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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Ahmed Abdullah on March 01, 2008, 02:39:01 AM

Title: What is meant when it is said that a molecule (e.g diene or poliene) is excited?
Post by: Ahmed Abdullah on March 01, 2008, 02:39:01 AM: We know that when an atom is excited electron is elevated from the ground state. But I can't really figure what is an excited molecule..
Is it something like , hmm....the molecular orbital is enlarged so that the shared electrons can enjoy extra freedom or something else?
I think you have understood the problem.
Title: Re: What is meant when it is said that a molecule (e.g diene or poliene) is excited?
Post by: Borek on March 01, 2008, 04:35:42 AM: We better start from the very beginning - what does it mean that atom is excited? What happens to the excited electron?
Title: Re: What is meant when it is said that a molecule (e.g diene or poliene) is exci
Post by: Ahmed Abdullah on March 01, 2008, 04:49:30 AM: Quote from: Borek on March 01, 2008, 04:35:42 AM
We better start from the very beginning - what does it mean that atom is excited? What happens to the excited electron?

1. At least one of the electron absorbs energy and rises from the ground state.

2. Excited electron remains in some higher energy level (as long as it is an excited atom).

I like your approach BOREK. Can you be more help?
Title: Re: What is meant when it is said that a molecule (e.g diene or poliene) is exci
Post by: Borek on March 01, 2008, 05:13:22 AM: Quote from: Ahmed Abdullah on March 01, 2008, 04:49:30 AM
2. Excited electron remains in some higher energy level (as long as it is an excited atom).

And in terms of orbitals?
Title: Re: What is meant when it is said that a molecule (e.g diene or poliene) is exci
Post by: Ahmed Abdullah on March 01, 2008, 10:00:31 PM: Quote from: Borek on March 01, 2008, 05:13:22 AM
Quote from: Ahmed Abdullah on March 01, 2008, 04:49:30 AM
2. Excited electron remains in some higher energy level (as long as it is an excited atom).

And in terms of orbitals?
electrons jump from one orbital to other; where the latter is somehow more energetic than the other. Right?
Title: Re: What is meant when it is said that a molecule (e.g diene or poliene) is excited?
Post by: Borek on March 02, 2008, 03:52:51 AM: Exactly. Now, molecules are not different - just instead of atomic orbitals they have molecular orbitals.
Title: Re: What is meant when it is said that a molecule (e.g diene or poliene) is excited?
Post by: Ahmed Abdullah on March 02, 2008, 04:36:58 AM: we know atoms can be ionized by absorbing photons of relevant energy.
So molecules like Cl2 should be able to undergo similar process to give rise to: Cl2 + photon-------> 2Cl+
Is it reality?
Title: Re: What is meant when it is said that a molecule (e.g diene or poliene) is excited?
Post by: Borek on March 02, 2008, 04:48:22 AM: You have to decide what you are talking about - 2Cl⁺ are not an excited Cl₂.
Title: Re: What is meant when it is said that a molecule (e.g diene or poliene) is excited?
Post by: Ahmed Abdullah on March 02, 2008, 05:17:40 AM: absorbing sufficient energy- atoms can eject out electron from the orbital so that the electron is no longer under the influence of the nucleus; that is a positive ion.
From the analogy I think similar thing can happen to electron pair in a molecular orbital.
Title: Re: What is meant when it is said that a molecule (e.g diene or poliene) is excited?
Post by: Borek on March 02, 2008, 05:47:02 AM: Quote from: Ahmed Abdullah on March 02, 2008, 05:17:40 AM
From the analogy I think similar thing can happen to electron pair in a molecular orbital.

Not necesarilly an electron pair - it may happen to a single electron leading to ion like H₂⁺. But this is completely new entity, it is NOT an excited molecule. So you have to decide if you are asking about excited molecule, or ionized molecule.
Title: Re: What is meant when it is said that a molecule (e.g diene or poliene) is exci
Post by: Yggdrasil on March 02, 2008, 07:57:56 AM: Often light can break chemical bonds in a molecule without ionizing the substance (i.e. without ejecting any electrons). For example, Cl₂ can break down into two chlorine radicals (Cl^.).