explain what is expected by the atom mass of one element. Calculate the atomic mass of an facet from the masses and also relative percentages of the isotopes of the element.

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In ptcouncil.netistry we very rarely address only one isotope of one element. We usage a mixture the the isotopes of an facet in ptcouncil.netical reactions and also other facets of ptcouncil.netistry, because every one of the isotope of an facet react in the exact same manner. That means that us rarely need to worry around the mass of a specific isotope, yet instead we require to understand the typical mass of the atom of an element. Utilizing the masses of the different isotopes and also how plentiful each isotope is, us can discover the median mass the the atom of one element. The atomic mass of an facet is the weighted typical mass that the atom in a naturally emerging sample the the element. Atom mass is generally reported in atom mass units.


Calculating atom Mass

You deserve to calculate the atom mass (or typical mass) of an element listed you know the relative abundance(the fraction of an aspect that is a given isotope), the element"s naturally arising isotopes, and also the masses of those different isotopes. We deserve to calculate this by the adhering to equation:

< extAtomic mass = left( \%_1 ight) left( extmass_1 ight) + left( \%_2 ight) left( extmass_2 ight) + cdots>

Look very closely to see how this equation is used in the complying with examples.


Example (PageIndex1): Boron Isotopes

Boron has actually two naturally emerging isotopes. In a sample of boron, (20\%) the the atoms space (ceB)-10, i m sorry is an isotope that boron through 5 neutrons and also mass of (10 : extamu). The other (80\%) that the atoms are (ceB)-11, which is one isotope of boron with 6 neutrons and a mass of (11 : extamu). What is the atom mass the boron?

Solution

Boron has two isotopes. Us will use the equation:

< extAtomic mass = left( \%_1 ight) left( extmass_1 ight) + left( \%_2 ight) left( extmass_2 ight) + cdots onumber>

Isotope 1: (\%_1 = 0.20) (Write all percentages as decimals), ( extmass_1 = 10) Isotope 2: (\%_2 = 0.80), ( extmass_2 = 11)

Substitute these into the equation, and we get:

< extAtomic mass = left( 0.20 ight) left( 10 ight) + left( 0.80 ight) left( 11 ight) onumber>

< extAtomic mass = 10.8 : extamu onumber>

The mass of an typical boron atom, and also thus boron"s atomic mass, is (10.8 : extamu).


Example (PageIndex2): Neon Isotopes

Neon has actually three naturally occurring isotopes. In a sample of neon, (90.92\%) that the atoms space (ceNe)-20, which is one isotope of neon with 10 neutrons and also a mass of (19.99 : extamu). An additional (0.3\%) that the atoms are (ceNe)-21, i m sorry is an isotope that neon v 11 neutrons and a massive of (20.99 : extamu). The final (8.85\%) the the atoms are (ceNe)-22, i beg your pardon is an isotope that neon v 12 neutrons and a mass of (21.99 : extamu). What is the atomic mass of neon?

Solution

Neon has actually three isotopes. Us will use the equation:

< extAtomic mass = left( \%_1 ight) left( extmass_1 ight) + left( \%_2 ight) left( extmass_2 ight) + cdots onumber>

Isotope 1: (\%_1 = 0.9092) (write all percentages as decimals), ( extmass_1 = 19.99) Isotope 2: (\%_2 = 0.003), ( extmass_2 = 20.99) Isotope 3: (\%_3 = 0.0885), ( extmass_3 = 21.99)

Substitute these into the equation, and also we get:

< extAtomic mass = left( 0.9092 ight) left( 19.99 ight) + left( 0.003 ight) left( 20.99 ight) + left( 0.0885 ight) left( 21.99 ight) onumber>

< extAtomic mass = 20.17 : extamu onumber>

The mass of an median neon atom is (20.17 : extamu)


The regular table provides the atom mass of each element. The atomic mass is a number the usually appears below the element"s price in every square. Notice that the atomic mass the boron (symbol (ceB)) is 10.8, which is what we calculated in instance (PageIndex1), and also the atomic mass the neon (symbol (ceNe)) is 20.8, i m sorry is what us calculated in example (PageIndex2). Take time to notification that no all periodic tables have actually the atom number over the element"s symbol and also the mass number listed below it. If girlfriend are ever confused, remember the the atomic number should constantly be the smaller of the two and also will it is in a whole number, while the atom mass should constantly be the bigger of the two and will be a decimal number.


Exercise (PageIndex1)

Chlorine has actually two naturally emerging isotopes. In a sample the chlorine, (75.77\%) that the atoms space (ceCl)-35, with a fixed of (34.97 : extamu). An additional (24.23\%) the the atoms space (ceCl)-37, v a fixed of (36.97 : extamu). What is the atom mass that chlorine?

Answer 35.45 amu

Summary

one element"s atomic mass is the weighted average of the masses that the isotopes of an element An element"s atom mass can be calculated noted the family member abundanceof the element"s naturally arising isotopes and the masses that those isotopes room known. The regular table is a convenient way to summarize information around the various elements. In enhancement to the element"s symbol, many periodic tables will likewise contain the element"s atomic number and the element"s atomic mass.

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Henry Agnew(UC Davis)


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