Ionization Energy for Be vs B

[Angel Shen 1E]

Why does boron (B) have a lower first ionization energy than beryllium (Be), even though it has a higher atomic number? What causes this exception in the trend of increasing ionization energy across a period? And do other elements within the same group also fall under this exception?

[Siya Bhardwaj 1C]

In boron, the outer electron is in the 2p orbital while beryllium's outer electron is in the 2s orbital. The 2s orbital is more stable and lower energy than the 2p orbital, so it's energetically favorable for boron to lose its only 2p electron.