You weigh two different objects on the same scale in the same location on earth. the weights are different. what must be different about the two objects?

The weight of an object is the measure of of the gravitation force that acts on an object. When you weigh the objects on the scale, you are measuring the amount of force they feel due to the gravity of the earth at that point. Near the surface of the earth, the weight of an object is calculated by the formula,   where is the mass, is the acceleration due to gravity. Since is constant at the location where the object's weights are measured, the only way their weights can be different is if they have different masses.

the atomic mass is the bottom number on the element.

-steel jelly