[LeaX]

In past papers I often get questions wrong because I don't say there are permanent dipoles (which are from ionic compounds I think?) What is the value for the difference in electro-negativity which differentiates between ionic and covalent?

[ThumbsUp]

'Permanent dipoles' is a vague term used to describe fully ionic compounds, as fully ionic compounds are when two ions (one positive and negative) are attracted to each other.

Dipoles suggest that there is a slight change in two atoms' charge, so one becomes slightly more positive and the other slightly more negative (delta positive/delta negative) due to the position of electrons in he bond.

The closer the electronegativies of two atoms, the more covalent they will be. I.e. a bond between two identical atoms must be 100% covalent. Hence the bigger the difference in electronegativities the more ionic the compound.

[chembob]

(Original post by LeaX)
In past papers I often get questions wrong because I don't say there are permanent dipoles (which are from ionic compounds I think?) What is the value for the difference in electro-negativity which differentiates between ionic and covalent?

There is no set numerical value for a difference in electronegativity that gives an ionic bond, you just need the difference to be significant.

For example the difference in the Pauling electronegativities for H and Li, 0.98 and 2.20 respectively is relatively large (seeing that electronegativities go from about 0.7 to just under 4.0) so you would expect there to be a fully ionic bond between the two in LiH.

NB. This is because the H 1s orbital and the Li 2s orbital are vastly different in energies and so no orbital overlap can occur. The Li 2s electron is simply transferred to the H 1s orbital instead (this is more energetically favourable), resulting in H- and Li+, i.e there will be electrostatic/coulombic attraction between the two.

On the other hand, say you had a C=O double bond as found in a ketone. You would have learnt that the two atoms are held together by a sharing of electrons thus forming a covalent bond. However O is more electronegative than C and so has a tendency to pull the bonding electrons closer to itself, away from C. This leads to a 'separation' of charge and hence a small amount of ionic character due to electrostatic attraction. Hence a C=O double bond is much stronger than a C=C double bond.

[Zygroth]

(Original post by LeaX)
In past papers I often get questions wrong because I don't say there are permanent dipoles (which are from ionic compounds I think?) What is the value for the difference in electro-negativity which differentiates between ionic and covalent?

You don't need to know any of this electronegativity stuff so don't bother learning any of it. With permanent dipoles (these describe covalent compounds, not ionic), you just need to check 1) whether your molecule is symmetric, and 2) whether the atoms involved are identical. If you have a symmetric molecule (CO2, CH4 etc), then no matter what your bonds are, there wont be a permanent dipole because they all cancel each other out, so then all you need to know is if your atoms involved are different - if they are, you have a permanent dipole.

[LeaX]

(Original post by Zygroth)
You don't need to know any of this electronegativity stuff so don't bother learning any of it. With permanent dipoles (these describe covalent compounds, not ionic), you just need to check 1) whether your molecule is symmetric, and 2) whether the atoms involved are identical. If you have a symmetric molecule (CO2, CH4 etc), then no matter what your bonds are, there wont be a permanent dipole because they all cancel each other out, so then all you need to know is if your atoms involved are different - if they are, you have a permanent dipole.

So, basically, as long as a covalent bond doesn't have equal electronegativities (like in O2, Cl2, etc...) then it will have permanent dipoles? If that is the case, then what bonds get instaneous induced dipoles?

[thegodofgod]

(Original post by LeaX)
So, basically, as long as a covalent bond doesn't have equal electronegativities (like in O2, Cl2, etc...) then it will have permanent dipoles? If that is the case, then what bonds get instaneous induced dipoles?

Aren't those Van der Waals' forces - instantaneous dipole, induced dipole - due to the movement of electrons?

[Zygroth]

(Original post by LeaX)
So, basically, as long as a covalent bond doesn't have equal electronegativities (like in O2, Cl2, etc...) then it will have permanent dipoles? If that is the case, then what bonds get instaneous induced dipoles?

Yep - as long as the two atoms are different, one will pull the electrons towards it more than the other, which is a dipole. Just make sure to check for symmetry as the overall molecule might not have a dipole.

As for induced dipoles, they come from electrons rather than bonds so everything with electrons will have them - when electrons move around, they wont be evenly distributed about the nucleus which leads to some charge separation (a dipole), which will induce a dipole in the next molecule (since the positive side of the molecule will attract electrons of the next molecule and the negative side will repel them)

[YaaahSarah]

The name's bond - Ionic bond.
I shall have atoms given to me and not shared .

i then realise its six form ... i am sorry OP