Theoretical element “A” has four isotopes. Use the data in the table to calculate the average atomic mass of the unknown element A. All steps of the calculations must be shown for credit.

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.  

colorless gas, exhibiting a blue glow when placed in an electric field

b) solid

its a type of rock