What is the relative formula mass of a compound?

To find the relative formula mass (M r) of a compound, you add together the relative atomic mass values (A r values) for all the atoms in its formula.

Explanation: Find the M r of carbon monoxide, CO. Find the M r of sodium oxide, Na 2O. The relative formula mass of a substance, shown in grams, is called one mole of that substance.

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.

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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.