Ionization Energy

The chemical behavior of atoms and ions is greatly affected by how easy or difficult it is to remove their electrons, especially the outermost electrons which participate in chemical bond formations.

The energy required to remove an electron from a gaseous atom in its ground state is called the first ionization energy and is given in kJ/mol. The energy required to remove the next electron is called the second ionization energy, and so on.

Moving down a column, the ionization energies decrease. Recall that the highest principal quantum number of valence electrons increases down the column leading to larger atomic sizes. Thus, the farther the outermost electrons are, the easier they are to remove.

For main-group elements, the ionization energy increases across the period. The reason lies in the increasing atomic number, where valence electrons experience a higher effective nuclear charge making the removal of outermost electrons more difficult. This explains why chlorine has a higher ionization energy than sodium, for example. Generally, ionization energy is a minimum for an alkali metal and rises to a peak with each noble gas.

Transition metals display a small increase in the ionization energy across the period, and the f-block elements show an even smaller change.

But there are some exceptions to consider.

Boron has a smaller ionization energy than beryllium, even though it is farther to the right on the periodic table. Beryllium has lower energy 2s electrons, whereas boron has a higher energy 2p electron making its removal energetically more favorable.

Another exception is oxygen, which has lower first ionization energy than nitrogen. Compared to nitrogen, oxygen has four p-electrons, and removing one electron eliminates the electron-electron repulsion. Thus, less energy is required for the ionization. These exceptions are observed in succeeding periods too.

Electron removal from cations is more difficult than from neutral atoms. Generally, the successive ionization energies increase for elements.

Consider potassium. The second ionization energy is significantly higher, as it involves the removal of a core electron from an ion with a noble gas configuration.

Similarly, for calcium, there is a high increase from the second to third ionization energy as a core electron is removed from a cation with a noble gas configuration.