A sample of lead is found to contain three isotopes: 23.5% of the sample has a mass of 204 amu, 24.1% has a mass of 207 amu, and the remainder has a mass of 208 amu. What
is the average atomic mass of lead?
0 206.3 amu
207.2 amu
0 82.0 amu
206.8 amu

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.