Symbol is $A_r$ $^1$
Because atoms are very light, in order to quantify the weight scientists compared the weight to Carbon-12. For example, Hydrogen is 1 twelfth the weight of Carbon-12 and Magnesium is twice the weight of Carbon-12. If Hydrogen has a $A_r$ of 1 then Magnesium has a $A_r$ of 24.
The following formula is essentially the weighted average of all of the masses isotopes of an element.$^3$
$$ A_r ={\%_1\text{mass}_1 + ... + \%_n\text{mass}_n} $$
$%_1$$\%_n$ is the percent abundance of the isotope(in decimals, 20% would be 0.2).
$\text{mass}_n$ is the mass of the isotope.$^3$
Symbol is $M_r$ $^1$
$M_r$ is the sum of $A_r$ in a molecule. $^1$
$\% = \frac{\text{mass of component}}{\text{total mass of susbtance}}\times100$
$\%$ is the percentage composition of a component in a substance$^1$
Substance: $\text{CuSO}_4$
Component: $\text{O}$
$\%\text{ of O}= \frac{\text{total mass of Oxygen}}{\text{total mass of CuSO}_4}\times100\\ \%\text{ of O}=\frac{16\times4}{64+32+16\times4}\times100\\ \%\text{ of O}=\frac{64}{160}\times100\\ \%\text{ of O}= 40\%$
Avogadro proposed it.$^1$
$1\text{ mole}=6.02\times10^{23}$