Chemistry A-Level
4s before 3d
What 4s before 3d means in Chemistry A-Level, as defined for AQA.
Definition of 4s before 3d
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Reference: AQA
The 4s sub-shell lies just below 3d in energy so it fills first, but once the 3d sub-shell is occupied it drops below 4s.
Topics that use 4s before 3d
- Electron configuration (AQA A-Level)
Related terms
- First ionisation energy
- Group 2 trend
- ion configurations
- ionisation equations
- orbital capacities
- Period 3 trend
- sub-shell filling order
Using 4s before 3d in the exam
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