Ionization Energy – Definition, Formula, and Factors

What is Ionization Energy?

The minimum amount of energy that an electron in an atom requires to come out of the influence of the nucleus is called ionization energy. In other words, the measure of difficulty that an electron faces in removing from an atom or ion due to nuclear forces is called the ionization energy. Therefore, the ionization energy gives the measure of strength or binding force by which an electron is held in an atom or ion. Ionization energy is sometimes also known as ionization potential.

Alternatively, the amount of energy supplied to an isolated atom to extract out its most loosely bound valance electron to form a positive atom or ion is referred to as ionization energy. The ionization energy is measured in electronvolts (eV) or kJ/mol.

Ionization energy basically gives an idea about the reactivity of chemical compounds, and therefore, we may use it to determine the strength of chemical bonds. An electric discharge tube is used to measure the ionization energy.

Depending on the ionization of molecules, the ionization energy can be of the following two types −

  • Adiabatic ionization energy
  • Vertical ionization energy

For any atom X, we have the following relation,

$$\mathrm{X+Ionization \: Energy\rightarrow X^{+}+e^{-} }$$

Bohr’s Atomic Model and Ionization Energy

Ionization energy can be explained by Bohr’s atomic model. According to Bohr’s atomic model, an atom has several paths, called orbits, for electrons to go around the nucleus. Each of these orbits is at a fixed distance from the nucleus and represents fixed energy. Thus, the electron in any orbit will have the energy of the orbit. However, an electron can absorb energy and move to the next orbit of higher energy. If there is more energy available to absorb by the electron, the electron will leave the atom.

Calculation of Ionization Energy

To understand how to calculate the ionization energy, consider an atom that consists of an electron revolving around a positively charged nucleus. This electron is bound to the nucleus due to the coulombic force of attraction and therefore the electron can have a fixed energy level. According to Bohr’s atomic model, this energy is given by,

$$\mathrm{E_{n}=-13.6\times \frac{Z^{2}}{n^{2}}\, \, eV=-\left (2.18 \times 10^{-18} \right )\times \frac{Z^{2}}{n^{2} }\:\:J/atom}$$

Where, Z is the atomic number and n is the number of the orbit.

The ionization energy is the amount of energy required to take an electron from its ground state or most stable state to infinity. Thus, if we assume 0 eV as the reference at infinity, then the ionization energy is given by,

$$\mathrm{IE = 0 - E(0)}$$

First and Second Ionization Energy

The amount of energy required to remove the first electron from a neutral atom is called the first ionization energy. Numerically, the first ionization energy is equal to the orbital energy of the electron but has an opposite sign.

The amount of energy required to remove the second electron from the positively charged atom or ion is called the second ionization energy.

It is to be noted that once the first electron is extracted from the atom, the electrostatic force of attraction increases on the rest of the electrons. Therefore, removing the second electron is more difficult. Consequently, the second ionization energy will be greater than the first ionization energy. Similarly, the third ionization energy is larger than the second ionization energy and so on.

Ionization Energy in Periodic Table

In the periodic table, when we go from left to right across the elements, the ionization energy increases due to decreasing atomic radius. Thus, the ionization energy of an electron increases with the increasing atomic number.

However, when we go from top to bottom, the ionization energy of decreases due to increasing atomic radius.

Factors Affecting Ionization Energy

Following are some of the factors that affect the ionization energy −

  • Ionization energy is affected by the size of the atom, i.e. the ionization energy decreases with the increase in the size of the atom and vice-versa.

  • The nuclear charge, i.e. charge on the nucleus affects the ionization energy. Where, the more the nuclear charge, the more ionization energy is required to remove the electron.

  • To remove an electron from an atom of the stable electronic configuration, more ionization energy is required.

  • It is more difficult to withdraw an electron from an inner shell due to the coulombic force of attraction, hence more ionization energy is required.