A graph of first ionisation enthalpy (y-axis) against atomic number (x-axis) for the elements lithium to neon shows a generally rising zig-zag line with two small dips: one at boron, which lies below beryllium, and one at oxygen, which lies below nitrogen. Which explanation of the two dips is correct?
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Boron's outer 2p electron is shielded by the 2s electrons and is held less tightly than beryllium's 2s electrons. In oxygen, one 2p orbital holds a pair whose mutual repulsion makes one of them easier to remove than an electron from nitrogen's half-filled 2p³.
- A.
Atomic radius decreases across a period, so B is smaller than Be and O is smaller than N.
- B.
A 2s electron penetrates closer to the nucleus than a 2p electron, and four electrons in three 2p orbitals cannot all be unpaired.
- D.
Boron has one 2p electron (2p¹) and oxygen has four (2p⁴); neither subshell is half-filled or full.
NCERT: Class 11 Chemistry, Chapter 3