Who was the first person to come up with the idea of the "atom?"

Democritus

Explanation:

Democritus first introduced the idea of the atom almost 2500 years ago. Democritus was an important philosopher however if your talking about how the atom is today look at first rudimentary   pudding model by JJ Thomson first design then its Neils  Bohr whos model we look at today

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.