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Among the following, the energy of 2s orbital is lowest in:
- Li
- H
- Na
- K
Correct answer: K
Solution
Orbital energy scales as -13.6*Z_eff^2/n^2, so for a fixed shell (2s) the energy becomes more negative as nuclear charge rises. Among H, Li, Na, K the 2s electrons feel the greatest effective nuclear charge in K (Z=19), so 2s is lowest in energy in K.
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