This is not a linear trendm the ionisation energy of boron being unexpectedly less than that for beryllium, but this is due to the 2s orbital being totally filled in beryllium, whereas boron has one electron in a 2p orbital as well, and the 2s orbitalis shielded much more than the 2p orbital, which gives boron a lower ... read more
According to periodic trends, boron should have a higher ionization energy but because its valence electron (2p1) is shielded by the 2s electrons, less energy is required to remove the the 2p electron(s) from a boron atom than is requried to remove the 2s electron from a beryllium atom. read more
When you remove an electron from beryllium, you are taking away an electron from the 2s orbital. When you remove an electron from boron, you are taking an electron from the 2p orbital. The 2p electrons have more energy than the 2s, so it is easier to remove them as they can more strongly resist the effective nuclear charge of the nucleus. read more