Gen Chem_Ch 6_19th_Exceptions in Electron Configuration

Overview

Dr. Reza Latifi explains why chromium and copper depart from the expected Aufbau electron configurations, using chromium’s [Ar] 4s¹ 3d⁵ configuration and copper’s [Ar] 4s¹ 3d¹⁰ as examples. He connects these exceptions to the added stability associated with half-filled or fully filled d subshells, and shows that transition-metal ions lose 4s electrons before 3d electrons.

Key takeaways

Chromium’s observed ground-state configuration is [Ar] 4s¹ 3d⁵, not the predicted [Ar] 4s² 3d⁴, because a half-filled 3d subshell is especially stable.
Copper’s ground-state configuration is [Ar] 4s¹ 3d¹⁰, as transferring one 4s electron completes its 3d subshell.
For transition-metal cations, remove electrons from the outer 4s subshell before 3d; consequently Cr⁺ is [Ar] 3d⁵ and Cr²⁺ is [Ar] 3d⁴.
The electron-transfer exceptions reflect an energy balance: the stabilization from a half-filled or filled d subshell can outweigh the cost of promoting an electron from s to d.
Molybdenum provides another half-filled-subshell example, with [Kr] 5s¹ 4d⁵ rather than the simple expected [Kr] 5s² 4d⁴.

Chapters

0:00 Chromium’s 4s-to-3d Shift and Ion Configurations

Chromium has 24 electrons; its expected [Ar] 4s² 3d⁴ configuration differs from its actual [Ar] 4s¹ 3d⁵ configuration.
The highest principal quantum number remains n = 4, placing chromium in period 4; counting its 4s and 3d electrons gives group 6.
When chromium is ionized, 4s electrons are removed before 3d electrons: Cr⁺ is [Ar] 3d⁵ and Cr²⁺ is [Ar] 3d⁴.
The [Ar] core represents 18 core electrons.

3:00 Copper and Molybdenum Show Related Configuration Exceptions

Copper’s expected 4s² 3d⁹ arrangement shifts one 4s electron into 3d, producing [Ar] 4s¹ 3d¹⁰.
The copper shift leaves the d subshell fully filled, while chromium’s shift creates a half-filled 3d⁵ subshell.
Molybdenum follows a similar pattern: instead of 5s² 4d⁴, its configuration is [Kr] 5s¹ 4d⁵.

4:00 Why Half-Filled and Filled d Subshells Stabilize Atoms

Moving an electron from 4s to 3d costs energy because the electron enters a higher-energy subshell.
Chromium gains a half-filled 3d⁵ subshell, with one electron in each of the five d orbitals.
Copper gains a fully filled 3d¹⁰ subshell; these symmetric arrangements can provide enough stabilization to offset the transfer’s energy cost.
The stability benefit comes with a tradeoff: the 4s subshell changes from two electrons to one.

Transcript excerpt

0:11 [music] >> Hello and welcome back to another lecture from general chemistry. In this lecture, we're going to go through some exceptions in the electron configuration for specific groups. Why do we get them and what's the science behind it? We start with a chromium. Chromium it has a 24 electrons and so the expected electron configuration for chromium should be this. Should be argon which represent 18 electrons and then we get 4s with two electrons and the 3d should be four because those two represent 20 electrons and that's going to leave us with four electrons in d orbital. This is the expected, but the real one is we get argon 4s with only one electron in it while the 3d it has five electrons from it. So

1:00 technically one electron was taken from s and given to the d orbital. Does it change anything about the valence electrons or say the the period at all? Again, the highest n in this case is equal to four. So the element belong to the fourth period. Now when it comes to valence electrons we say go with this uh valence electron go with ns plus nd or n minus one d technically. d So we have 4s with one electron and then we say 3d it has five electrons. So we go one plus five that's going to give us uh uh six. So technically chromium belong to group six and but we say b cuz we want to say this is not main metal or

1:48 main element is a transition metal. Okay, how about taking electron from it? Chromium is equal to argon 4s 1 3d 3d5. You you to get If you want to take one electron from it, just one, that's going to leave you with argon 4S 1 - 1 it will

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Related notes

Summary, takeaways, and chapters were generated by AI from the video's transcript and may contain errors. The video belongs to its creator, Dr. Reza Latifi at Oklahoma State University, OSU.