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In class, Professor Lavelle said that increasing the strength of dispersion forces explains why F2 and Cl2 are gases while Br2 is a liquid. What do dispersion forces have to do with the state of these diatomic molecules?
The more electrons a molecule has, the more polarizable it is. Because there are more dispersion forces happening in larger molecules because of the number of electrons, the interactions are likely to be a liquid substance over a gaseous substance. This is evident in why F2 is a gas and Br2 is a liquid.
Br2 is a liquid because Br is a larger atom than F and Cl. The larger the atom, the stronger the dispersion force between them. So since F2 and Cl2 are smaller and have less attraction between them, they're held less closely together and are gases at room temperature. Br2 is a liquid because the Br2 molecules are more attracted to each other and are held closer together.
larger atoms have more attractive forces between them because they have more electrons, so they they will have stronger dispersion forces holding them closer together and will therefore be liquid at room temp rather than gases.
Dispersion forces are synonymous with the term dipole-induced dipole-induced interactions, which as the name suggests deals with the polarzability of an atom. In the case of Cl2 and F2, both molecules have a significantly smaller atomic radius and have less electrons compared to Br2 which indicates that the electrons in Cl2 and F2 packed closer together whereas Br2 are further away from the nucleus. The idea behind dipole-induced dipole-induced interactions is that a temporary negatively charged region of an atom repels away the electrons of another atom thus revealing a region with a slight positive charge therefore creating the interaction between the temporary positive and negative region. The larger the molecule/atom the the greater the polarizability hence the more attractive force, and thus Bl2 having a stronger polarizability and stronger attraction . Therefore as the polarizability and the strength of the dipserion force increases, molecules would be seen increasing from gas to solid.
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