How do orbital energies change across the periodic table?

Atomic orbital energies generally become lower, meaning more negative, as nuclear charge increases across the periodic table, but the specific ordering of subshells can change between elements.

Think of each atom as a tiny house with rooms called orbitals where electrons live, and each room has different comfort levels called energy. As you move to atoms with more protons, the nucleus pulls electrons in tighter so the rooms get cozier and their energy numbers go down. However, some rooms on the same floor, like the three p and three s or three d orbitals, can swap how comfortable they are because of how close they are to the nucleus and how they shield each other. That means the usual order you learn, such as one s before two s or three p after three s, can shift for different elements, especially when d and f orbitals start to interact with higher principal levels.

Students often assume a single universal order always applies, which leads to mistakes when predicting electron filling for heavier atoms; check actual subshell energies for the element instead of relying on one list. Another common error is forgetting that increased nuclear charge lowers orbital energies overall but does not prevent specific subshell crossings, so be careful when moving across a period into transition metals. A quick fix is to visualize both principal energy level and subshell penetration and shielding, and to consult measured or computed orbital energies for tricky cases.

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Orbital Energy Trends Across the Periodic Table

1. Atoms are tiny houses with orbitals
2. More protons pull electrons tighter
3. Orbital energies decrease across periods
4. Subshell comfort can swap between elements
5. d and f orbitals interact with higher levels
6. Check actual subshell energies for elements

Common Orbital Energy Mistakes

1. Assuming one universal subshell order
2. Ignoring subshell crossings in heavy atoms
3. Neglecting penetration and shielding effects

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