Ionization Energy

In the physical or chemical sciences, the minimum energy required for removing the most loosely bounded electron of an isolated or noble gaseous atom's or molecule's shell is known as the Ionization energy of the atom or molecule. This energy isn't actually a form of energy, but it is a term for the energy that is required during a chemical reaction or physical process. It may seem that ionization energy is just like other topics that are studied in physics or chemistry but actually, the importance of this topic is much more. Ionization energy is very helpful in many physics and chemistry perspectives as it helps calculate the exact amount of energy required for removing an electron. By calculating the Ionization energy of an atom or molecule, one can find out the reaction in which the given atom is involved is exothermic or endothermic? It is also helpful in finding out the energy that will be released or consumed during a given chemical reaction or physical process. The following article discusses all the basic concepts related to Ionization energy and ionization energy trends in the periodic table.

Introduction to Ionization Energy

As already discussed, the basic definition of ionization energy states that 'it is the amount of energy required for removing the loosely bounded electron present in the outermost shell of an atom.' But this is only the basic definition of ionization energy, and on a practical note, it is actually more than that. In fact, ionization energy can be calculated for every electron present in an atom or molecule. Following is the quantitative expression of describing the ionization energy of an atom:

A(g) + energy => A+(g) + e-

In the expression given above, some terms are used, which are explained below:

• A(g) = An atom or molecule in the isolated or neutral gaseous state
• energy = Ionization energy of the atom or molecule
• A⁺(g) = Positively charged atom or molecule after releasing an electron
• e^- = Loosely bounded electron released from the outermost shell of the atom

We can use this expression to calculate the ionization energy for an atom, and it is also helpful in finding out the nature of the reaction or process. Ionization energy is always positive for neutral state atoms or electrons, which means that the reaction that will take place for removing the electron will be an endothermic reaction (type of reaction where energy will be consumed during the reaction for yielding byproducts). So, it can be said that the less the distance between an atom's nucleus & the electron present in the outermost shell, the higher the ionization energy of the atom.

• Ionization Potential: The 'ionization potential' term is an older and obsolete term that was previously used to represent the ionization energy of an atom. This term was in use because previously, some older methods of measuring ionization energy (that used an ionization sample and acceleration of electron value) were in use, and from such values, the ionization potential term came up.

Expressing the Ionization energy:

Ionization energy is the concept that is used both in physics and chemistry, but it is very interesting to see that it is expressed in different metric units in both fields. In physics, the ionization energy is expressed in Joule (J) or electron volt (EV), whereas in chemistry, it is expressed as Kilojoules per mole (KJ/mol) or Kilocalories per mole (Kcal/mol) which means that total energy required to ionize a mole of atoms or molecules.

N^th ionization energy of an atom:

The nth ionization energy of an atom or molecule is usually referred to as the total energy required for removing the nth electron present in the given atom. It can also be defined by the total energy required for removing the most loosely bounded electron from the given atom or molecule bearing 'n-1' charge. Following is the example which can be used to understand the first nth ionization energies of a given 'A(g)' atom:

o 1^st Ionization Energy of A(g) atom:

A(g) + energy (IE 1st) => A+(g) + e-

o 2^nd Ionization Energy of A(g) atom:

A+(g) + energy (IE 2nd) => A2+(g) + e-

o 3^rd Ionization Energy of A(g) atom:

A2+(g) + energy (IE 3rd) => A3+(g) + e-

o n^th Ionization Energy of A(g) atom:

A(n - 1)+(g) + energy (IE nth) => An+(g) + e-

It is also interesting to know that the first ionization energy will always be less than the second ionization energy, the second ionization energy will always be less than the third ionization energy, and so on. Following is the increasing order of the ionization energy of an atom:

IE (1st) < IE (2nd) < IE (3rd) < .. < IE (nth)

Influencing factors for Ionization Energy

The ionization energy of an atom depends upon many factors, and most of them are very crucial for defining how much energy is required for removing the most loosely bounded electron from the atom's outermost shell. Therefore, studying these factors becomes even more important, and having proper knowledge of these factors provides an idea of the nth ionization energy of an atom. All the factors that influence the ionization energy of an atom can be categorized into the following two types:

1. Major Influencing factors
2. Minor Influencing factors

Both of these types include many factors depending to what extent they affect the ionization energy of an atom and every factor that affects the ionization energy of an atom fall either in one of these two types.

