The atomic number is the atomic mass number of neutrons protons plus the neutrons number of protons

The atomic number of an atom is determined by the number of protons present in the atom that is the atomic number and number of protons are same for an atom. It is unique for every atom.

Atomic mass: It is nearly equals to the sum of mass of subatomic particles present in the atom that is sum of proton and neutron.

Number of neutrons: in an atom, the number of neutrons are always equal to the number of protons. If the number of neutrons are different for a number of proton then it is said be an isotope of that atom.

Protons plus neutrons: Sum of number of protons and number of neutrons is said to be the mass number of that atom.

Hence, the atomic number is the number of protons.

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.

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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.