Difference Between First and Second Ionization Energy

Main Difference – First vs Second Ionization Energy

Ionization energy is the amount of energy needed by a gaseous atom in order to remove an electron from its outermost orbital. This is the ionization energy because the atom gets a positive charge after the removal of an electron and becomes a positively charged ion. Each and every chemical element has a specific ionization energy value. This is because atoms of one element are different from atoms of another element. The first and second ionization energies describe the amount of energy required by an atom in order to remove one electron and another electron, respectively. The main difference between first and second ionization energy is that the first ionization energy has a lesser value than the second ionization energy for a particular element.

Key Areas Covered

1. What is First Ionization Energy
      – Definition, Trends in the Periodic Table
2. What is Second Ionization Energy
      – Definition, Trends in the Periodic Table
3. What is the Difference Between First and Second Ionization Energy
      – Comparison of Key Differences

Key Terms: First Ionization Energy, Ionization, Second Ionization Energy, Shells

What is First Ionization Energy

First ionization energy is the amount of energy required by a gaseous, neutral atom to remove its outermost electron. This outermost electron is located in the outermost orbital of an atom. Therefore, this electron has the highest energy among other electrons of that atom. Hence, the first ionization energy is the energy required to discharge the highest energy electron from an atom. This reaction is essentially an endothermic reaction. This can be given in a reaction as follows.

X_(g)    →    X_(g)⁺   +    e^–

This concept is associated with a neutrally charged atom since neutrally charged atoms are composed only of the original number of electrons that the element should be composed of. However, the energy required for this purpose depends on the type of element. If all electrons are paired in an atom, it requires a higher energy. If there is an unpaired electron, it requires a lower energy. However, the value also depends on some other facts. For example, if the atomic radius is high, a low amount of energy is required because the outermost electron is located far from the nucleus. Then the attraction force between this electron and the nucleus is low. Therefore, it can easily be removed. But if the atomic radius is low, then the electron is highly attracted to the nucleus. Then it is hard to be removed from the atom.

The periodic table of elements shows a certain pattern or a trend of varying the first ionization energy throughout its periods. When going down a group of the periodic table, the first ionization energy decreases since the atomic radius increases down the group.

Figure 1: Trend of First Ionization Energy in the Periodic Table of Elements

The above image shows how the first ionization energy is varied throughout a period. The noble gasses have the highest first ionization energies because these elements have atoms that are composed of completely filled electron shells. Therefore, these atoms are highly stable. Due to this stability, it is very difficult to remove the outermost electron.

What is Second Ionization Energy

Second ionization energy can be defined as the amount of energy required to remove an outermost electron from a gaseous, positively charged atom. Removal of an electron from a neutrally charged atom results in a positive charge. This is because there aren’t enough electrons to neutralize the positive charge of the nucleus. Removing another electron from this positively charged atom will require a very high energy. This amount of energy is called the second ionization energy. This can be given in a reaction as below.

X_(g)⁺   →    X_(g)⁺²    +     e^–

Second ionization energy is always a higher value than the first ionization energy since it is very difficult to remove an electron from a positively charged atom than from a neutrally charged atom; this is because the rest of the electrons are highly attracted by the nucleus after removing one electron from a neutral atom.

Figure 2: Differences between First, Second and Third Ionization Energies in Transition Metals

The above image shows the differences between first, second and third ionization energies. This difference occurs because removing electrons becomes difficult with the increase of the positive charge. Moreover, when electrons are removed, the atomic radius is decreased. It also makes it difficult to remove another electron.

Difference Between First and Second Ionization Energy

Definition

First Ionization Energy: First ionization energy is the amount of energy required by a gaseous neutral atom to remove its outermost electron.

Second Ionization Energy: Second ionization energy is the amount of energy required by a gaseous positively charged atom to remove an outermost electron.

Value

First Ionization Energy: The first ionization energy is comparatively a low value.

Second Ionization Energy: The second ionization energy is comparatively a high value.

Starting Species

First Ionization Energy: First ionization energy is defined regarding a neutrally charged atom.

Second Ionization Energy: Second ionization energy is defined regarding a positively charged atom.

End Product

First Ionization Energy: The end product is a +1 charged atom after the first ionization.

Second Ionization Energy: The end product is a +2 charged atom after the second ionization.

Conclusion

Ionization energy values are important in determining the reactivity of chemical elements. It is also helpful in determining whether a chemical reaction would occur or not. The ionization energy sometimes acts as the activation energy for a certain reaction. The main difference between first and second ionization energy is that first ionization energy is a lower value than second ionization energy for a particular element.

References:

1. “Ionization energy.” PURDUE Science. Available here. Accessed 22 Aug. 2017.
2. Libretexts. “Ionization Energy.” Chemistry LibreTexts, Libretexts, 14 May 2017, Available here. Accessed 22 Aug. 2017.

Image Courtesy:

1. “First ionization energies” (CC BY-SA 3.0) via Commons Wikimedia
2. “Transition Metals Ionization Energies” By Oncandor – Own work (CC BY-SA 4.0) via Commons Wikimedia