Complex sentence: The r process contributes to the heaviest four 131Xe, 132Xe, 134Xe and 136Xe, and is totally responsible for the heaviest two.The s process contributes to the middle five isotopes, 128,129,130,132Xe.
Split into simpler sentences: The r process contributes to the heaviest four 131Xe, 132Xe, 134Xe and 136Xe, and is totally responsible for the heaviest two of those. By contrast the s process contributes to the middle five isotopes, 128,129,130,132Xe.