What is the trend in ionization energy across period 3 in the periodic table?

Ionization energy generally increases across period 3 because the nuclear charge increases but the shielding of the outer electrons remains relatively the same. This means that the electrostatic force of attraction between the outer electrons and the nucleus is becoming greater, so more energy is needed to remove the electrons. Since the elements are in period 3, this means that all of them have an outer electron in energy level 3. However, the trend has two anomalies.

The first is between Mg and Al, because the outer electron of Mg is in the orbital 3s, whereas that of Al is in 3p. The 3p electron has more energy than the 3s electron, so the ionization energy of Al is actually less than that of Mg. This makes sense because the 3p electron requires less energy to be removed from the atom. Remember that if an electron has more energy, it needs less external energy to be removed from the atom because having more energy makes it more unstable.

The second anomaly is between P and S. Although both have outer electrons in the 3p orbital, that of P is unpaired (3p³) but that of S is paired (3p⁴). 3p orbitals can fit up to 6 electrons, but if there are 3 or less electrons, they are unpaired and take up a "space" just for themselves. Since the fourth electron in 3p⁴ is paired, it will experience a repulsive force from the third electron. This makes it easier to remove, and therefore the ionisation energy of S is less than that of P.