Question

what is the mass in amu of 7 electrons, 7 neutrons and 7 protons? report this answer 2 places past the decimal point.

Answer 1

Since this a neutral atom with 7 protons, you can find the mass in amu from the Periodic Table. 7 protons means the atomic number of this atom Is 7. You'll find the mass in that element's square.

Answer 2

You should look up (or work out for yourself) the mass of an electron, a proton and a neutron, in atomic mass units.
Then add up these three masses and multiply the result by 7.

Now, if you combine 7 electrons, 7 protons and 7 neutrons to make a nitrogen atom, you will find , if you look it up, that the mass of a nitrogen atom is slightly different to the answer you worked out above.

That difference in mass between the building blocks of an atom, and the atom itself, is the source of the tremendous amounts of energy that can be released in nuclear reactions.