Question

calculating atomic mass
isotopes that are common in nature have greater importance in atomic mass calculations, just like tests may be assigned greater importance in calculating your final grade.
to calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.
example 1
the percent abundances of the isotopes of magnesium are shown in the table below.
isotopes\tpercent abundance
magnesium - 24\t79%
magnesium - 25\t10%
magnesium - 26\t11%
total percentage=
calculate the relative atomic mass of magnesium.
isotope\t% abundance\tmass (amu)\tmass contribution
magnesium - 24\t79%\tx\t=

magnesium - 25\t10%\tx\t=

magnesium - 26\t11%\tx\t=

total\t\t\ttotal:

Explanation:

Step1: Convert percentages to decimals

Magnesium - 24: $79\%=0.79$, Magnesium - 25: $10\% = 0.1$, Magnesium - 26: $11\%=0.11$

Step2: Multiply mass of each isotope by its abundance

The mass of Magnesium - 24 is approximately 24 amu, its mass contribution is $24\times0.79 = 18.96$ amu.
The mass of Magnesium - 25 is approximately 25 amu, its mass contribution is $25\times0.1=2.5$ amu.
The mass of Magnesium - 26 is approximately 26 amu, its mass contribution is $26\times0.11 = 2.86$ amu.

Step3: Calculate the total atomic mass

$18.96+2.5 + 2.86=24.32$ amu

Answer:

24.32 amu