Van der Waals, Induced Dipole, Permanent Dipoles?

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Posted Sun 31st March, 2013 @ 18:43 by Zahara Nathanie

4 Answers

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Van de Waals; are the weakest attraciton and they form between all molecules. they are caused by one molecule becoming charged by a temporary shift of electtrons closer to one end of the molecule. This happens constantly in different molecules. When one molecule has the temporary charge, it induces (forces) other molecules to also become temporary charged as the electrons close to one end push the electrons of other molecules away due to repulsion. These forces break and form all the time. They do not last very long as they are only temporary. (These are also called induced dipoles and London forces). These form between nonpolar and polar molecules.

Permanent dipoles are when there is a slight difference between electronegativity between the ends of the molecules creating poles. therefore there is permanent attractions between the molecules (so like van de waals only permanently because there is a greater electronegativity difference).

Hydrogen bonds are the strongest attraction and they form between molecules containing oxygen, nitrogen and flourine. So water and other such molecules contain H bonds. These are very strong permanent dipoles, which only form when those elements are present.

Hope this  helps. Ask if you need more detail.

Answered Sun 31st March, 2013 @ 21:35 by Tilly - Team GR
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Intermolecular forces (Permanent dipole, instantaneous-induced dipole and hydrogen bonds) are forces of attraction BETWEEN MOLECULES. People get confused by the term inter, which is between 2 or more seperate molecules. E.g: Between 2 Chlorine molecules. Also, these forces occur ONLY between SIMPLE MOLECULES.

These forces occur due to either 1) Large differences between electronegativities between 2 DIFFERENT atoms and 2) Uneven distribution of electrons at any given moment.

Now, we all know about electronegativity, it is the ability of an atom to attract pairs of BONDED electrons in a covalent bond. Flourine is the most electronegative element there is. Permanent dipoles occur due to this. Lets take HCl, Chlorine is far more electronegative, so it attracts the electrons between H and Cl to itself. This gives Cl and slightly negative charge, we say it is polarised and HCl is dipolar. Because it is a permanent dipole, Cl will always be partially negative. Now, in a sample of HCl, the partially negative Cl will be attracted to the pratially positive H in ANOTHER molecule and vice versa. Because this is a permanent dipole, the attraction between the 2 HCl's will be VERY VERY strong. 

Exam question: Explain why HCl has a permanent dipole?

There is a permanent dipole due to large differences in electronegativity between H anc Cl. Chlorine is more electronegative so it attracts the bonding pairs of electrons in the covalent bond to itself making chlorine partially negative.

Some molecules are not polar, which means the atoms have similar electronegativities. But, we all know electrons orbit the nucleus with different energies. Therefore, electrons can be on one side of the atom at any give moment. We say there can be an uneven distribution of electrons in an atom/molecule. Lets take Cl2 this time, there is no difference of electronegativity right? They are both the same. However, at any moment, electrons can be on one side of the molecule, so one chlorine is slightly electronegative and the other is partially positive, this is called instaneous dipole.

Now, in a sample of Cl2, if one Cl2 becomes instantly dipole, it can INDUCE a dipole in the next neighbouring Cl2. The partially negative chlorine can repel the electrons in another Cl2 to make it dipolar. This is called Instaneous- induced dipole. It is a bit confusing but try drawing it out, +Cl-CL-  +Cl-Cl- 

You need to remember this is very weak, these forces are constatntly being broken and made. 

Dont get it mixed up with chemical bonds, Chemical bonds occur within two or more attoms to achieve a full outer shell, they are: Ionic, Covalent and metallic. Intermolecular bonds are: Permanent dipole, Instaneous-induced dipole and hydrogen bonds. They occur naturally. Chemical bonds are waaaaaaaaaaaay more stronger than intermolecular forces. REMEMBER!

Answered Sun 31st March, 2013 @ 23:34 by Azimul
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Forces that occur in covalent bonds?

Answered Sun 31st March, 2013 @ 21:22 by attiqa
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But what are the differences between them?

Answered Sun 31st March, 2013 @ 21:26 by Zahara Nathanie