The total number of protons and neutrons in the nucleus of an atom

Is the mass number

Explanation:

Every atom is usually identified by using two different numbers:

- Atomic number: generally labelled with Z, corresponds to the number of protons in the nucleus, so Z = p

- Mass number: generally labelled with A, corresponds to the number of protons+neutrons in the nucleus, so A = p + n

To find the number of neutrons in a nucleus, therefore, we can do

n = A - Z

what experimental evidence led scientists to change from the previous model to this one?

a few of the positive particles aimed at a gold foil seemed to bounce back.

the colors of light emitted from heated atoms had very specific energies.

experiments with water vapor showed that elements combine in specific proportions.

cathode rays were bent in the same way whenever a magnet was brought near them.