Ionization energy definition
The study of ionization energy trends in the periodic table is an important article in inorganic chemistry for school and college courses.
Electrons are raised to higher energy levels by the transfer of energy from external sources. If energy transfer to electrons sufficient, electrons go completely out of the influence of the nucleus of an atom.
Thus the ionization energy defined as the amount of energy required to remove the most loosely bound electron or the outermost electron from an isolated gaseous atom of an element in its lowest energy state or ground state to produce a cation.
So the process of ionization is an endothermic process during the ionization process, energy consumed by atoms.
Ionization generally represented I or IE and measured in electron volt or kilocalories per gram atom.
What is an electron volt simple definition?
One electron volt is the energy consumption by an electron falling through a potential difference of one volt. Electron volt simply represented eV.
∴ 1 eV = charge of an electron × 1 volt
= (1.6 × 10-19 coulomb) × (1 volt)
= 1.6 × 10-19 Joule
1 eV = 1.6 × 10-12 erg
The ionization energy of hydrogen
The energy transfer for removing an electron from energy levels of the hydrogen atom called ionization.
Simply the energy corresponding to the transition from n = ∞ to n = 1 gives the ionization energy of the hydrogen atom.
So the ionization energy of the hydrogen
where n1 = 1 and n2 = ∞.
∴ EH = 2.179 × 10⁻¹¹ erg
= 2.179 × 10⁻¹⁸ Joule
∴ EH = 13.6 eV
Second and third ionization energy
The electrons are removed in stages one by one from an atom.
- The amount of energy consumption for removal of the first electron from a gaseous atom called its first ionization.
M (g) + IE1 → M+ (g) + e
- If the energy consumption for removal of the second electron from a cation called second ionization.
- Similarly, we have third, fourth ionization.
M+3 (g) + IE4 → M+4 (g) + e
The ionization energy of the helium atom
The ground state electronic configuration of helium 1S2.
Second ionization potential means the removal of the second electron from the 1S orbital against the nuclear charge of +2.
= Z2 × IEH
Thus second ionization energy of helium
= 22 × 13.6 eV
= 54.4 eV
Ionization energy trend in the periodic table
In the periodic table ionization of an atom influenced by the following factors
- Atomic radius.
- Change on the nucleus or atomic number.
- Completely-filled and half-filled orbitals.
- Shielding effect of the inner electrons.
- Overall charge on the ionizing species.
Atomic radius and the periodic table
Greater the atomic radius or the distance of an electron from the positive charge nucleus of an element, the weaker will be the attraction. Hence the energy consumption for the removal of the electron lower.
If an atom raised to an excited state by promoting one electron to a higher energy level, the excited electron more easily detached because the distance between the electron and nucleus increases.
Ionization energy and the periodic table
The Atomic radius decreases from left to right along a period of the periodic table because of the increasing charge on the nucleus of an atom. Thus when we move left to right along with period normally ionization energy increases.
But when we moving from top to bottom in a group the ionization energy of the elements decreases with the increasing size of the atom.
Atomic number and ionization energy
With the increasing atomic number charge on the nucleus increases and more difficult to remove an electron from an atom. Hence grater would be the value of ionization.
Normally the value of ionization increases in moving from left to right in a period since with the increasing atomic number the change on the nucleus also increases.
With the increase in the electrostatic attraction between the outermost electrons and the nucleus of an atom ionization energy also increases. Thus remove an electron from the nucleus is more difficult.
Half filled and completely filled orbitals
According to Hund’s rule, an atom having half-filled or completely filled orbital comparatively more stable. Hence for such an atom, more energy consumption to remove an electron.
Thus the ionization of such an atom relatively difficult than expected normally from their position in the periodic table.
Exceptions to the ionization energy trend
Few exceptions in the value of the ionization trend in the periodic table explained on the basis of the half-filled and completely filled orbitals.
The ionization energy of group-15 elements is higher than the group-16 elements and group-2 elements are higher than the group-3 elements in the periodic table.
The ionization energy of nitrogen
Boron and nitrogen in the second period and magnesium and phosphorus in the third period have a slightly higher value of ionization energy than those normally expected.
Nitrogen and phosphorus in group-15 elements with atomic number 7 and 15 have the electron configuration
1S2 2S2 2P6 3S2 3P3
But the removal of an electron from half-filled 2P and 3P suborbital of nitrogen and phosphorus consumed more energy.
Removal of an electron from the group-2 element of beryllium and magnesium with completely-filled S-subshell consumed more energy.
Shielding effect and ionization
The study of electrostatic attraction between the electrons and nucleus shows that an outer electron attracted by the nucleus and repelled by the electrons of the inner shell.
The combined effect of this attractive and repulsive force acting on the outer electron experiences less attraction from the nucleus. This is known as the shielding effect.
Thus a larger number of electrons in the inner shell, lesser the attractive force for holding outer electron.
The radial distribution functions of the S, P, d subshell show that for the same principal quantum number the S-subshell most shielding than the p-subshell and least shielding the d – orbital.
But as we move down a group, the number of inner-shells increases and hence the ionization tends to decreases.
Ionization of atoms and the periodic table
The greater the charge on the nucleus of an atom the more energy consumption for removing an electron from the atom.
If more energy consumed, then more energy required for removing an electron from the atom.
But with the increasing atomic number electrostatic attraction between the outermost electrons and the nucleus of an atom increases. Thus the ionization of an atom difficult.
Again the values of ionization energy generally increase in moving left to right in a period. Therefore the nuclear charge of an element also increases in the same direction.
But due to the presence of a completely filled and half-filled orbital of beryllium and nitrogen, the ionization of beryllium and nitrogen slightly higher than the neighbor element boron and oxygen.
Thus the ionization energy trend in the periodic table for the second period
Charge of an atomic nucleus
An increase in the overall charge on the ionizing species (M+, M+2, M+3, etc) will enormously influence the ionization. During ionization electron withdrawal from a positively charged species more difficult than from a neutral atom.
But the first ionization of the elements varies with their positions in the periodic table.
In each of the tables, the noble gas has the highest value and the alkali metals the lowest value for the ionization energy.
Describe how ionization energy impacts ionic bonding
The study of the ionization of the element in a particular group of the periodic table is essential for the properties of the elements.
Thus lithium, sodium, potassium, rubidium, and cesium or alkali metal in the periodic table with a low value of ionization energy, point to the high reactivity of alkali metals for the formation of the ionic bonding.