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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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irregularities of mass in the periodic table
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
Calculating Relative Mass

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)

100

slide3

EXAMPLE # 1

Using the information given below about Lithium, find its relative mass.

  • amu% Abundance
  • Li-6 6u 7.42%
  • Li-7 7u 92.58%

Lithium

Mass = (amu * % abundance) + (amu * % abundance)

100

  • Relative Mass = (6u * 7.42) + (7 * 92.58)
  • 100
  • Mass = 6.93u
slide4

EXAMPLE # 2

Using the information given below about oxygen, find its relative mass.

  • amu% Abundance
  • O-16 16u 99.762%
  • O-17 17u 0.038%
  • O-18 18u 0.200%

Oxygen

  • = (16u * 99.762) + (17 * 0.038) + (18 * 0.200)
  • 100
  • Relative Mass = 16.008
slide5

Step # 1

Find % abundance of second isotope…. 100% - 98.89 = ?

= 1.11%

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

  • Relative Mass = (amu * %abun.) + (amu * %abun.)
  • 100
  • 12.01113719 = (12u * 98.89) + (?* 1.11%)
  • 100

Atomic mass = 13.00335

13.00335

1.11%

slide6

TRICKY!

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….

  • % Abundance
  • Si-28 99.762%
  • Si-29 0.038%
  • Si-30 0.200%

NOTE: The question only gave the neutrons….so you add them to the protons!

slide7

Step # 2

Plug information into formula and solve

  • % Abundance
  • Si-28 92.2%
  • Si-29 4.70%
  • Si-30 3.09%
  • = (28u * 92.2) + (29u * 4.70) + (18u * 3.09)
  • 100

Relative Mass =27.7352u

slide8

Key Points to Remember

Calculating relative mass:

Relative Mass =

amu x % abundance + amu x % abundance

100

Atomic mass

activities and resources
Activities and Resources
  • Worksheet # 2
  • Student Study Guide, Module I p. I-50
  • Other Activities:
    • ANSTO (Australian Nuclear Science and Technology Organization)
    • http://www.ansto.gov.au/ari/brochures_misc/rad1.html
references
References
  • Student Study Guide – Physical Science 416/436 - MEQ
  • Science Quest – Grenier, Daigle, Rheaume – 1998, Cheneliere
  • Google Images
  • Animation Factory