When elements bond together, they can do so ionically, metallically or covalently (depending on the properties of the elements - only metals bond metallically, and only non-metals bond covalently). These are strong bonds, and require more energy respective to other bonds for the bond to break apart.
They can also form weak bonds, such as the excitingly named permanent dipole-permanent dipole interactions, the similarly named permanent dipole-induced dipole interactions, the interestingly-named London forces, and the rather dull-named hydrogen bonds. These are a collective which involve intermolecular forces (which act between molecules in liquids and solids) and, armed with a textbook, the internet, and a keyboard, I'm going to try and write about these forces for revision purposes (but shared with the internet).
Hydrogen bonding
Hydrogen bonds with many elements, but should it bond with fluorine, oxygen or nitrogen, hydrogen bonds will form. This is due to the high electronegativity of these elements, which makes them more likely to attract the shared electrons in a bond. Hydrogen's electronegativity is relatively low, at about 2.20 on the Pauling scale, whereas fluorine's is at 3.98 - which is about as high as you can get on the Pauling scale (which goes from 0-4). Therefore, there's a greater difference in electronegativity, so a dipole is induced as the electronegativities are so different.
A dipole is a separation of charge - specifically equal and opposite charges on either end of the bond. In this case, the difference in electronegativity between hydrogen and nitrogen, oxygen and fluorine causes the bond to be polar (for a dipole is induced) covalent (because these are non-metals sharing electrons). Therefore, the bonds are stronger, requiring more energy to be broken and thus higher melting and boiling points. Compared to other compounds containing bonds between hydrogen and other elements of the same group (such as H-Cl compared to H-F), the melting point of H-F dwarfs the following melting points, as can be seen on this graph:
Eventually, the melting points do eventually start to increase as the bonds do get stronger as you go down Group 17.
Van der Waals Forces - Keesom, Debye and London
Hydrogen bonds are a unique case involving dipole-dipole interactions, of which there are two - permanent dipole-permanent dipole forces, and induced dipole-permanent dipole forces. The former is where two molecules, already with permanent dipoles, will be attracted to each other - as always in charge, like charges repel, opposite charges attract. The greater the polarity, the greater the dipole, therefore the stronger the force. (These interactions are also known as Keesom forces.)
But what are these charges? Focusing on the example of an H-F bond, fluorine has a greater electronegativity than hydrogen, so it will attract more shared electrons - therefore, it can be represented with a negative charge (δ-) - whereas hydrogen would be represented with a positive charge (δ+).
What about induced dipole-permanent dipole forces? Here, a permanent dipole will induce a dipole in another molecule, which makes it polar and thus forms an attraction between them. These are also known as Debye forces.
Compared to their sibling - hydrogen bonds - these interactions are much weaker, so less energy is required to break apart these bonds - however both are much weaker than stronger bonds such as ionic bonds.
London Forces
Upon learning about these forces I searched up the origin of the name, hoping there may have been a chemistry-related place to visit in South London - instead, it's actually named after Fritz London, a German physicist.
Either way, these are also known as dispersion forces, wherein the constant movement of electrons causes a shift in charge across a molecule, and therefore can result in an instantaneous dipole forming. This sets off a chain reaction of sorts, where dipoles are induced amongst adjacent molecules, and the molecules are thus attracted to each other. These are the weakest type of intermolecular force.
"Bond/s" word count - 24
"Force/s" word count - 14
Comments
Post a Comment