Irregularities Of Mass In The Periodic Table. Elements do not have their atomic masses as whole numbers. The atomic mass of an element is the weighted average of all the different isotopes of that element This is also known as Relative Mass. Calculating Relative Mass.
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To calculate relative mass do the following:
STEP # 1:
You need to know the relative abundance of each isotope (expressed as a percent %)
and the atomic mass of each isotope.
Use the formula below to solve for relative mass.
STEP # 2:
Relative Mass =
(amu * % abundance) + (amu * % abundance)
Using the information given below about Lithium, find its relative mass.
Mass = (amu * % abundance) + (amu * % abundance)
Using the information given below about oxygen, find its relative mass.
Find % abundance of second isotope…. 100% - 98.89 = ?
EXAMPLE # 3
The atomic mass (relative mass) of carbon is 12.01113719 amu. The chart below shows data on the two isotopes of carbon. What is the atomic mass of the second isotope of carbon?
Use the formula to solve for the atomic mass (working backwards)
Step # 2
Atomic mass = 13.00335
EXAMPLE # 4
Silicon (Si) has 3 naturally occurring isotopes. One isotope has 14neutrons and a % abundance of 92.2, another isotope has 15 neutrons and a % abundance of 4.70, and finally the last isotope has 16 neutrons and a % abundance of 3.09.
What is the approximate average atomic mass of Silicon?
Step # 1
Get the atomic mass for each isotope….
NOTE: The question only gave the neutrons….so you add them to the protons!
Plug information into formula and solve
Relative Mass =27.7352u
Calculating relative mass:
Relative Mass =
amu x % abundance + amu x % abundance