Nthe periodic table, the atomic mass of oxygen is listed as 15.9994 amu. based on this, one can deduce that the most common of oxygen contains a total number of protons plus neutrons equalling

- Isotope for the first blank and sixteen for second blank.

Explanations:- The sum of protons and neutrons makes the mass number of the element. Since atomic mass is the average of isotopes of the element, it is not a whole number but the mass number is a whole number as it is the sum of number of protons and neutrons.

15.9994 is almost 16 means the mass number for the most common isotope of oxygen is 16.

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.