Aparticle with 10 protons, 11 neutrons, and 12 electrons has a mass number of:

For any given element the mass number can be found by adding the protons and the neutrons.

In this case its 10 protons plus 11 neutrons which gives us 21 as 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.