Protons plus neutrons plus electrons equals mass number?

Answer 1

The atomic weight/mass is the average weight of all the isotopes of that element. Adding the number of protons and neutrons together does equal to the atomic weight, but only if you round the decimal.

For example the element sulfur (atomic number 16) has an Atomic Mass of 32.066. This is not a whole number, but that does not mean that sulfur has 16.066 neutrons. Instead, you would just say that the atomic mass is 32 amu and subtract the number of protons, which is the same as the atomic number, from the atomic mass. Sulfur therefore has 16 neutrons, protons, and electrons.

Answer 2

Although atoms have electrons as well as protons and (in most cases) neutrons, an electron weighs approximately 2000 times less than a proton, whereas protons and neutrons have approximately the same weight, so if you add up the protons and neutrons you will get a good measurement of the mass of an atom.

Answer 3

no just protons + neutrons but always number of electrons= number of protons( in an atom)

Answer 4

The mass number of an atom is the sum of the number of protons and neutrons in its nucleus. Electrons contribute negligible mass to the atom, so they are not included in the mass number calculation.