Use the periodic table to find the molar mass of
each of the following elements.
Sodium (Na):
1
g/mol
Oxygen (0)
g/mol
Carbon (C):
g/mol

I can give you a clue of getting mass. if it is the atomic number is even multiply by 2 but if the atomic number is odd multiply by 2 and add 1

sodium atomic number = 11 so mass = 11*2 +1 = 23

oxygen. atomic number = 8 so mass = 8*2 = 16

carbon atomic number = 6 so mass = 6*2 = 12

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.