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how does the octet rule applies to covalent bonds?

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3 Answers

The octet rule isn't really a reason why.  It's simply an observation as to what happens.  The full answer is rather complex, but it mainly amounts to two properties:  ionization energy (how much energy it takes to remove an electron) and electron affinity (the ability to gain electrons).

Simply put, alkali metals have relatively low ionization energies, so it is easy to remove electrons from the neutral atoms.  They have unfavorable electron affinities, so they don't like to gain electrons.

On the other side of the periodic table, halogens are the opposite.  They have favorable electron affinities, so it is favorable to gain electrons to form negative ions.  They have very high ionization energies, so it is very difficult to remove electrons.

Once an atom has lost electrons to form an octet, the outermost electrons are in a lower shell, closer to the positively charged nucleus, so are much harder to remove.  This is why they stop at the noble gas configuration.

The other end is a bit trickier, but once an electron has been added to a neutral atom, a negatively charged ion is formed.  Adding a negative to a negative is always unfavorable (think of bringing two north poles on a magnet together).  The reason it sometimes happens has to do with the very favorable formation of solid crystals and the efficiency of packing that results.  In any case, once the octet is obtained, not only are you adding a negative to a negative, you are trying to add it to a higher level shell, farther from the nucleus, so there is very little attraction holding onto the electron.

 

Now, looking back at your original question, I notice that you are asking two completely different questions.  The title of the question asks about covalent bonds, while the text asks about ionic bonds.

Halogens (elements in Group 17) have only one electron too fill the outermost shell and make the octet complete.  Their valence number is 7.

Alkali metals (elements in Group 1) have only one electron to give up.  They need 7 more (usually from a Group 17 element but can be from another group if there is more than one Group 1 element in the chemical formula).  Their valence number is 1

 

To add to Cristina,

The octet rule states that atoms look to have eight electrons in the outermost or bonding shell. Halogens have seven electrons in their outermost shell as an example fluorine has nine electrons, two of which or in the first orbital and seven in the second orbital. Fluorine would like an electron to complete this shell Now Sodium has 11 electrons and that means 2 in shell one, 8 in shell 2 and a lone electron in shell three.  So Sodium needs seven to complete shell 3. Thus by sodium bonding with fluorine, they both complete the outermost shell with eight electrons.

Art

Comments

This answer is misleading.  Sodium doesn't get seven electrons from fluorine.  It's much easier to give up one electron, and get an octet that way, than to gain seven electrons (I hate to use the word impossible, but if ever it fits, sodium gaining seven electrons would be the case).  

Maybe a better way to say it would be, that the halogen gets the extra electron and thus completes an octet in the shell it had almost full already, whereas the sodium gives up that lone electron and thereby empties the 3rd shell, but still has the full 2nd shell.
And anyway, neither of the atoms would do *anything* if they weren't stabilized by the subsequent condensation of the ions formed. Atoms are stable species, unless they have an opportunity to lose the energy liberated by 1) condensation into a solid (metallic, vanderWaals, or such attractions), (2) formation of covalent bond(s) or (3) passage of electron(s) accompanied by ionic pair or usually lattice formation.
 
Naked atoms are rather rare species, on Earth at any rate, convenient as it may be to imagine them. Metallic group I and II metals, perhaps. The closest molecular approximations are perhaps radicals, such as triplet carbenes, which are indeed reactive species. But anytime there's (free) energy to be liberated by a reaction, and not a prohibitive activation energy, things will happen.
I believe this point was meant to show that when sodium forms the +1 ion it is doing so to allow it to bond with a halogen ion of -1 which gains an electron but each elements octet is filled.  This is an ionic bond not covalent which is sharing of electrons to form the octet.

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