What are the exceptions to the periodic trends in ionization energy?

The exceptions to the periodic trends in ionization energy are the first ionization energy of beryllium is higher than that of boron and the first ionization energy of nitrogen is also higher than that of oxygen.

Explanation:

Taking a close look at the figure of first ionization energies, it clearly shows that the first ionization energy of beryllium is higher than that of boron and the first ionization energy of nitrogen is also higher than that of oxygen.

This is as a result of Hund's rule and electron configuration. For example,  the first ionization potential electron of beryllium is obtained from a 2s orbital while that of boron comes from a 2p electron. However, for oxygen and nitrogen, their electrons are obtained from 2p orbitals. While spin is uniform for all 2p electrons of nitrogen, it is different for oxygen.

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"standard molar enthalpy:

h2o ( liquid water ) : - 285.8 kj/mole

h2o ( water vapor ) : - 241.8 kj / mole.

δhrxn = - 241. 8 - ( - 285.8 )= -241.8 + 285.8 = 44.0

answer: 44.0 kj"