Wikipedia:Reference desk/Archives/Science/2023 September 14

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September 14

How Do Small Changes In Composition Change The Properties Of Molecules And Atoms So Much?

Please do been helping me understand. ~~Alex Salazar 13:31, 14 September 2023 (UTC)

Can you give an example? It’s hard to know what constitutes a “small” change in composition in your mind. Also, it isn’t even always true. Sodium hydroxide and potassium hydroxide differ in 1/3rd of their atoms, and have a 40% difference in mass, but in most applications are practically interchangeable as long as the same number of moles of the salts are used. —OuroborosCobra (talk) 14:16, 14 September 2023 (UTC)

No Need for Been Attack. I will See You if I may Figure Out How This Goes. For Me I Been Suppose There Is A Duality Of Arsenic Acid And Carboxygenic Gas. ~~Alex Salazar 50.237.188.108 (talk) 14:43, 14 September 2023 (UTC)

That's a good question and one which those who design drugs and other commercially-important chemicals would love to know the full answer to! The nearest article on this subject may be Quantitative structure–activity relationship but is a bit technical for non-specialists. Roughly speaking, for drugs and pesticides the analogy of the lock and key model is usually quoted. Mike Turnbull (talk) 15:02, 14 September 2023 (UTC)

I do been wanting to tell you thank you for the citation of me, Mike Turnbull. 50.237.188.108 (talk) 15:54, 14 September 2023 (UTC)

I'm going to disagree a little bit here. That analogy works well for larger molecules (large organic compounds or full on macromolecules, like proteins), but it isn't always so easily apparent for smaller molecules or even small active sites within proteins or enzymes. The example that Alex is talking about, between what is mostly carbon dioxide and arsenic acid, that's not at all what I would consider a "small" composition change, since we have a huge elemental difference, geometry difference, etc., but what if we made it arsenic acid vs phosphoric acid? They are structurally very similar, their elemental composition is identical except for the core atom (arsenic vs phosphorous), but in terms of overall properties, this doesn't seem to have much of an impact. Their pKas are nearly identical, their melting points are fairly close, etc. However, one is extremely toxic, and the other (in moderation of course) is fairly benign and not an uncommon intentional food additive, such as in carbonated beverages. The difference is that arsenic is a metalloid, and phosphorous is strictly a non-metal. Metals and metalloids tend to love to bind to sulfur (some nice inorganic chemistry reasons that I won't get into, but look to electron configurations, d-orbitals, in other words, electron wavefunctions and quantum stuff that isn't apparent at all in a "lock and key model."). It's similarity to phosphorous allows it to bind in place of phosphoric acid in some important areas not related to sulfur, and its binding to sulfur (such as thiols in many amino acids) completely disrupts the function of vital enzymes. Put that together, and the citric acid cycle quickly breaks down, you stop making ATP, and you die. --OuroborosCobra (talk) 14:27, 15 September 2023 (UTC)