Type 1: Major or most Influencing factors of Ionization Energy:

These types of factors include those factors that affect the ionization energy of an atom up to a major extent, or it can be said these factors are majorly responsible for the high or low ionization energy of an atom. This type includes many common influencing factors such as an electronic charge on the atom, stability of the nucleus, number of electrons in the outermost shell, and many others. Following is the detailed explanation of most influencing factors that affect the ionization energy of an atom up to a major extent:

1. Number of electrons in the outermost shell: The total number of electrons present in the outermost shell of an atom affects the atom's ionization energy up to a major extent. If only one or two atoms are present in the outermost shell, it is comparatively easy to remove the electron from the atom as they are very loosely bound with the atom's nucleus. Therefore, if fewer electrons are present in the outermost shell, less ionization energy would be required to remove the electron from the atom or vice-versa.
2. Effective nuclear charge on the atom: If the effective nuclear charge (Zeff) is more, it means that the most loosely bounded electron in the outermost shell is already removed from the atom. The greater the penetration and magnitude of electron shielding of an atom, the easier it would be to remove the electron from the atom's outermost shell. This is because when an atom's penetration and electron shielding magnitude is greater, it means that electrons are less tightly bound in the electron shell of an atom. Therefore, the higher the nuclear charge of an atom, the more ionization energy is required to remove an electron from the outermost shell or vice versa.
3. Nuclear Charge: Nuclear Charge works exactly opposite to the effective nuclear charge of an atom because the higher the nuclear charge, the more tightly the electrons will be held with the atom's electron shells. Therefore, if the nuclear charge of an atom is high, it means that more ionization energy is required to remove an electron from the atom or vice-versa.
4. Number of Electron Shells: The number of electron shells present in an atom is the major influencing factor for the atom's ionization energy, the same as the number of electrons present in the atom's outermost shell. More the number of electron shells present in the atom, more loosely the electrons of the outermost shell will be bounded with the nucleus. The high number of electron shells increases the size of the atom, which results in more distance between the outermost shell's electron and the nucleus of the atom. And, it will ultimately result in less nuclear attraction between these two entities, which makes it easier to remove the electron from the outermost shell of the atom. Therefore, if more electron shells are present in the atom, less ionization energy is required to ionize the electron and remove it from the atom's shell.
5. Stability of the molecule or atom: Here, stability is referred to how stable is the electronic configuration of an atom. If the electronic configuration is more stable, it means that electrons of that atom are more tightly bound with the nucleus. Thus, removing an electron from atoms having a more stable electronic configuration will require more ionization energy or vice-versa.
   These all are the most influencing factors for the ionization energy of an atom, and a change in any of these factors will change the ionization energy of an atom up to a major extent.

Type 2: Minor Influencing factors of Ionization Energy:

This type of factors includes those factors that affect the ionization energy of an atom only up to a minor extent, or it can be said that these factors are not majorly responsible for high or low ionization energy of an atom, but a change in these factor's values can cause minor changes in the ionization energy of an atom. This type includes minor influencing factors, which are not as common as the other type of influencing factors. This type of factor includes pairing energies of electrons, Scandide contraction, Lanthanide & Actinide contraction, relativistic effects, and many others. Following is the detailed explanation of minor influencing factors that affect the ionization energy of an atom only up to some minor extent:

