The process of ionization energy (IE) is fundamental to understanding how atoms interact and form chemical bonds. Ionization energy is defined as the minimum energy required to remove one electron from a neutral, isolated atom in its gaseous state. This initial energy measurement is called the first ionization energy, or \(\text{IE}_1\). Atoms, however, are capable of losing more than just a single electron, which leads to the concept of successive ionization.
Defining Successive Ionization Energy
Successive ionization energy refers to the energy required to remove each subsequent electron after the first one has already been removed. There are multiple successive ionization energies, such as the second (\(\text{IE}_2\)), third (\(\text{IE}_3\)), and so on, up to the total number of electrons in the atom. The removal process is sequential, always targeting the resulting positive ion.
This sequential nature is represented by simple chemical equations. For example, the removal of the first electron from a neutral atom \(\text{M}\) requires \(\text{IE}_1\): \(\text{M}(\text{g}) \rightarrow \text{M}^+(\text{g}) + \text{e}^-\). The second ionization energy, \(\text{IE}_2\), is the energy needed for the reaction \(\text{M}^+(\text{g}) \rightarrow \text{M}^{2+}(\text{g}) + \text{e}^-\). The third ionization energy, \(\text{IE}_3\), continues this trend, removing an electron from the doubly charged ion: \(\text{M}^{2+}(\text{g}) \rightarrow \text{M}^{3+}(\text{g}) + \text{e}^-\). Each successive step involves removing an electron from a species that carries an increasingly higher positive charge.
The General Trend of Increasing Energy
The energy required always increases with each subsequent electron removal (\(\text{IE}_1 < \text{IE}_2 < \text{IE}_3[/latex], and so on). This predictable increase is due to the fundamental change in the electrostatic forces within the atom. As each electron is removed, the remaining electrons are held tighter by the same number of positively charged protons in the nucleus. The ratio of positive nuclear charge to negative electron charge increases, resulting in a greater net attraction on the remaining electrons. The remaining electrons are therefore closer to the nucleus and experience a higher effective nuclear charge, requiring more energy to overcome the stronger electrostatic pull. For instance, removing an electron from a singly charged positive ion ([latex]\text{M}^+[/latex]) is more difficult than removing one from a neutral atom ([latex]\text{M}[/latex]). Similarly, removing an electron from a doubly charged ion ([latex]\text{M}^{2+}[/latex]) is harder still. This general, gradual upward trend in energy is expected as the ion becomes more and more positively charged.
Using Ionization Jumps to Determine Electron Configuration
While successive ionization energy generally increases, its most practical application comes from observing the massive, non-linear jumps in energy that occur. These discontinuities in the otherwise smooth trend provide direct evidence for the atom’s internal structure and electron configuration. The huge jump in energy signals a transition from removing a valence electron to removing an electron from an inner, complete electron shell.
Valence electrons are the outermost electrons, and they are relatively easy to remove compared to core electrons, which are closer to the nucleus and shielded less effectively from its charge. For an element like Magnesium (Mg), which has two valence electrons, the first and second ionization energies are relatively low and close to each other. However, the third ionization energy ([latex]\text{IE}_3\)) is drastically higher than \(\text{IE}_2\).
For example, if an element had successive ionization energies of \(\text{IE}_1 = 500\), \(\text{IE}_2 = 1,000\), \(\text{IE}_3 = 1,500\), and \(\text{IE}_4 = 7,000\) (all in \(\text{kJ/mol}\)), the jump between \(\text{IE}_3\) and \(\text{IE}_4\) is significant. This massive jump indicates that the fourth electron is being removed from a more stable, inner electron shell. The data shows that the first three electrons were the valence electrons, and the fourth was the first core electron.
By identifying the point at which this large energy jump occurs, scientists can definitively determine the number of valence electrons an atom possesses. In the example above, since the jump occurs after the third electron is removed, the element must have three valence electrons, placing it in Group 13 of the Periodic Table. This makes successive ionization data a powerful tool for confirming the electron shell structure of any element.