An unsaturated solution is one that an unsaturated solution is one that contains the maximum concentration of solute possible, and is in equilibrium with undissolved solute has no double bonds contains no solute has a concentration lower than the solubility contains more dissolved solute than the solubility allows

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.