How is the mass of the atom calculated? by adding the number of protons and electrons
by adding the number of protons and neutrons
by subtracting the number of electrons from the number of protons
by adding the number of neutrons and electrons

By adding the number of protons and neutrons

Explanation:

Protons and neutrons have very similar masses and are used in calculating the mass of an atom. One would calculate the mass of an atom by doing the (mass of a proton) x (# of protons) + (mass of a neutron) x (# of neutrons). Electrons have such a small mass that their mass is considered negligible  and therefore is not included in mass calculations for atoms.

mass of a proton: 1.67262 × 10−27 kg

mass of a neutron: 1.67493×10−27 kg

mass of an electron: 9.10938 × 10-31 kg

i think it's c. i'm not sure but i think so.

explanation:

let the different atoms be x and y. each with four protons and same mass will be represented as follows.

mass number is number of protons + number of neutrons.

            and  

since, it is given that one has a negative charge while the other is neutral. therefore, there will be increase in number of protons and the atom will be represented as follows.

according to periodic table, mass number of beryllium is 9 and it has atomic number 4 so, the atoms will be and .  

the protons and neutrons will be located inside the nucleus whereas the electrons will be revolving around the nucleus.