Ionic bond A bond formed when electrons are shared--forms molecules
B. weak electrostatic bond formed by the attraction between a hydrogen and an oxygen, fluorine or nitrogen atom
C. pure substance with unique properties, formed when 2 or more elements combine
D. electrical attraction between opposite charged atoms or groups of atoms

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.