claudia_1e wrote:In class today, Dr. Lavelle discussed an "exception" in writing electron configurations... He said "Half full d5 and full d10 subshells have lower energy". So in the example of Cr, the configuration is written as [Ar]3d5 4s1. Can anyone explain this to me? Why does the 4s shell not get filled first?
-The exceptions for Electron Configurations are for certain Transition Metals.
A. This phenomenon is prevalent in atoms with d and f electrons
1. Cr (Mo, too): [Ar} 4s^2 3d^4 is actually [Ar]4s^1 3d^5
2. Cu (Ag, Au too): [Ar]4s^2 3d^9 is actually [Ar]4s^1 3d^10
3. The reason this occurs is that these configurations are energetically favorable. This means that they have a lower energy.
C. This occurs for Cr, Mo, Cu, Ag, Au when these elements are neutral.
-Most s and p block elements become ions that have noble gas configurations
1. N: 1s^2 2s^2 2p^3 N^3-: 1s^2 2s^2 2p^6 , which is isoelectronic to Ne
-For transition metals, the outer s electrons are lost first because it takes too much energy to pull off all the s and d electrons. Therefore, when writing the electron configurations, always pull of the s first! Remember: every element has all energy levels, it just depends whether it's occupied or not.
Mn: [Ar}4s^2 3d^5 so Mn^2+ is [Ar]3d^5