Answer
Probability of finding light atom is 0 at $\phi = \pi$
Work Step by Step
The normalized wave function given in exercise 7B.3 is $\psi(\phi) = ({1\over 2π})^{1/2}e^{iϕ}$
The probability of finding a particle over a point is always zero.
To proof that consider a wave function $\psi(r)$ where is based on three dimensional coordinate.
$P = \int_{V_0}^{V_0} \psi ^2 d\tau$
Which is clearly 0.
