Then, when it comes across Zinc,Zn to Gallium, Ga, the trend of first ionisation energy decreases.
The electronic configuration of Zn is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 while the electronic configuration of V is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p1. In Zn, the fully-filled d orbital is more stable than Ga which is only partial filled. In Ga, the valence electron is in 4p shell, which is further from the nucleus, causing the nuclear attraction towards valence electron weaker than Zn.
Thus,it is easier to remove the electron during first ionisation energy in Ga compared to Zn. Therefore, the trend of first ionisation energy decreases from Zn to Ga. After Ga, the trend returns to increasing again.