Explain what is supposed by the atom mass of one element. Calculate the atomic mass of an facet from the masses and also relative percentages of the isotope of the element.

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In gaianation.netistry we very rarely attend to only one isotope of an element. We use a mixture that the isotopes of an aspect in gaianation.netical reactions and other aspects of gaianation.netistry, because all of the isotopes of an aspect react in the same manner. That way that us rarely need to worry about the mass of a certain isotope, but instead we require to understand the typical mass the the atoms of one element. Utilizing the masses of the various isotopes and also how abundant each isotope is, we can discover the typical mass the the atoms of an element. The atomic mass that an aspect is the weighted typical mass that the atoms in a naturally arising sample of the element. Atomic mass is frequently reported in atom mass units.

Calculating atomic Mass

You can calculate the atom mass (or median mass) of an element noted you understand the relative abundance(the fraction of an element that is a offered isotope), the element"s naturally occurring isotopes, and the masses the those various isotopes. We have the right to calculate this by the following equation:

\<\textAtomic mass = \left( \%_1 \right) \left( \textmass_1 \right) + \left( \%_2 \right) \left( \textmass_2 \right) + \cdots\>

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

Example \(\PageIndex1\): Boron Isotopes

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


Boron has two isotopes. Us will use the equation:

\<\textAtomic mass = \left( \%_1 \right) \left( \textmass_1 \right) + \left( \%_2 \right) \left( \textmass_2 \right) + \cdots \nonumber\>

Isotope 1: \(\%_1 = 0.20\) (Write all percentages as decimals), \(\textmass_1 = 10\) Isotope 2: \(\%_2 = 0.80\), \(\textmass_2 = 11\)

Substitute these right into the equation, and also we get:

\<\textAtomic mass = \left( 0.20 \right) \left( 10 \right) + \left( 0.80 \right) \left( 11 \right)\nonumber\>

\<\textAtomic mass = 10.8 \: \textamu\nonumber\>

The mass of an typical boron atom, and thus boron"s atom mass, is \(10.8 \: \textamu\).

Example \(\PageIndex2\): Neon Isotopes

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


Neon has three isotopes. Us will use the equation:

\<\textAtomic mass = \left( \%_1 \right) \left( \textmass_1 \right) + \left( \%_2 \right) \left( \textmass_2 \right) + \cdots\nonumber\>

Isotope 1: \(\%_1 = 0.9092\) (write all percentages together decimals), \(\textmass_1 = 19.99\) Isotope 2: \(\%_2 = 0.003\), \(\textmass_2 = 20.99\) Isotope 3: \(\%_3 = 0.0885\), \(\textmass_3 = 21.99\)

Substitute these into the equation, and also we get:

\<\textAtomic mass = \left( 0.9092 \right) \left( 19.99 \right) + \left( 0.003 \right) \left( 20.99 \right) + \left( 0.0885 \right) \left( 21.99 \right)\nonumber\>

\<\textAtomic mass = 20.17 \: \textamu\nonumber\>

The fixed of an median neon atom is \(20.17 \: \textamu\)

The periodic table provides the atom mass of each element. The atom mass is a number the usually appears below the element"s prize in every square. An alert that the atomic mass of boron (symbol \(\ceB\)) is 10.8, i beg your pardon is what we calculated in instance \(\PageIndex1\), and the atom mass the neon (symbol \(\ceNe\)) is 20.8, i m sorry is what we calculated in instance \(\PageIndex2\). Take it time to an alert that not all routine tables have the atomic number above the element"s symbol and the fixed number listed below it. If girlfriend are ever confused, remember that the atomic number should always be the smaller sized of the two and will be a whole number, while the atomic mass should always be the larger of the two and also will be a decimal number.

Exercise \(\PageIndex1\)

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

Answer 35.45 amu


an element"s atomic mass is the weighted typical of the masses the the isotopes of an facet An element"s atomic mass can be calculated provided the loved one abundanceof the element"s naturally occurring isotopes and also the masses that those isotopes room known. The regular table is a convenient way to summary information about the different elements. In enhancement to the element"s symbol, many periodic tables will also contain the element"s atom number and the element"s atom mass.

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