The observed general trend in ionization energies shows two slight discontinuities in each period. These discontinuities occur when moving from the end of one subshell to the beginning of the next subshell within a given period.
To understand why these discontinuities occur, let's examine the electron configuration at these points. In the periodic table, the electron configuration represents the arrangement of electrons in an atom's energy levels or orbitals.
Each subshell has a different energy level, denoted by the principal quantum number (n) and is represented by a series of letters (s, p, d, f). The energy of these subshells generally increases with increasing values of n. For example, the 3p subshell has a higher energy than the 2p subshell.
Now, as we move across a period from left to right, the number of protons in the nucleus increases, resulting in a stronger attractive force between the nucleus and the electrons. This increased nuclear charge tends to pull the outermost electrons closer to the nucleus, making it harder to remove them.
However, there are two exceptions to this general trend. In the transition from the end of the s-block to the beginning of the p-block, and from the end of the d-block to the beginning of the p-block, we observe a slight discontinuity in ionization energies.
Let's take an example of the second period. The electron configuration at the end of the 2s subshell is 1s^2 2s^2. At the beginning of the 2p subshell, the electron configuration starts with 1s^2, and then it fills the 2p subshell. During this transition, there is a slight decrease in ionization energy.
This decrease occurs because the 2p subshell is higher in energy than the 2s subshell. The outermost electrons in the 2p subshell experience less attractive force from the nucleus compared to the 2s electrons. Thus, it is relatively easier to remove an electron from the 2p subshell, resulting in a lower ionization energy.
Similarly, in the transition from the end of the d-block to the beginning of the p-block, there is another slight discontinuity. For example, in the fourth period, at the end of the 3d subshell, the electron configuration is 3d^10 4s^2. At the beginning of the 4p subshell, the electron configuration starts with 3d^10, and then the 4p subshell is filled. During this transition, there is another slight decrease in ionization energy.
This decrease occurs because the 4p subshell is higher in energy than the 3d subshell and experiences less attractive force from the nucleus. Therefore, it is relatively easier to remove an electron from the 4p subshell compared to the 3d subshell, resulting in a lower ionization energy.
In summary, these slight discontinuities in ionization energies occur due to the energy differences between the subshells. The higher energy of the next subshell allows for a relatively easier removal of electrons, resulting in a temporary decrease in ionization energy.