Ionization energy in periodic table
In learning chemistry, the periodic table trend of ionization energy or potential affecting mainly by the following factors,
- Atomic radius
- atomic number
- Charge on the nucleus
- Filled or half-filled orbitals
- Shielding electron
- Oxidation number of elements
What is ionization energy?
The electron is raised to a higher energy level by absorption of energy from external sources. If the ionization process continued, a stage comes where the electron goes fully out of the influence of the atomic nucleus.
The process where an atom produces a cation by the removal of the electron is defined as ionization energy. It can be calculated from the required energy to complete this process
Ionization energy equation
Electrons are raised to higher energy levels by the transfer of energy from external sources. But if energy transfers to electron sufficient, electrons go fully out of the influence of the nucleus of atoms.
Therefore, M (g) + IE → M+ (g) + e
The ionization energy or enthalpy of the periodic table element is an endothermic reaction in thermodynamics because during the process energy is consumed by atoms. It is generally represented as I or IE. It can be derived by unit electron volt (eV) per atom or kcal/mol or kJ/mol.
Electron volt to joule
For the conversion of electron volt to joule, first, we define electron volt. The energy required by an electron falling through a chemical potential difference of one volt is defined as an electron volt (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
First, second, and third ionization energies
- The amount of energy required for the removal of the first electron from a gaseous atom is called its first ionization energy,
M (g) + IE1 → M+ (g) + e
- The energy required for the removal of the second electron from a unipositive cation is called second ionization energy.
M+ (g) + IE2 → M+2 (g) + e
- Similarly, we have to define the term third, the fourth ionization potential of periodic table elements.
Ionization energy of hydrogen
The energy transfer for completely removing an electron from hydrogen energy levels is called the ionizing energy of a hydrogen atom. The ionized energy of hydrogen can be measured by calculating the energy difference for the transition of an electron from n = ∞ to n = 1.
From the Bohr model of hydrogen, the ionization potential of hydrogen,
IEH = 2.179 × 10−11 erg
= 2.179 × 10−18 Joule
= 13.6 eV
Ionization energy of helium
The electron configuration of helium 1s2. The second ionization potential finds by the removal of the second electron from the 1s orbital against the nuclear charge of +2.
The calculated IE of He from the Bohr energy equation,
IEHe = Z2 × IEH
= 22 × 13.6 eV
= 54.4 eV
Factors affecting ionization energy
The magnitude of the ionization energy of the periodic table elements depends on the following factors,
- Charge of the nucleus
- Atomic radius
- Half-filled and filled orbitals
- Shielding effect of electrons
Atomic radius trend
The atomic radius decreases from left to right along a period in the periodic table. Therefore, when we move left to right along a period in the periodic table, ionization potential trends normally increase because the atomic radius decreases
When we move from top to bottom in a group the value of the potentials of chemical elements decreases with the increasing size of the atom.
Charge of the nucleus
With the increasing atomic number, the charge on the nucleus increases, and more difficult to remove an electron from an atom. Normally the term ionization energy trend increases with the increasing atomic number or moving from left to right in a period of the periodic table.
Because with the increasing atomic number, the change in the nucleus also increases. Therefore, the electrostatic attraction between the outermost electrons and the nucleus of an atom also increases and the removal of an electron from the nucleus is more difficult.
Atomic radius and ionization energy
The greater the atomic radius of elements in the periodic table, the weaker will be the attraction. Hence the required energy for the removal of the electron is lower.
If an atom is raised to an excited state by promoting one electron to a higher quantum level, the excited electron is more easily detached because the distance between the electron and nucleus increases.
Half-filled and filled Orbitals
According to Hund’s rule, a half-filled or fully-filled orbital is comparatively low energy or more stable. For such an atom, more energy is required to remove an electron.
The ionization energy of such an atom is greater than the expected value calculated from the formula.
Shielding effect and ionization energy
Electrostatic attraction between the electrons and nucleus shows that an outer electron is attracted by the nucleus and repelled by the electrons of the inner shell. This combines attractive and repulsive force acting on the outer electron experiences less attraction from the nucleus. Thus this effect is known as the shielding effect.
The larger the number of electrons in the inner shell, the lesser the attractive force for holding the outer electron.
The radial distribution functions of the s, p, and d subshell show that for the same principal quantum number the s-subshell is most shielding than the p-subshell and least shielding the d-orbital. The shielding efficiency, s > p > d.
As we move down a group, the number of inner-shells increases, and hence the determined ionization potential tends is decreased. For example, the measure ionization potential trend for group-2 elements, Be > Mg > Ca > Sr > Ba.
Periodic Table Trends of Ionization Energy
The greater the charge on the nucleus of an atom the more energy is required for removing an electron from the atom.
With the increasing atomic number electrostatic attraction between the outermost electrons and the nucleus of an atom increases. Therefore, the ionizing of an atom is difficult in chemistry and the value of ionization energy generally increases in moving left to right in a period.
Ionization energy of second-period elements
The ionization trends in the periodic table for the second-period elements,
Li < B < Be < C < O < N < F < Ne
Exceptions of ionization energy trend
Few exceptions in the value of the ionization energy trends in the periodic table are explained based on the half-filled and fully-filled orbitals.
Group-15 elements have higher ionization potential than the group-16 elements and group-2 elements have higher than the group-3 elements in the periodic table.
Ionization energy of nitrogen and oxygen
Nitrogen and phosphorus in group-15 elements with atomic numbers 7 and 15. The electron configuration of these two elements,
N: 1s2 2s2 2p3
P: 1s2 2s2 2p6 3s2 3p3
Therefore, the removal of an electron from half-filled 2p and 3p-orbital of nitrogen and phosphorus required more energy.
Beryllium and oxygen
Removal of an electron from the group-2 element of beryllium (Be) and magnesium (Mg) with a fully-filled s-orbital required more energy.
Positively charged ions
An increase in the overall charge on the ionizing species (M+, M+2, M+3, etc) will enormously influence ionization.
During ionization, electron withdrawal from a positively charged species is more difficult than from a neutral atom. But the first ionization potential chart of the elements varies with their positions in the periodic table.
In each of the tables, the noble gas has the highest value and alkali metals have the lowest value for determining ionization energy.
Ionization energy and reactivity
In learning chemistry, the measure of ionization potential for a particular group is an impotent property of periodic table elements. Therefore, lithium, sodium, potassium, rubidium, cesium, and alkaline earth metal have a low value of ionization energy.
- In science, the reactivity of alkali alkaline metals for the formation of polar molecules with ionic bonding is greater than the other elements of the periodic table.
- The lower the value of ionization energy or potential, the greater the reducing power of the elements in the redox reactions because the removal of an electron from the chemical elements would mean oxidizing reaction.
- With the decreasing ionizing power, the basic properties of elements also increase, and acid properties decrease.
- The ionization energy chart was also used to calculate the bond energy, electronegativity, and electron affinity of the periodic table elements by Mulliken.