In chemistry, the most commonly used unit of concentration is

Molarity

Explanation:

Molarity

Molarity is the most common concentration unit involved in calculations dealing with volumetric stoichiometry. Molality (m): This unit of concentration relates the moles of solute per kilogram of solvent.

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.

            and  

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.