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:
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.
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.
Nth 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 1st Ionization Energy of A(g) atom:
A(g) + energy (IE 1st) => A+(g) + e-
o 2nd Ionization Energy of A(g) atom:
A+(g) + energy (IE 2nd) => A2+(g) + e-
o 3rd Ionization Energy of A(g) atom:
A2+(g) + energy (IE 3rd) => A3+(g) + e-
o nth 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:
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:
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:
Ionization Energy: Trends and Values
A general trend is that the value of N+1th ionization energy of an atom or molecule is always greater than its Nth 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 (Zeff), 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:
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:
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-1th 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: