Most people say that it is because the 3p orbital of chlorine when overlapping with the s orbital of hydrogen covers more area than when 4p orbital of bromine overlaps with the s orbital of hydrogen which makes bond between H and Cl more stronger than the bond between H and Br. But how can we say that the area of overlapping is more for 3p and s orbital compared to overlapping of 4p and s orbital?
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1$\begingroup$ "Less reactive" is not well defined. $\endgroup$– Ivan NeretinCommented Nov 21, 2018 at 18:44
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$\begingroup$ What do you mean? $\endgroup$– Blazar 3c273Commented Nov 22, 2018 at 7:03
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$\begingroup$ I mean, when you use these words without clarification, you can and will get misunderstood. $\endgroup$– Ivan NeretinCommented Nov 22, 2018 at 7:27
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$\begingroup$ I edited my question. I think it is more clear now. $\endgroup$– Blazar 3c273Commented Nov 22, 2018 at 7:39
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$\begingroup$ Yes, that's better. $\endgroup$– Ivan NeretinCommented Nov 22, 2018 at 7:41
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2 Answers
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You can think of the overlapping volume between the electron clouds, but a simpler form to show why this happens is when you consider the electrical potential between the atoms. Even tough this isn't an ionic compound, it has many characteristics that are similar.
If I remember correctly, this is the expression for the potential energy between charged particles: $E_p = k\frac{Q_1 \cdot Q_2}{d}$
Since the radius of $\ce{Br}$ is greater than the radius of $\ce{Cl}$, the potential energy involved in $\ce{H-Br}$ bond will be smaller than $\ce{H-Cl}$ bond.
Another thing that came to my mind, and I'm not sure, is that as the 4p cloud of $\ce{Br}$ is bigger than that of $\ce{Cl}$, the relative amount of the volume that is overlapped with the 1s cloud of $\ce{H}$ gets smaller with $\ce{Br}$ than with $\ce{Cl}$, which means the connection is weaker.
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$\begingroup$ How does potential energy of electrons in the outermost shell of an atom are even related to bond energy? Its just a part contributing in the bond energy. The interaction energy between electrons of one atom and the other atom and the interaction energy between electrons of one atom and protons of the other atom also play a role. By electrons I mean all the electrons, not just valence electrons. And for that you need to know the positions of electrons which is not possible because of the uncertainty principal. $\endgroup$ Commented Nov 22, 2018 at 18:36
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$\begingroup$ I am a bit confused in the last part. $\endgroup$ Commented Nov 22, 2018 at 18:49
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If you aren't convinced by the overlap area argument, then think of it in terms of polarization. The H+ cation will polarize Br- more as compared to Cl-, so HBr will possess a more covalent character as compared to HCl, or conversely, H-Cl bond will be more ionic than the H-Br bond. And as we know, ionic bonds are generally stronger than covalent bonds
answered Dec 23, 2018 at 4:13
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$\begingroup$ No. Ionic and covalent bonds are,made of the same stuff, electrostatic attraction between opposite charges with similar magnitude at similar distances. They are roughly equal in strength. $\endgroup$ Commented Dec 23, 2018 at 14:30
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$\begingroup$ @OscarLanzi Sorry,my bad. I guess I wanted to convey the idea that more electrostatic attraction will be present in H-Cl as compared to H-Br due to differences in charge/ radius ratio, and hence the charge from chlorine will be more than bromine in the formula of colombic force. $\endgroup$ Commented Dec 23, 2018 at 14:45
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$\begingroup$ @OscarLanzi Also, I used the word generally, so it is kind of true. chemistry.stackexchange.com/q/8281/54655 $\endgroup$ Commented Dec 23, 2018 at 14:45