The key difference between 18 electron rule and EAN rule is that 18 electron rule indicates that there have to be 18 valence electrons around the metal in coordination complexes in order to become stable whereas EAN rule describes that a metal atom has to obtain the electron configuration of the noble gas present in the same period in order to become stable.
Both 18 electron rule and EAN rule indicate that obtaining a noble gas electron configuration makes a metal atom stable. According to the 18 electron rule, we need to consider the valence electrons of the metal atom whereas according to EAN rule, we have to consider the whole electron content of the metal atom. However, both these terms are mainly discussed under organometallic compounds where we can find coordination complexes having transition metal atom at the centre, surrounded by ligands. These terms are applied for the central metal atom to see whether these complexes are stable or not.
CONTENTS
1. Overview and Key Difference
2. What is 18 Electron Rule
3. What is EAN Rule
4. Side by Side Comparison – 18 Electron Rule vs EAN Rule in Tabular Form
5. Summary
What is 18 Electron Rule?
18 electron rule is a concept in chemistry we use to determine the stability of a metal atom in an organometallic compound by determining whether it has 18 valence electrons. It is a simplified version of EAN rule. In the EAN rule, we have to consider the total number of electron of the atom, but here we only consider the number of valence electrons. The valence shell of a transition metal can be given in a general form as follows:
nd(n+1)s(n+1)p
The electron configuration of metal can hold a maximum of 18 electrons. Therefore, the noble gas electron configuration has all the 18 electron hoes filled with electrons. It is why we call this concept as 18 electron rule.
What is EAN Rule?
EAN rule is a concept in chemistry that states that if the central metal atom in an organometallic compound has the electron configuration of the noble gas present in the same period as the metal, then the complex is stable. The term EAN stands for Effective Atomic Number. Here, this concept considers the total number of electrons present in the metal atom. It is similar to the 18 electron rule because this also states that having the noble gas electron configuration makes the metal complex stable.
For example, let us consider a metal complex having the Fe2+ ion at the centre. The atomic number of iron is 26. Since this ion has a +2 charge, the total electron count will be 24. Therefore, if the ligands that bind with this metal atom donates 12 electrons to the metals ion so that the electron configuration of iron completes (to get the noble gas electron configuration=36 for the period where iron is in), then the metal complex becomes stable.
What is the Difference Between 18 Electron Rule and EAN Rule?
Both 18 electron rule and EAN rule indicates that obtaining a noble gas electron configuration makes them stable. However, the key difference between 18 electron rule and EAN rule is that 18 electron rule indicates that there have to be 18 valence electrons around the metal in coordination complexes to become stable, whereas the EAN rule describes that a metal atom has to obtain the electron configuration of the noble gas present in the same period to become stable.
The below infographic summarizes the difference between 18 electron rule and EAN rule.
Summary – 18 Electron Rule vs EAN Rule
Both 18 electron rule and EAN rule indicates that obtaining a noble gas electron configuration makes them stable. The key difference between 18 electron rule and EAN rule is that 18 electron rule indicates that there have to be 18 valence electrons around the metal in coordination complexes in order to become stable, whereas the EAN rule states that a metal atom has to obtain the electron configuration of the noble gas present in the same period in order to become stable.
Reference:
1. “24.3: The 18-Electron Rule.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.
Image Courtesy:
1. “18 electron rule example” By Derrekli – Own work (CC BY-SA 4.0) via Commons Wikimedia