It an element has 13 protons, 13 electrons and 15 neutrons what is its mass number?

28

Explanation:

The mass of an element, also called its atomic mass, can be calculated by adding the number of protons and neutrons.

P + N = mass

13 + 15 = 28

Therefore, the atomic mass is 28.

The number of electrons is generally not considered because they are so light, each of them weighs almost nothing.

the potential energy of the reactants is 200j.

from the energy diagram, the energy of the product formed is 350j; this means that, this reaction is an endothermic reaction, because it absorbs energy from its environment.