Sketch the radial distribution functions for the 3s, 3p, 4s, and 3d subshells. Then, use the principle you observed in your comparison of radial distribution functions from question 3 to explain the origin of the Aufbau principle for filling atomic orbitals in the familiar sequence: 3sfills first, then 3pand 4sbefore 3d orbitals.

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.  

it's all four options.