1. Electron Pairing Energies: The energies generated from the pairing of the electrons in the electron's shell of an atom are known as electron pairing energy. It is usually seen that atoms that have electron shells either completely filled or completely half-filled have higher electron pairing energy, and it results in some increment in the value of ionization energy of the atom. Thus, the higher the electron pairing in the electron shells of an atom, the higher the chances are that more ionization energy is required to remove an electron from such configuration.
2. Lanthanide and Actinide contraction: This contraction effect is shown in the atoms that follow Lanthanide and Actinide atoms in their respective rows. The contraction effect is basically an unprecedented shrinking in the atom's electron shells towards the nucleus. Thus, these contraction effects are also playing a crucial role in increasing the ionization energy of the atom, but since this effect is not seen in all the atoms of the periodic table, this effect is categorized under the minor influencing factors. The major reason for increasing ionization energy of an atom showing Lanthanide, Actinide, or Scandide contraction is that more nucleus charge is felt on the atom's outermost electron, making it tightly bound with the nucleus.
3. Relativistic effects: The term 'relativistic effects' comes from the quantum mechanics where this term was first originated. This effect is commonly seen in heavier elements, such as elements that have an atomic number of more than 70. Therefore, electrons, heavier elements or atoms that have an atomic number of more than 70, approach the speed of light due to the relativistic effect, which makes the atomic radius of such elements smaller than other elements. Thus, the ionization energy of atoms that shows a relativistic effect is usually on the higher side because their electrons are approaching the speed of light which makes it more difficult to remove them.
   These are some minor influencing factors for the ionization energy of an atom, and the ionization energy of all atoms is minorly changed because of these effects or factors. These effects can change the ionization energies of a particular atom or some atoms up to a major extent, but since the presence of these effects causes a minor difference in ionization energy of all atoms, these effects are categorized under this type of factor.

Ionization Energy: Trends and Values

A general trend is that the value of N+1^th ionization energy of an atom or molecule is always greater than its N^th ionization energy. The major reason behind this phenomenon is very simple. When an electron is removed from the atom's shell, the effective nuclear charge on all other electrons increases, and this ultimately results in more ionization energy requirement for removing the second electron from the same electron shell of the atom molecule (It should also be noted that ionization energies of anions are usually lower than those of cations due to difference in their effective nuclear charge). When an electron is removed from higher nuclear charge (Z^eff), greater forces of electrostatic attraction is observed on the electron, which makes it more difficult to remove the electron, and therefore, more energy is required to ionize this electron and pull it out from the atom's outermost electron shell. One more point that plays a crucial role here is that if all the electron from the atom's outermost electron shell is removed, the next electron would be present in a lower electron shell. Thus, now the distance between the lower shell's electron and atom's nucleus is decreased, resulting in more force on the electron than all electrons present in the atom's outermost shell. Therefore, a sudden increase in ionization energy is also seen in many atoms when an electron is removed.

Trends in ionization energy of atoms according to the periodic table:

A common trend (increase or decrease) can be seen in the ionization energies of the atom while moving from left to right or up to down in the periodic table.

Usually, it is seen that ionization energy is always increasing in the periodic table while moving from left to right. It means that the ionization energy of an atom having atomic number 2 is greater than an atom having atomic number 1. But there are many kinds of exceptions present in this case that arise for many reasons.

One of the most common reasons, which cause exceptions in this trend is stable electronic configuration (including noble gases). As noble gases are inert or unreactive in nature, it requires a very high amount of ionization energy to remove an electron from the atom's outermost shell. Noble gases or elements have all electron shells occupied completely, and therefore, they possess more electrostatic force on each electron. Therefore, it is very common that ionization energies of noble gases are always higher than atoms having atomic numbers more than these noble gases. Following are some common examples of exceptions caused by this reason:

• Lithium [Li] (IE) < Helium [He] (IE),
• Sodium [Na] (IE) < Neon [Ne] (IE),
• Potassium [K] (IE) < Argon [Ar] (IE), etc.

Also, unlike the common cases, noble gases have very high first ionization energy, but the second ionization energy is very low. Commonly, we see the opposite of this where elements with second ionization energy are much higher than the first ionization energy. After removing the first electron from the noble elements' electron shell, their electronic configuration becomes unstable, making it easier to remove the second electron from these noble elements. Therefore, after removing an electron from the noble element, the noble element becomes highly reactive, and it becomes uncontrollable to stop it from reacting with other atoms. Following is the illustration of this exception that is found in noble gases elements:

• Helium [First ionization energy] >> Helium [ Second Ionization Energy],
• Neon [First ionization energy] >> Neon [ Second Ionization Energy],
• Argon [First ionization energy] >> Argon [ Second Ionization Energy], etc.

