The atomic number of an element best corresponds to which of the following?
O The mass of an atom of that element.
O The number of electrons orbiting the nucleus of an atom of that element.
O The number of protons in the nucleus of an atom of that element.
O The number of neutrons in the nucleus of an atom of that element.

The number of protons in the nucleus of an atom of that element.

Explanation: the atomic number of an element is simply the proton number of that element. This is the most reason while the ion of a particular element still retain its atomic number whether it gains or loss electrons. For example, sodium atom (Na) has 11 electrons, 11 protons and 12 neutrons. Sodium ion (Na+) on the other hand has 10 electrons because it loss one electron, 11 proton and 12 neutrons. If the atomic number was to be determined by the number of electrons, then the atomic number of sodium atom and sodium ion would have been different. So the best way to determine the atomic number is by the number of protons. Neutron number will only bring about isotope and not atomic 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.