When dealing with metal cations, the process of electron removal follows a specific order based on the principal quantum number, denoted as n. The n value indicates the shell number or energy level of the electrons. For instance, in the electron configuration of an atom, such as 1s², 2s², 2p⁶, 3s², and 3p⁶, the numbers preceding the subshell letters (s and p) correspond to the n values.

In this example, the 1s² electrons are in the first shell where n equals 1, while the 2s² and 2p⁶ electrons are in the second shell where n equals 2. Similarly, the 3s² and 3p⁶ electrons are located in the third shell where n equals 3.

When forming a cation, electrons are removed starting from the highest shell number. Therefore, if we begin to remove electrons, we would first target the 3p⁶ electrons before addressing the 3s² electrons. This systematic approach ensures that electrons are removed in the order of their energy levels, prioritizing those in the outermost shell.