Another common reason for the exception in this trend is that atoms have completely half-filled electron shells, making them less reactive than the succeeding atom. The best example for this reason of exception is Oxygen [O] and Nitrogen [N]. If following the trend, the ionization energy of Oxygen should be greater than the ionization energy of Nitrogen, but this is not true. Actually, the first ionization energy of nitrogen is higher than the first ionization energy of Oxygen. The only reason behind this is the completely half-filled p-shell of nitrogen. Due to this completely half-filled p-shell, Nitrogen exerts more electrostatic force on its electrons, and therefore it requires more ionization energy to remove an electron. Whereas this is not the case in Oxygen, and therefore, it becomes slightly easier to remove the first electron from Oxygen compared to Nitrogen.

The trend in the Ionization Energy while moving down the group:

Another common trend which one can see in ionization energy is that ionization energy gradually decreases while moving down in a particular group. While moving down in any particular group of the periodic table, one can see a decrease in the nth ionization energy of that group's atoms. Therefore, it is common to see that the ionization energy of Sodium [Na] is higher than those of Potassium [K] or any other succeeding atoms of Group 1 of the periodic table. This happens because when the atomic number increases, the number of electrons increases in the atom, and thus, the size of the atom also increases. This increase in the size of the atom increases the atomic radius of that atom which means that the distance between electrons and nucleus of the atom will be more than the previous atom of the same group. Increased distance between the outermost shell's electrons and the nucleus of the atom will result in a less electrostatic force on the outermost electron. When less electrostatic force is enforced on the atom's outermost shell's electron, less amount of energy will be required to remove that electron which is more in the case of the preceding atom of the same group. And, it is very rare to see any exception in this trend. This is because all the atoms of a particular group follow the same type of electronic configuration, and therefore, only the size of the atom plays here a crucial role in defining the stability of the atom.

Large jumps in the nth ionization energy of an atom:

All the atoms suddenly see a high jump in their nth ionization energy value compared to their n-1^th ionization energy. For example, the value of 2nd ionization energy (4560) of Sodium [Na] is very much higher than the first ionization energy of it (496). As it can be seen that suddenly the ionization energy of sodium increases to 10 times an electron is already removed from the outermost shell. The same trend is seen in the cases of all other atoms. Like in Magnesium [Mg], its third ionization energy value is very higher than the second ionization energy. The major reason behind this trend is that when an atom loses a particular number of electrons, it becomes more stable by achieving the noble gas electronic configuration. Like in the case of Sodium, when it loses an electron from its outermost shell, its electronic configuration becomes very much similar to the electronic configuration of Neon [Ne]. And, therefore, a sudden rise is seen in the 2nd ionization energy of sodium compared to the 1st ionization energy value. The same is in the case of Magnesium; it also achieves the noble electronic configuration of Neon after losing two electrons from its outermost shell. This trend is very common in every atom of the periodic table, and it only depends upon the number of electrons present in the outermost shell.

Ionization Energy of Molecules

The ionization energy of molecules is also defined by the term 'total amount of energy required to remove an electron from the atom.' The ionization energies don't follow the same trends, which is commonly seen in the ionization energies of the atoms. The trends that are very common in the periodic table are also not followed by the ionization energies of molecules. The ionization energy of the molecules is of two types which are defined by the following two terms:

1. Adiabatic Ionization Energy: The amount of energy required to remove an electron from the outermost shell of a neutral molecule's atom is known as Adiabatic Ionization Energy of the molecule. The value of adiabatic ionization energy is calculated by finding the difference between the vibrational ground state of the positive ion of the molecule and the vibrational ground state of the neutral atom.
2. Vertical Ionization Energy: The transition that is seen in the adiabatic ionization energies of the molecule when the molecule is in the ground state and when the molecule is in an excited state, is known as Vertical Ionization Energy of the molecule. This transition in the ionization energy of molecules is referred to as 'vertical' because it is represented by a completely vertical line on a molecule's potential energy or ionization energy diagram. The transition in the molecular ionization energies is seen because of the possible changes in their molecular geometry. These changes in their molecular geometry arise due to dynamic ionization in the vibrational ground state of the molecule. The molecular transition of a diatomic molecule (a molecule formed by the combination of two same atoms) is defined by the length of their bond if the single bond is present between them.
   Mostly the vertical ionization energies of molecules are always higher than their adiabatic ionization energies, but, in many cases, it is seen that their adiabatic ionization energies are higher than vertical ionization energies.