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 

Francisco pays for the clothes in the store with pesos...He is in Mexico