So polyatomic ions are several atoms covalently bonded to one another but having a positive or negative charge because the total number of valence electrons from the atoms cannot produce a stable structure. with one or more electrons added or removed, a stable structure results.
example: nitrate
knowing this, why do compounds need to be neutral while polyatomic ions not. why don't compounds follow the same strategy as polyatomic ions such that they add or remove electrons to make themselves stable?

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.  

oxygen, selenium, tellurium, and polonium.

they're all from the same group as sulfur (group 16).

5.6⋅10−^19  j

do you need the explaination?