Why is there a non-uniformity in even the reasons that explain exceptions in the trends in chemistry? [closed]








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I have been studying the periodic table and several properties of atoms like the Ionization Energy, Atomic Radii, Electron gain enthalpy, Electronegativity etc.


Now, each property has somewhat of a "continuous trend" for left to right(periods) and top to bottom(groups) but there are "a lot" of exceptions as well. There are a few reasons that explain those exceptions but I have been wondering why those reasons do not apply to other elements of the periodic table as well.


For eg- Neon, Beryllium and Nitrogen have the least Electron Affinity in their period with the reason being stated that they have half and fully filled orbitals but on the other hand in the period just below them one would expect to see the same patter of exceptions but now Magnesium, Argon have the least Electron Affinity in their period and Phosphorus which should've been with Magnesium and Argon is in the middle of the trend of increasing Electron Affinity. This shows that even the pattern in the reasons for exceptions break and there are more and more such examples.


So, finally I want to ask that **why is there a disuniformity in even the reasons that explain exceptions in trends in chemistry?** **Why do the reasons just apply to some elements conveninetly and not to others?** Is there any way to explain every single exception there is?(in atleast the trends of the terms I mentioned above like Electron Affinity and Electronegativity).




Exceptions exist for periodic trends and the explanations for those exceptions vary (if such explanations exist at all) because individual atoms are highly intricate and complex systems that are often not amenable to simple models, and complex models can't always be connected to straightforward verbal explanations.


A single electron around a nucleus is sufficiently simple that the solution to the Schrödinger equation for hydrogen-like atoms and the development of the associated quantum theory was possible. One of the greatest accomplishments in 20th century physics was this ability to explain with great mathematical precision a huge number details about an object in the real world, one hydrogen-like atom. It wasn't until the early 21st century that very good mathematical models were developed to account for the additional level of complexity resulting from the presence of the second electron in helium. None of those mathematical models of helium can or should be taught to students in high school at the level where verbal explanations are given for periodic trends.


When mathematical theory is sufficiently well developed, we may be able to mimic every physical property of individual multi-electron atoms, but I don't believe verbal explanations will ever be available in the future for every detail about every periodic trend because the mathematical theory will be too complex for such explanations.


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It is certainly a good thing to be curious, and if you are, people will want you to show that you are motivated and able to look for solutions and bring new ideas. Your question contains several properties, you should have limited the question or your research to one particular property. Some of them are, while others are really pathological. The properties that arise from orbital energies, such as ionisation energies and electron affinities, arise from a complicated ground state of a many-body electronic system. Until now, there is no specific method that can accurately and efficiently predict these properties for all systems. An intuitive understanding is limited to rules and a few exceptions. If you try to work out a rule for these exceptions, you will arrive at another rule with another set of exceptions, and so on. The most accurate description will be an infinite number of rules, each rule described by a specific well-behaved function that maps the configuration of your element to a specific result. The methods for solving this kind of problem are worked out in this way.


As this reasoning is well known, and your question consists of many different properties, this may be one reason why you have probably received more downvotes than answers.


As this argument is well known, and your question has many different properties, this may be the main reason why you have probably received more downvotes than answers, rather than an inability to answer.


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**Chemistry is complicated; simple trends should not be expected to be universal**


The expectation that trends in bulk chemical properties should be simple or clean assumes that only one or a very small number of factors cause the trends. But, if that were true, chemistry would be a very dull subject (which is the same as saying it would be physics and a particularly simple for of physics).


In reality, most observed chemical properties, even of elements never mind molecules) are the result of multiple interacting factors. The wavefunctions of electrons in atoms, for example, are only mathematically soluble for the simplest possible atom (a single hydrogen atoms with just one electron). In anything more complex, there are multiple interactions between electrons and then other interactions between multiple nuclei and multiple electrons. Given that, universally simple trends should not be expected.


Without a complete model of all the factors involved in electron-electron and electron-nucleon interactions it is impossible from first principles to know which particular interaction dominates in each case. We can observe some rules and trends, but we can't easily know when other factors will become important enough to break the trend.


This is one reason why chemistry is a rich enough subject to be interesting.


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