Thoughts about Group VII

So is it intermolecular forces or is it inter-atomic forces?

One of the more puzzling thing in the learning of Group VII elements is trying to understand when do we use intermolecular forces and when do we use inter-atomic forces to explain phenomenon observed in this group.

1. Intermolecular forces at work.

When dealing with hydrogen halides (HX), when we are trying to account the different boiling points, we are explaining in terms of differing strength of intermolecular forces. Changing the state of HX from liquid to gas, ensures that the substance retain its molecular form - it is still HX when it changes from liquid to gas. As energy is needed to change the state, some form of attraction must be broken. Hence, it is the Van der Waals interaction between the HX molecules.

Down the group, HX gets larger, the electron cloud becomes more polarisable, hence induced dipole interaction becomes stronger.

2. Inter-atomic forces at work.

However, when we discuss about the thermal stability and the acidic strength of HX, we are basically talking about the following pairs of equation:

2HX (g) -> H₂ (g) + X₂ (g) &
HX -> H⁺ + X^-

The equation shows clearly that the H-X bond has to be broken. Hence, the ease of thermal decomposition and the acidic strength HX is determined by the strength of the H-X single bond. The stronger the bond, the more thermal stable the HX will be and the weaker acid it will be.

Although, in conclusion, although there are differing strength in the acidic strength of HX, they are generally strong acids as they do dissociate completely.

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^{Article written by Kwok YL 2010.}

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