Ionization energy is a fundamental measure that quantifies the amount of energy required to detach the most loosely held electron from a neutral, gaseous atom. For oxygen, an element central to biological and atmospheric processes, this energy requirement is a specific, measurable value. Understanding this property is crucial for explaining oxygen’s reactivity and its tendency to form compounds.
Understanding Ionization Energy
Ionization energy (IE) is defined as the minimum energy necessary to remove the most loosely bound electron. This process is always endothermic, requiring an input of energy to overcome the attractive force of the positive nucleus. The chemical process for the first ionization is represented by the equation: \(X_{(g)} + \text{energy} \rightarrow X^+_{(g)} + e^-\), where \(X\) is the neutral atom.
This property directly reflects how strongly the nucleus pulls on its outermost electrons. Atoms with tightly held electrons have a high ionization energy, while those with loosely held electrons have a low value. Ionization energy is commonly expressed in kilojoules per mole (\(\text{kJ/mol}\)) or electron volts (\(\text{eV}\)).
The Measured Values for Oxygen
The first ionization energy of oxygen is the most commonly cited value, representing the energy required to convert a neutral oxygen atom (\(\text{O}\)) into a singly charged positive ion (\(\text{O}^+\)). This value has been experimentally determined to be \(1313.9 \text{ kJ/mol}\), which corresponds to approximately \(13.6 \text{ eV}\).
The second ionization energy, which removes an electron from the \(\text{O}^+\) ion to form a doubly charged \(\text{O}^{2+}\) ion, is significantly higher, requiring \(3388.3 \text{ kJ/mol}\) of energy. The reaction is represented as \(\text{O}^+_{(g)} + \text{energy} \rightarrow \text{O}^{2+}_{(g)} + e^-\).
Subsequent ionization energies increase with each electron removed because the remaining electrons are held more tightly. For example, the third ionization energy for oxygen is \(5300.5 \text{ kJ/mol}\). Successive values reveal crucial information about the atom’s internal shell structure.
Electron Shells and Successive Ionization
Oxygen has an atomic number of 8, with electrons arranged in the configuration \(1s^2 2s^2 2p^4\). The first six electrons removed are the valence electrons residing in the outer \(n=2\) shell. The removal of these valence electrons shows a gradual, non-uniform increase in energy.
The first ionization energy is slightly lower than that of its neighbor nitrogen due to electron-electron repulsion within the \(2p\) orbitals. Oxygen’s \(2p\) orbital contains one pair of electrons, and the repulsion between these paired electrons makes one of them easier to remove than anticipated.
The most significant observation occurs after the sixth electron is removed. A massive jump in required energy occurs when attempting to remove the seventh electron because the process must now break into the inner, core shell (\(n=1\)). These core electrons are much closer to the nucleus and experience a much higher effective nuclear charge, as there is no inner electron shell shielding them. This dramatic increase confirms the completion of an inner electron shell.