Why do atoms make bonds? [duplicate]








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[Fundamental forces behind covalent bonding](/questions/710/fundamental-forces-behind-covalent-bonding)

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I have been studying chemistry for a long time in school till now, but, what I recently realised I do not understand is why do atoms really make bonds, why do they want to gain or lose electrons or why do they want to attain noble gas configurations when it results in putting a charge on them and comparing to the original condition, let's say oxygen has 8 electrons and 8 protons, why isn't this stable?


This question came to me when I was trying to understand why the ionisation enthalpy can never be negative and how an atom like Na only exhibits it's electropositivity when provided with some energy and is stable until then.


Thank you in advance for your answer


PS: I have realised in the past few days that I really understand very little chemistry:(




I'd say for the same reason why an apple fells to the ground. Because there is an energy gain. Atoms and electrons follow the electromagnetic forces through which they interact, in the same way the apple and our planet do. Just in the case of the apple it is a two body problem with classical mechanics. In the case of bonds it is a few (many) body problem. This makes it much more difficult to visualize it and/or to have an intuitive picture. Luckily we have math and numerical simulations to help us with that.


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We never know why, but we can study when they do and when they don't make bonds. And mostly (on earth) they do make bonds.



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> [...] why do they want to gain or lose electrons or why do they want to attain noble gas configurations when it results in putting a charge on them and comparing to the original condition, let's say oxygen has 8 electrons and 8 protons, why isn't this stable?
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A single (isolated) oxygen atom will keep is electrons, as will a single sodium atom. Making ionic compounds from isolated atoms (a thought experiment), there is usually a net cost of transferring electrons to make the anions and cations, and a large gain when these cations and anions combine to form ionic compounds. The cost is also offset when cations and anions are solvated in water. Mostly, the atoms in elements are already bonded in some way (metallic bonds for metals, covalent bonds for non-metals), so when compounds form from elements, you also have to consider breaking those bonds. Some elements react with many other elements, while others (like noble metals such as gold or platinum) don't.


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**Because some molecules have lower energy that the isolated atoms they are made of**


Isolated atoms are usually stable if they don't interact with anything else. But, when things interact, you need to start thinking about what can happen and one way to do this is to understand the energy levels of the isolated atoms compared to the molecule.


In some cases when isolated atoms interact, nothing happens. Helium atoms, for example, just bounce off each other and no bonds are formed.


But two isolated hydrogen atoms, if they meet, will often produce a stable hydrogen molecule. When this happens, energy is released and we can measure how much. The hydrogen molecule, we say, has *lower energy* than two isolated hydrogen atoms. This we call a chemical bond.


But why is a molecule lower in energy than two isolated atoms? It is partially about electromagnetic forces (two positive protons attract two negative electrons in a hydrogen molecule). But knowing whether the *net* forces lead to a bond involves a lot of detailed calculations and you need a fair amount of quantum mechanics to even approximate the answer. The summary of the answer is that the amount of energy involved in the electromagnetic field of two isolated hydrogen atoms is a lot more than the energy involved in a hydrogen molecule. This isn't true for two helium atoms, which is why they keep to themselves and don't form bonds. But the details of those calculations are advanced.


A great deal of chemistry is about observing when and how bonds form and deriving general rules from observation that explain what happens without having to do those calculations (which are, in general, extremely hard). But the general rule is all about whether the amount of energy involved is lower with a bond than it is with isolated atoms.


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