The periodic table lists two main numbers for each element. the atomic number is an integer that equals the number of protons. the number of neutrons is not given in the periodic table because it will vary with different isotopes. the mass number of an element is the sum of the protons and neutrons. to find the number of neutrons, the atomic number must be subtracted from the mass number.

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.