## Order or orbitals

Giovanni Anguiano-Gutierrez 3L
Posts: 52
Joined: Thu Jul 25, 2019 12:17 am

### Order or orbitals

Concerning when we do electron configuration, why does Lavelle put the d-orbital before the s-orbital? For example, if we're finding Arsenic it'd usually look like [Ar]4s2 3d10 4p3, however, Lavelle would write it as [Ar]3d10 4s2 4p3

Tanmay Singhal 1H
Posts: 143
Joined: Sat Jul 20, 2019 12:16 am

### Re: Order or orbitals

half and full shells are more stable

Julie Park 1G
Posts: 100
Joined: Thu Jul 25, 2019 12:15 am

### Re: Order or orbitals

I believe that we're supposed to write the electron configuration such that it is in a least-to-most energy order

Example from slides:
Sc: [Ar] 3d^1 4s^2

Once the 4s orbital has been occupied and electrons begin to fill the 3d state, the 3d state is considered to have less energy than 4s

That's why it would be Sc: [Ar] 3d^1 4s^2
and not Sc: [Ar] 4s^2 3d^1

Note that....
Lower energies 1s 2s 2p 3s 3p 3d 4s 4p higher energies

Posts: 49
Joined: Wed Sep 18, 2019 12:20 am

### Re: Order or orbitals

Orbital filling concerns Aufbau's principle. Essentially, in their ground state electrons fill orbitals in order of increasing energy, lower first, and higher last.
The energy of orbitals follows the (n+l) rule. The 4s orbital has n=4 and l=0, 4+0=4, and the 3d orbital has n=3 and l=2, 3+2=5. Thus, 4s has a lower energy in comparison to the 3d orbital.

HOWEVER, when electrons are placed into 3d, the relative energies of the two sub-levels fluctuate, allowing it to be so that the 3d electrons are actually lower in energy than the 4s electrons, which is why they are written first.

Key takeaway: the 4s sub-level is only lower in energy if there are no electrons in the 3d orbitals.

MeeraBhagat
Posts: 95
Joined: Sat Aug 24, 2019 12:15 am

### Re: Order or orbitals

Can someone explain the reason for why copper and chromium are exceptions to the normal filling rule?

Jillian C 4C
Posts: 51
Joined: Sat Jul 20, 2019 12:16 am

### Re: Order or orbitals

When writing electronic configurations, the order goes from lowest energy to highest energy, and the 3d orbital has lower energy than 4s, so the 3d goes before the 4th level.