I have recently learned that pure ionic and covalent bonds are just the extremes of a spectrum of bonds from this article from Chemguide. But I can't seem to square this with my understanding of how bonding works. In ionic compounds like $\ce{NaCl}$ for example, the difference in electronegativity is so great that it seems that electrons are transferred from $\ce{Na}$ to $\ce{Cl}$, so they both gain octet structures. Similarly, in covalent compounds like $\ce{H2}$, the difference in electronegativity is $0$, so the electrons are shared between the elements, so both gain octet structures. But in between these extremes, are the electrons shared, transferred, or a mix of both? And if it's a mix of both, how can atoms fill their octet if the electrons are "partially transferred"?
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$\begingroup$ They are shared but the electrons are more likely to be found nearer to the more electronegative atom.. $\endgroup$– Safdar FaisalAug 16, 2020 at 8:59
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$\begingroup$ @Safdar So the spectrum of bonds goes from covalent, to polar, to ionic, and in covalent and polar bonds the electrons are shared between atoms, while in ionic bonds, one atom has control of both electrons in the electron pair(i.e electrons are "transferred")? $\endgroup$– sukessh velusamyAug 16, 2020 at 10:01
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$\begingroup$ Yes.. That would be correct. $\endgroup$– Safdar FaisalAug 16, 2020 at 10:23
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$\begingroup$ Recently, there was a similar question which was answered with electron density maps (chemistry.stackexchange.com/questions/138377/…). Perhaps this complementary perspective is helpful for you, $\endgroup$– ButtonwoodAug 16, 2020 at 17:48
1 Answer
In a covalent bond between two atoms with an electronegativity difference, the bonding electrons are closer to the more electronegative atom in the bond. This causes a partial negative charge to form on the more electronegative atom and a partial positive charge forms on the less electronegative atom. This causes two poles to form in the compound called a dipole. Thus, there is a degree of ionic character observed in the covalent bond.
Now you need to understand charge density. In ionic bonds, cations and anions are next to one another. If the cation is small in size and has a large charge it has a high charge density. If it is bonded to an anion of large radius. The outer electrons of the anion are not firmly bonded to the nucleus and are attracted by the strong electric field of small highly charged cation. This causes an increase in electron density between the two ions. Thus, a degree of covalent character is conferred on the ionic bond.
Now, to answer why covalent compounds are covalent and ionic compounds are ionic is answered by the following phenomenon. Strength of dipole is measured by its dipole moment. Dipole moment is product of separated electric charge and distance between them. It has the unite Debye which has the dimensions of coulomb meter.
Now ionic compounds have quite high dipole moments like 7.0D etc. while even the most polar covalent compounds like HF have a dipole moment of 1.91D. This shows that even the most polar covalent compounds are a long way from being ionic.
So electrons are not "partially transferred". They are either transferred or shared. The sharing may become unequal or unbalanced causing bond polarity. The electron transfer may result in ions forming with different charge densities leading to attraction of electrons towards the strong electric field. The main objective is to reach a point where there is maximum stability in the compound.
Following link gives further insight into polarity and dipole moments if you find difficulty in understanding the answer.
