HOMO-LUMO energy gap of compounds

In summary, I believe that option A is the correct option because as the energy difference between two atomic orbitals increases, their interaction, and thus the value of E (or E*) decreases.
  • #1
Amitkumarr
20
4
Homework Statement
Which of the following statements is/are correct?

(a) The molecular orbitals are essentially identical to the relevant atomic orbitals at very high energy differences.

(b) Dominantly ionic compounds of group I have HOMO-LUMO energy gap in visible region generally.

(c) Coloured covalent diatomic molecules have HOMO-LUMO energy gap in ultraviolet region.

(d) All of the above.
Relevant Equations
No relevant equations.Based on Molecular orbital theory.
According to me Option A is a correct option because as the energy difference between two atomic orbitals increases,
their interaction, and thus the value of E (or E*) decreases. Here, E is the energy difference between the bonding molecular orbital and the atomic orbital of lower energy , while E* is the energy difference between the antibonding molecular orbital and the higher-energy atomic orbital. As these values decrease, the molecular orbitals become more and more similar to the atomic orbitals from which they are derived.
But what should be the correct answer for options B and C? The HOMO-LUMO energy gap of the given compounds lies in which region?
Note: Answer is given as option A.Why options B and C are wrong?
 

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  • #2
Ionic compounds will be colored if the HOMO-LUMO bandgap is of the same energy as visible light. What does this mean to you? What color are Group I ionic compounds, generally? If that bandgap is of the same energy of a photon outside the visible region, will it be colored?
 
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  • #3
chemisttree said:
Ionic compounds will be colored if the HOMO-LUMO bandgap is of the same energy as visible light. What does this mean to you? What color are Group I ionic compounds, generally? If that bandgap is of the same energy of a photon outside the visible region, will it be colored?
Group I ionic compounds are generally colorless so,their HOMO-LUMO band gap must lie outside the visible region.If that bandgap is of the same energy of a photon outside the visible region,they will not be colored,right?
Whereas,covalent diatomic molecules which are colored must have their HOMO-LUMO band gap in the visible region.

So,is there any specific region where the HOMO-LUMO band gap of Group I ionic compounds lie(like ultraviolet, infrared etc.)?
 
  • #4
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  • #5
chemisttree said:
That’s a good question. Try it for yourself!

Remember, E = hv
Thanks a lot for helping me and for giving me the link.
 

1. What is the HOMO-LUMO energy gap of a compound?

The HOMO-LUMO energy gap of a compound refers to the difference in energy between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the compound. It is a measure of the compound's ability to undergo electronic transitions and is an important factor in determining its optical and electronic properties.

2. How is the HOMO-LUMO energy gap calculated?

The HOMO-LUMO energy gap can be calculated using various methods, such as density functional theory (DFT) or time-dependent DFT. These methods use mathematical equations to predict the electronic structure of the compound and determine the energy levels of the HOMO and LUMO.

3. What factors affect the HOMO-LUMO energy gap of a compound?

The HOMO-LUMO energy gap is influenced by several factors, including the molecular structure, the type and number of atoms present, and the presence of functional groups. Additionally, external factors such as temperature and pressure can also impact the energy gap.

4. Why is the HOMO-LUMO energy gap important in organic chemistry?

The HOMO-LUMO energy gap is a crucial concept in organic chemistry as it provides insight into the reactivity and stability of a compound. Compounds with smaller energy gaps are more reactive and can undergo electronic transitions more easily, while compounds with larger energy gaps are more stable and less likely to react.

5. How does the HOMO-LUMO energy gap affect the color of a compound?

The HOMO-LUMO energy gap is closely related to the color of a compound. Compounds with smaller energy gaps tend to absorb light in the visible region, giving them a more intense color. On the other hand, compounds with larger energy gaps are colorless as they do not absorb visible light. This relationship is used in the field of organic chemistry to design and synthesize compounds with specific colors for various applications.

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