Which pairs of elements within a period are exceptions to the general increase in first ionization energy across the period?

• A. Be and B, N and O
• B. C and N, F and Ne
• C. B and C, O and F
• D. Li and Be, Na and Mg

Answer: (A)

Verified step by step guidance

1. Recall that the first ionization energy generally increases across a period from left to right due to increasing nuclear charge and decreasing atomic radius, which makes it harder to remove an electron.
2. Understand that exceptions to this trend occur because of electron configurations and subshell stability, particularly involving half-filled and fully filled subshells.
3. Identify that the first exception is between Beryllium (Be) and Boron (B): Be has a filled 2s subshell, while B starts filling the 2p subshell, which is higher in energy and less stable, causing B to have a slightly lower ionization energy than Be.
4. Identify the second exception between Nitrogen (N) and Oxygen (O): N has a half-filled 2p subshell (three unpaired electrons), which is relatively stable, while O has one paired electron in the 2p subshell, causing increased electron-electron repulsion and a slightly lower ionization energy for O compared to N.
5. Conclude that these exceptions (Be and B, N and O) are due to subshell electron configurations affecting the ionization energy trend across the period.