Ionization energy (IE) is a fundamental concept in chemistry, representing the energy required to remove an electron from a gaseous atom. The process of electron removal is sequential, meaning the energy needed for the first electron differs from the second, and so on. Understanding the specific value for the second ionization energy (\(IE_2\)) provides deep insight into an element’s electron configuration and chemical behavior. This article explains the definitions and principles that govern the determination of the second ionization energy.
Defining First and Second Ionization Energy
Ionization energy is the minimum energy required to remove the most loosely bound electron from an isolated gaseous atom or ion. The first ionization energy (\(IE_1\)) corresponds to the removal of a single electron from a neutral atom (\(X\)), creating a unipositive ion (\(X^+\)). This process is represented by the chemical equation \(X(g) \to X^+(g) + e^-\).
The second ionization energy (\(IE_2\)) is the next sequential step. It is the energy required to remove a second electron from the already positively charged ion (\(X^+\)). This removal creates a dipositive ion (\(X^{2+}\)) and is represented by the equation \(X^+(g) \to X^{2+}(g) + e^-\). Both \(IE_1\) and \(IE_2\) are endothermic processes, meaning the energy value is always positive.
Principles Governing Successive Ionization Energies
A general principle in chemistry is that each successive ionization energy is always greater than the one preceding it, meaning \(IE_2\) is always larger than \(IE_1\). The primary reason for this increase is the concept of effective nuclear charge. The nuclear charge, which is the number of protons in the nucleus, remains constant during both the first and second ionization steps.
When the first electron is removed, the remaining electrons are held by the same positive nuclear charge but now experience less electron-electron repulsion. Removing a second electron from a positively charged species, \(X^+\), is significantly more difficult because the remaining electrons are pulled inward more strongly by the nucleus than they were in the neutral atom. This higher net positive attraction on the remaining electrons results in a smaller ionic radius and requires substantially more energy to overcome.
The magnitude of the jump between successive ionization energies can reveal the element’s electron shell structure. A particularly large increase in ionization energy occurs when the removal of an electron requires breaking into a new, inner electron shell. Inner-shell electrons are much closer to the nucleus and experience significantly less shielding from the few remaining outer electrons. For instance, an element like Magnesium (Group 2) has two valence electrons, and its third ionization energy (\(IE_3\)) is extremely high because it involves removing an electron from the stable, full inner shell. Analyzing where the largest jump occurs in the successive ionization energies is a reliable method for determining the number of valence electrons an element possesses, which corresponds to its group number on the periodic table.
Experimental Determination and Data Analysis
The determination of ionization energy, including the second ionization energy, is an experimental process. The most common and precise method involves a technique called Photoelectron Spectroscopy (PES). In this method, a beam of high-energy photons, often in the X-ray range for core electrons or UV range for valence electrons, is directed at a gaseous sample of the element.
The photons transfer their energy to the electrons in the gaseous atoms or ions, causing the electrons to be ejected. The instrument then measures the kinetic energy of these ejected electrons. The binding energy, which is equivalent to the ionization energy, is determined by subtracting the measured kinetic energy of the electron from the known energy of the incident photon.
To specifically determine \(IE_2\), scientists must first create a sufficient concentration of the unipositive ion (\(X^+\)) within the gaseous sample. This is often achieved by bombarding the sample with high-energy electrons or photons to generate the \(X^+\) ions, which are then subjected to a second ionization event. The spectral peaks in the PES data correspond to the different energy levels from which electrons are removed.
For a neutral atom, \(IE_1\) shows up as one peak; for the \(X^+\) ion, \(IE_2\) appears as a second distinct peak corresponding to a higher binding energy. By analyzing the position of these peaks, the exact energy required to remove the second electron can be quantified. Furthermore, the analysis of the magnitude of the successive ionization energies is a practical way to identify an unknown element.