What is Effective Atomic Number?
The effective atomic number is a very common term used in the coordination chemistry branch. The effective atomic number 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 an 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 Effective Atomic Number Rule
In the 1920s, N.V. Sidgwick observed that the metal present in a complex like metal carbonyl, for example [Ni(CO4)] has the same valence electron count as that of noble gas that terminates the periodic table, of which the metal is a part. Though “the inert gas” rule was coined by the scientist to indicate stability, it is now referred to as the 18 –electron rule or EAN Rule.
The representation of the atom is done as Zeff. It indicates the number of protons that the electron in a metal effectively sees due to the screening by the inner-shell electrons. Generally, the EAN of the given central atom is numerically equal to the atomic number of the noble gas that is present in the same period to which the given central metal atom belongs. 36 (Krypton), 54 (Xenon), and 86 (Radon) are noble gases that generally fall under the consideration of the EAN rule.
The noble gases that belong to the 3rd, 4th and 5th periods are generally considered for the EAN rule. The elements that generally belong to the 3rd, 4th and 5th periods are the transition metals, hence, the central atom of the complex formed is generally composed of the transition element. Thus, the central metal atom of the complex generally belongs to the 3rd, 4th and 5th periods of the d-block element.
Effective Atomic Number in Coordination Compounds
In the above definition, we explain the 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 ligand.
The Formula of Sidgwick EAN Rule
EAN = Atomic number (Z) – Oxidation number + 2 × Coordination number
EAN= Z – x + 2nL
Z= Atomic number of the metal in the complex
x = Oxidation state of the metal in complex
n= Number of the ligands
L = Number of coordinate bonds formed by 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.
Significance of the Effective Atomic Number
The major significance of the effective atomic number are listed below:-
The effective atomic number helps us in understanding why electrons, when further apart from the nucleus, are weakly bound to it.
The stability of the coordinate compound is explained with the help of the effective atomic number.
In the case of the non-classic complexes like that of the metal carbonyl complex, the effective atomic number is more valid. Hence this rule explains the stability, oxidising and reducing character of carbonyl compounds. Though it is found to be invalid for most of the complexes, it is seen to be valid for all the cases of the carbonyl complexes. No ligands act as a three electron donor.
EAN Rule Examples
1. Explain Effective Atomic Number for Iron (Fe) in Fe (CO)5.
Ans. 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 + the 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
Ans. 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 + the 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]
Ans. 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 + the 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 configurations, the iron in this compound will accept an electron. Therefore, it will act as an oxidizing agent.
Did You Know?
The 18 electron rule follows the noble gas configuration concept but still it is different from the EAN rule.
Not every coordination species follow this rule.
FAQs on Effective Atomic Number
1. What is the Effective Atomic Number (EAN)?
The Effective Atomic Number (EAN) is the total number of electrons surrounding the central metal ion within a coordination complex. This count includes the metal ion's own electrons plus the electrons donated by the ligands. The concept was introduced by N.V. Sidgwick to explain the stability of these compounds.
2. How is the Effective Atomic Number (EAN) calculated?
The Effective Atomic Number can be calculated using a straightforward formula:
EAN = Z - ON + 2 × CN
Where:
- Z is the atomic number of the central metal atom.
- ON is the oxidation number of the central metal atom.
- CN is the coordination number of the complex, which is the total number of coordinate bonds formed by the ligands.
3. What is the EAN rule in coordination chemistry?
The EAN rule, also known as Sidgwick's theory, states that a coordination complex is considered to be particularly stable if the Effective Atomic Number of its central metal atom is equal to the atomic number of the noble gas found in the same period. The most common target EANs for transition metals are 36 (Krypton), 54 (Xenon), and 86 (Radon).
4. Calculate the EAN for the central metal atom in [Co(NH₃)₆]³⁺.
To calculate the EAN for cobalt (Co) in [Co(NH₃)₆]³⁺, we follow these steps:
- The atomic number of Cobalt (Z) is 27.
- The oxidation state of Co (ON) in the complex is +3.
- The coordination number (CN) is 6, as there are six monodentate NH₃ ligands.
- Using the formula: EAN = Z - ON + 2 × CN
- EAN = 27 - 3 + 2 × 6 = 24 + 12 = 36.
This EAN of 36 matches the atomic number of the noble gas Krypton (Kr), suggesting the complex is stable according to the EAN rule.
5. What is the main significance of determining the EAN of a complex?
The primary significance of the EAN is to predict the stability of coordination compounds. A complex that satisfies the EAN rule (i.e., its EAN matches a noble gas configuration) is generally predicted to be stable. It is especially useful in explaining the stability, bonding, and properties of organometallic compounds, particularly metal carbonyls.
6. Why do some stable coordination compounds not follow the EAN rule?
The EAN rule is a useful guideline but not an absolute law. Many stable complexes do not adhere to it because other factors also significantly influence stability. These factors include:
- Ligand field effects: The geometry and electronic structure determined by ligand interactions can confer stability regardless of the EAN.
- Steric hindrance: The physical size of ligands can prevent the metal from achieving the ideal coordination number required by the rule.
- Overall charge of the complex: The distribution of charge can also be a more dominant factor in the stability of a complex.
7. Is the Effective Atomic Number (EAN) the same as the Effective Nuclear Charge (Zeff)?
No, they are two completely different concepts.
- The Effective Atomic Number (EAN) refers to the total number of electrons around a central metal atom within a coordination complex.
- The Effective Nuclear Charge (Zeff) refers to the net positive charge experienced by an electron in a single atom, which accounts for the shielding effect of inner-shell electrons. EAN applies to complexes, while Zeff applies to individual atoms or ions.
8. What is the difference between the EAN rule and the 18-electron rule?
While both rules are used to predict the stability of complexes, they have a different focus. The EAN rule considers the total number of electrons on the central metal, aiming to match the electron count of a noble gas (e.g., 36, 54, 86). The 18-electron rule is a more specific version primarily for transition metals, focusing only on the valence shell electrons (ns, np, (n-1)d orbitals). A complex that has 18 valence electrons is considered stable. For many transition metal complexes, satisfying the 18-electron rule is equivalent to achieving the EAN of the next noble gas.
9. How can the EAN rule help predict if a complex will act as an oxidising or reducing agent?
The EAN rule can provide insight into a complex's redox behaviour.
- If a complex has an EAN just short of a noble gas configuration (e.g., 35), it has a strong tendency to accept an electron to reach the stable 36 count. This makes the complex an oxidising agent.
- Conversely, if a complex has an EAN slightly above a noble gas configuration (e.g., 37), it may readily lose an electron to achieve the stable 36 count. This makes it a reducing agent.
