A simple model to explain the isotope effect in the Walden exchange reaction is suggested. It is developed in the spirit of the line-of-centers models, and considers a hard-sphere collision that transfers energy from the relative translation to the desired vibrational mode, as well as geometrical properties and steric requirements. This model reproduces the recently measured cross sections for the reactions of hydrogen with isotopic silanes and older measurements of the substitution reactions of tritium atoms with isotopic methanes. Unlike previously given explanations, this model explains the effect of the attacking atom as well as of the other participating atoms. The model provides also qualitative explanation of the measured relative yields and thresholds of CH3T and CH2TF from the reaction T + CH3F. Predictions for isotope effects and cross sections of some unmeasured reactions are given.