Any tips for a isotopes? I have a benchmark tomorrow.

Isotopes is the average of the most common masses of a certain atom found in nature.

You have different masses from having different amounts of neutrons in the atom. (The electrons and neutrons can leave/come but the protons always stay the same)

The imbalance of neutrons and protons is what makes the atoms radioactive.

The formula for isotopes / Atomic Mass is: (M₁) (P₁) + (M₂) (P₂) + ... = atomic mass

P=percent found in nature

M=atomic mass