Practice question · Multiple choice
Boron has a LOWER first ionisation energy than beryllium, breaking the general trend. Why?
Hints
- Compare which orbital each element loses its electron from.
- Beryllium removes from a filled 2s; boron removes from a single 2p, which is higher in energy.
Show the answer
C. Boron’s outermost electron is in a higher-energy 2p orbital, not the filled 2s
Why
Beryllium () loses an electron from a filled, lower-energy . Boron () loses its lone electron, which sits higher and is more shielded, so it costs less despite the larger . (Pairing repulsion is the explanation for the oxygen dip, not this one.)
Practise Periodic Properties
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