The quantity (ij = 2Bij - Bij - Bjj plays a role in the thermodynamics of gas mixtures at low pressures: see, e.g., Eq. (14.4) for ϕi. This quantity can be positive or negative, depending on the chemical nature of species i and j. For what kinds of molecular pairs would one expect (ij to be
(a) Positive;
(b) Negative;
(c) Essentially zero?