar]3d74s2
Since there are 9 electrons after the noble gas configuration of Argon, Cobalt has 9 valence electrons.

Sorry for the confusion, hope that helps clear things up!

[HimaniMadnawat3L]

Valence electrons are any electrons after the closest noble gas to the element you are looking at. Cobalt will have 9 valence electrons since the d level electrons are counted. D level electrons are slightly higher in energy than the next level's s orbital. For exmaple: 3d is at a higher energy level than 4s, so 4s will fill before 3d. So, even though d is in the n=3 level, they energy is high enough to have 4s fill before the 3d.

[taywebb]

This really cleared things up for me! Does this mean that zinc has 12 valence electrons?