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# More about isotopes - PowerPoint PPT Presentation

More about isotopes. Atomic mass vs average atomic mass or atomic weight. Atomic Mass. The mass of a specific isotope of an element expressed in atomic mass units (amu) The atomic mass is the total mass of the protons, neutrons and electrons in the atom Atomic mass = mass of an atom

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## PowerPoint Slideshow about 'More about isotopes' - kaseem-hawkins

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### More about isotopes

Atomic mass vs average atomic mass or atomic weight

Atomic Mass
• The mass of a specific isotope of an element expressed in atomic mass units (amu)
• The atomic mass is the total mass of the protons, neutrons and electrons in the atom
• Atomic mass = mass of an atom
• This is what we refer to in our isotope expression, eg.
Average Atomic Mass (also called Atomic Weight)

Atomic Number

Symbol of element

Atomic Weight

1

H

1.008

This is the number that you see in the periodic table.

This is the weighted average of the all of the isotopes of Hydrogen

What is a Weighted Average?
• The isotopes of an element do not occur with equal frequency or amounts
• For example, the relative abundances for the three carbon isotopes are:
• carbon-12 98.9%
• carbon-13 1.1%,
• carbon-14 <001%.
• The "average" mass for the atoms of an element is dictated by the most abundant or common isotope
• The atomic weights in the periodic table = weighted averages
• Tabulated atomic weight value doesn't match any actual atom, but is closer to the weight for the most common isotope
• We’ll follow the steps of how to calculate a weighted average next….
Calculating Weighted Average

weighted average = ( decimal fraction A) mass A + ( decimal fraction B) mass B

• Carbon-12 atomic mass = 12 amu; abundance = 98.9%
• Carbon-13 atomic mass = 13 amu; abundance = 1.1%
• Carbon-14 atomic mass = 14 amu; abundance = 0.001%

Atomic weight = (0.989 * 12) + (0.011 * 13) + (0.00001 * 14) = 12.011

6

C

12.011

Average Atomic Mass Worksheet

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