The discontinuity in the trend of first ionization energies between Mg and Al, as well as P and S can be explained by looking at their electron configurations and the underlying principles of atomic structure.
First, let's start with the ionization energy trend across the periodic table. Generally, the first ionization energy increases from left to right across a period. This is because, as you move from left to right, the atomic number of the elements increases, so does the number of protons in the nucleus. This increased nuclear charge tends to hold the electrons more tightly, making it harder to remove them, thus increasing ionization energy.
Now, specifically focusing on the discontinuity between Mg and Al, and P and S, we need to consider the electron configurations of these elements.
Magnesium (Mg) has the electron configuration of 1sĀ² 2sĀ² 2pā¶ 3sĀ², while Aluminum (Al) has the configuration of 1sĀ² 2sĀ² 2pā¶ 3sĀ² 3pĀ¹. The important thing to note here is that Al has an extra electron in its 3p orbital compared to Mg. When removing an electron from Mg, you are taking it from the 3sĀ² orbital, which is closer to the nucleus and more tightly held. However, when removing an electron from Al, you are taking it from the 3pĀ¹ orbital, which is further away from the nucleus and less strongly held. Therefore, it requires slightly less energy to remove an electron from Al compared to Mg, resulting in a lower first ionization energy for Al.
Similarly, phosphorus (P) has the electron configuration of 1sĀ² 2sĀ² 2pā¶ 3sĀ² 3pĀ³, while sulfur (S) has the configuration of 1sĀ² 2sĀ² 2pā¶ 3sĀ² 3pā“. Here also, removing an electron from P means taking it from the 3p orbital, which is closer to the nucleus and more tightly held, whereas, for S, you are taking it from the 3pā“ orbital, which is further away from the nucleus and less strongly held. Thus, it requires slightly less energy to remove an electron from S compared to P, resulting in a lower first ionization energy for S.
In summary, the discontinuity in the trend of first ionization energies between Mg and Al, as well as P and S, can be explained by considering the electron configurations and the position of the electrons being removed.