1. We can look up isotopic abundance for Mg and find:
QUESTION:
The mass of one atom of Mg-26 is closest to A u, B u, C u, D u
The mass given in a typical periodic table (24.3) is the average for a naturally-occurring sample.
We can look up isotopic abundance for Mg and find:
Mg-24. Mass=23.99 u, Abudance=78.99%
Mg-25. Mass=24.99 u, Abundance=10.00%
Mg-26. Mass=25.98 u, Abundance=11.01%
Average Mass
= (0.7899)(23.99 u) + (0.1000)(24.99 u) (0.1101)(25.98 u)
= 24.3 u, or 24.3 Dalton
QUESTION:
The mass of one atom of Mg-26 is closest to A u, B u, C u, D u
PAUSE CLICK
Referring to a typical periodic table, we find that the atomic number of magnesium is 12, and its
atomic mass is 24.3 atomic mass units.
Therefore, choice D is wrong.. 12, the atomic number, is the number of protons.
Choice A is also wrong.
The mass given in a typical periodic table (24.3) is the average for a naturally-occurring sample.
The actual mass of atom in atomic mass unit is very, very close to its mass number.
So the correct answer is choice C. The mass of an atom of magnesium 26 is very close to 26.
CLICK
We can look up isotopic abundance for Mg and find the following:
magnesium 24 has a mass of atomic mass units, and has a relative abundance of 78.99%. This means that about 8 out of 10 magnesium atoms
you’ll find in nature have a mass of about 24 amu.
We will also find that magnesium 25 has a mass of atomic mass units. It has a relative abundance of 10 percent.
One out of every 10 magnesium atoms in nature has a mass of about 25 amu.
Magnesium 26, has a mass of atomic mass units and a relative abundance of about 11 percent.
CLICK From this information, we can find the average mass by multiplying each mass by the relative abundance and taking the sum.
The Average Mass is equal to ...
the mass of magnesium 24
HIGHLIGHT 23.99u
multiplied by its relative abundance
HIGHLIGHT
plus the mass of magnesium 25
HIGHLIGHT u
HIGHLIGHT
plus the mass of magnesium 26
HIGHLIGHT u
multiplied by its relative abundance.
If we did this, we come up with the number that is typically shown on a periodic table...
24.3 atomic mass units, or 24.3 daltons.
CLICK PAUSE END RECORDING

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08