Ionization Potential

All of theoretical chemistry related to atomic and molecular reactions is quantum physics, which studies the energy changes associated with electrons and provides a system for probability calculations of atomic and molecular orbitals. One of the important concepts associated with atomic physics is the ionization potential of an atom. The aim of this article is to provide a clear explanation of what is ionization potential of an atom.

Before we go ahead and talk about ionization potential, let me brush up some of the basics of atomic structure. An atom consists of a central nucleus containing positively charged protons and neutrons, which is surrounded by electrons revolving around it in orbitals with specific energy levels. The electrons are bound to the atomic nucleus, because of the electromagnetic force of attraction between the electrons and protons inside the nucleus.

To break free from the attractive force of the atomic nucleus, an electron requires energy which may be supplied from external sources. For every atom, the energy levels for each atomic orbital are different. With that background, we are ready for a discussion of what is ionization potential.

What is Ionization Potential?

The ionization potential or ionization energy is defined as the total amount of energy required to free an electron revolving in the outermost shell of any atom. Alternatively, chemists also define it as the total energy required to liberate one mole of electrons, from one mole of a particular atom. The units used for measurement of ionization potential are more than one.

In physics, we prefer using electron volts (eV) per atom as a unit for ionization potential, while in chemistry, the ionization potential is measure in terms of kJ / mol (Kilojoules per Mole). For example, the ionization potential of Hydrogen is 13.5984 eV. The lower the ionization energy, higher is the tendency of an atom to become a reducing agent and more reactive it is. On the other hand, higher is the ionization energy, higher is the tendency to become an oxidizing agent and less reactive it is.

What is the Ionization Potential Equation?

To be able to measure the ionization potential for any atom, one must solve the Schrodinger equation from quantum physics, to calculate the energy levels of that atom. A complete analytical solution for any atom, other than Hydrogen is not possible, an estimate of the ionization potential energy can be made on the basis of an approximate model. The following equation can be used to estimate the ionization potential energy of an electron in any shell of the atom.

Ionization Potential (eV) = 13.6 eV x (Z2 / n2)

where Z is the atomic number of the atom and n is the orbital number. This formula can only provide you with an approximate value. A more precise value can only be obtained from experiments.

The energy required to liberate one electron from an atom’s outer shell of electrons is known as first ionization potential, while the energy required to liberate the second one is known as the second ionization potential. Second ionization potentials are always higher than first ionization potentials. That is because, more you take away the electrons from its outermost shells, more tightly does an atom hold the rest, which are left.

As we saw before, the key point to understand is that higher the ionization potential of an atom, greater will be the energy required to remove an electron from inner level orbitals of the atom. In conclusion remember that atoms with low ionization potential lose electrons easily and are hence good reducing agents. On the other hand, atoms with high ionization potentials are more likely to be oxidizing agents. As an exercise, take a look at the ionization potential table for all elements and guess which elements are likely to be reducing or oxidizing agents. Hope this article has left no doubt in your mind about what is the ionization potential of an atom.

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