[DoNotKnow]

Hi!!

So noob question. How to know which elements are stable??

Thanks!!

[Corribus]

Define stable.

[DoNotKnow]

Like for example Boron isn't that reactive. How to know that??

[Corribus]

Everything is relative. Take your example, boron. Isn't chemically reactive? According to what reference? Boron is very rare as a pure element, so clearly it must be relatively reactive, otherwise you'd find elemental boron everywhere. We might describe carbon as stable compared to boron, but unstable compared to nitrogen. Moreover, even within carbon itself - some forms of carbon are more stable than others (compare graphene to carbyne to diamond, all pure forms of carbon), and even THEN you can't generalize, because some elemental forms are more stable under some conditions than other. Consider tin, which has two predominant forms: β-tin is metallic and stable above ~ room temperature, but α-tin, often considered a nonmetal, is the stable form at lower temperature. Any of these observations can be explained individually, but to explain all observations simultaneously in a single, simple way, is not possible. Unless you say something trivial, like, "because of thermodynamics". But is that helpful?

Asking why some elements are reactive and others aren't is too general a question. It's like asking, "why are some people more angry than others?"

[DoNotKnow]

Thanks man make sense, but why is group VII more reactive than group IV does that have to do anything with electrons?

[Arkcon]

Thanks man make sense, but why is group VII more reactive than group IV does that have to do anything with electrons?

That is a very apt statement, pretty much everything to do with chemistry is because of the electrons.  However, without more specifics, that statement doesn't say much.

[Corribus]

Well, practically everything in chemistry has something to do with electrons. I still have to say that it's an overly broad question.

Group VII vs. Group IV: do you mean, isolated atoms, pure elemental substances, or what? A general observation is that atoms prefer to form completed octets - although this in itself isn't a very satisfying explanation. To wit: both fluorines in F₂ have full octets, as do all the carbons in diamond. Fluorine gas, though, is certainly more reactive than diamond. So this in itself isn't a good explanation. There are a lot of ways you could explain these differences. We must also again define what we mean by "stability". There is thermodynamic stability and kinetic stability. Mixtures of gasoline and oxygen are not thermodynamically stable, but they are kinetically stable. The phase of the material also makes a big difference: gasoline is far more reactive when it is aerosolized versus when it is a liquid, which is why it is nebulized in an engine prior to ignition. Therefore, even based on phase alone, comparisons between Group 4 carbon (diamond) and Group 7 (fluorine) are not necessarily fair. Diamond is a solid and fluorine is a gas. Based on that alone, we might expect varying degrees of reactivity.

Back to the point, when comparing one element to another, what we might mean to do is compare the strenghts of bonds between atoms. This certainly has much to do with electronic structure, and we may even begin to answer such a question in a general way. But speaking of reactivity in isolation still doesn't make much sense. With the exception of some decomposition reactions, must reactions require at least TWO molecules to tango. Fluorine may be a reactive gas, but only in the presence of other reactants. A vial of pure fluorine will last indefinitely. So, in almost all cases, we must specify the conditions and context to understand a vaguely defined concept like "stability".

So, in short, while we may typically think of the Group VII elements as "reactive", and generally this has to do with the weakness of covalent bonds between atoms in this group (compared to more polar/ionic bonds between atoms in this group and atoms in other groups), which in turn has a lot to do with electron-electron repulsion, it's better and more instructive to consider more specific comparisons.

[DoNotKnow]

Thanks man!!

[sbf5]

You don't make it clear as to what you mean by "stable".
If your definition of unstable is "reacts easily", then you'll notice that the close to each side you grow, the less stable the element becomes.
If your definition of unstable is "emits particles", then you'd have to take a look at a periodic table that displays atomic masses. Take, for instance, lead. You'll notice that its atomic mass has no bracket around it. This indicates that it has at least one stable isotope. In fact, lead is the last element on the periodic table to have this. Every element after lead is "unstable", in this sense. However, technetium (atomic #43) and promethium (atomic #61) have no stable isotopes and have brackets around their atomic masses.