Effective Atomic Number

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What is Effective Atomic Number?

The EAN full form in chemistry is an effective atomic number. The effective atomic number is a very common term used in the coordination chemistry branch. The effective atomic number of chemistry can be represented as EAN chemistry in short. Effective atomic number theory was the first theory that explained the concept behind the complex compound formation. This theory was given by Sidgwick. Therefore, this theory is known as the Sidgwick rule.


EAN in chemistry is used for complex compound formation. Let’s come to the main question? What is effective atomic number? The effective atomic number is the number that tells about the total number of electrons present around the central metal in a complex compound.


The Formula of Sidgwick EAN Rule

EAN = (z - x) + (C.N. * 2)

EAN = (z - x) + (L * D * 2)

In the above formula, z represents the atomic number of the central metal ion.

X represents the oxidation number of the central metal ion.

L represents the total number of ligand bound to the central metal atom.

D represents the denticity of the ligand.


Sidgwick EAN Rule Tells about:

  • Stability of coordination compound.

  • The metal ion in a coordination complex will continue accepting the electrons till the total number of electrons in the metal ion becomes equal to the atomic number of the noble gas of that series.

Effective Atomic Number in Coordination Compounds

In the above definition, we explain effective atomic number concept. This concept explains the stability and the possibility of complex compound formation. According to this concept, only that complex compound can be formed that will attain the noble gas configuration. 

D-block element atomic number

Scandium (Sc)- 21

Titanium (Ti)- 22

Vanadium (V)- 23

Chromium (Cr)-24

Manganese (Mn)- 25

Iron (Fe)- 26

Cobalt (Co)- 27

Nickel (Ni)- 28

Copper (Cu)- 29

Zinc (Zn)- 30

Krypton (Kr)- 36

The Nobel gas close to this series is 36. All these elements are less stable than the krypton. Therefore, all these above-mentioned elements will try to attain this electronic configuration. For this noble gas configuration, these elements will form a complex compound with different types of the ligand. 


EAN Rule Examples

1. Explain Effective Atomic Number for Iron (Fe) in Fe (CO)5

The oxidation state of iron is zero.

The atomic number of iron is 26.

Carbonyl (CO) is a monodentate ligand.

EAN for the iron will be = an Atomic number of iron + total number of electrons donated by the ligand.

EAN of iron (Fe) = 26 + 5 *2

EAN of iron (Fe) = 26 + 10

EAN of iron (Fe) = 36.

36 is the noble gas electronic configuration.


2. Explain Effective Atomic Number for Iron (Fe) in [Fe (NH3)6]+2

The oxidation state of iron is + 2.

The atomic number of iron is 26.

Number of electrons in Fe+2 = 24

Ammonia (NH3) is a monodentate ligand.

EAN for the iron will be = an Atomic number of iron + total number of electrons donated by the ligand.

EAN of iron (Fe) = 24 + 6 *2

EAN of iron (Fe) = 24 + 12

EAN of iron (Fe) = 36.

36 is the noble gas electronic configuration.


3. Explain Effective Atomic Number for Iron (Fe) in K3 [Fe (CN)6]

The oxidation state of iron is + 3.

The atomic number of iron is 26.

Number of electrons in Fe+3 = 23

Cyanide (CN) is a monodentate ligand.

EAN for the iron will be = an Atomic number of iron + total number of electrons donated by the ligand.

EAN of iron (Fe) = 23 + 6 * 2

EAN of iron (Fe) = 23 + 12

EAN of iron (Fe) = 35.

35 is not the noble gas electronic configuration.

Not satisfying EAN rule. To attain thirty-six electronic configuration, the iron in this compound will accept an electron. Therefore, it will act as an oxidizing agent.


Did You Know?

  • Do you know that the 18 electron rule follows the noble ga configuration concept but still it is different from the EAN rule.

  • Not every coordination species follow this rule.

FAQ (Frequently Asked Questions)

Question: What is the EAN Rule?

Answer: EAN rule is a theory given by Sidgwick. It is the theory that gives the idea about the stability of the coordination compound that forms. It represents the total number of electron present around the nucleus of the central metal ion in the coordination sphere. It can be calculated by the formula given below:

EAN = (z - x) + (C.N. * 2)

EAN = (z - x) + (L * D * 2

Question: What will be the Effective Atomic Number for the [Co (NH₃)₆]⁺³

Answer: The effective atomic number for [Co (NH₃)₆]⁺³ will be

The oxidation state of cobalt is + 3.

The atomic number of cobalt is 27.

Number of electrons in Co+3 = 24

ammonia (NH₃) is a monodentate ligand.

EAN for the iron will be = an Atomic number of iron + total number of electrons donated by the ligand.

EAN of cobalt (Co) = 24 + 6 * 2

EAN of cobalt (Co) = 24 + 12

EAN of cobalt (Co) = 36.

36 is a noble gas electronic configuration.