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This diagram shows the symbol for helium, “H e.” The number to the upper left of the symbol is the mass number, which is 4. The number to the upper right of the symbol is the charge which is positive 2. The number to the lower left of the symbol is the atomic number, which is 2. This number is often omitted. Also shown is “M g” which stands for magnesium It has a mass number of 24, a charge of positive 2, and an atomic number of 12.
The symbol for an atom indicates the element via its usual two-letter symbol, the mass number as a left superscript, the atomic number as a left subscript (sometimes omitted), and the charge as a right superscript.

Information about the naturally occurring isotopes of elements with atomic numbers 1 through 10 is given in [link] . Note that in addition to standard names and symbols, the isotopes of hydrogen are often referred to using common names and accompanying symbols. Hydrogen-2, symbolized 2 H, is also called deuterium and sometimes symbolized D. Hydrogen-3, symbolized 3 H, is also called tritium and sometimes symbolized T.

Nuclear Compositions of Atoms of the Very Light Elements
Element Symbol Atomic Number Number of Protons Number of Neutrons Mass (amu) % Natural Abundance
hydrogen 1 1 H
1 1 0 1.0078 99.989
1 2 H
1 1 1 2.0141 0.0115
1 3 H
1 1 2 3.01605 — (trace)
helium 2 3 He 2 2 1 3.01603 0.00013
2 4 He 2 2 2 4.0026 100
lithium 3 6 Li 3 3 3 6.0151 7.59
3 7 Li 3 3 4 7.0160 92.41
beryllium 4 9 Be 4 4 5 9.0122 100
boron 5 10 B 5 5 5 10.0129 19.9
5 11 B 5 5 6 11.0093 80.1
carbon 6 12 C 6 6 6 12.0000 98.89
6 13 C 6 6 7 13.0034 1.11
6 14 C 6 6 8 14.0032 — (trace)
nitrogen 7 14 N 7 7 7 14.0031 99.63
7 15 N 7 7 8 15.0001 0.37
oxygen 8 16 O 8 8 8 15.9949 99.757
8 17 O 8 8 9 16.9991 0.038
8 18 O 8 8 10 17.9992 0.205
fluorine 9 19 F 9 9 10 18.9984 100
neon 10 20 Ne 10 10 10 19.9924 90.48
10 21 Ne 10 10 11 20.9938 0.27
10 22 Ne 10 10 12 21.9914 9.25

Atomic mass

Because each proton and each neutron contribute approximately one amu to the mass of an atom, and each electron contributes far less, the atomic mass    of a single atom is approximately equal to its mass number (a whole number). However, the average masses of atoms of most elements are not whole numbers because most elements exist naturally as mixtures of two or more isotopes.

The mass of an element shown in a periodic table or listed in a table of atomic masses is a weighted, average mass of all the isotopes present in a naturally occurring sample of that element. This is equal to the sum of each individual isotope’s mass multiplied by its fractional abundance.

average mass = i ( fractional abundance × isotopic mass ) i

For example, the element boron is composed of two isotopes: About 19.9% of all boron atoms are 10 B with a mass of 10.0129 amu, and the remaining 80.1% are 11 B with a mass of 11.0093 amu. The average atomic mass for boron is calculated to be:

boron average mass = ( 0.199 × 10.0129 amu ) + ( 0.801 × 11.0093 amu ) = 1.99 amu + 8.82 amu = 10.81 amu

It is important to understand that no single boron atom weighs exactly 10.8 amu; 10.8 amu is the average mass of all boron atoms, and individual boron atoms weigh either approximately 10 amu or 11 amu.

Calculation of average atomic mass

A meteorite found in central Indiana contains traces of the noble gas neon picked up from the solar wind during the meteorite’s trip through the solar system. Analysis of a sample of the gas showed that it consisted of 91.84% 20 Ne (mass 19.9924 amu), 0.47% 21 Ne (mass 20.9940 amu), and 7.69% 22 Ne (mass 21.9914 amu). What is the average mass of the neon in the solar wind?


average mass = ( 0.9184 × 19.9924 amu ) + ( 0.0047 × 20.9940 amu ) + ( 0.0769 × 21.9914 amu ) = ( 18.36 + 0.099 + 1.69 ) amu = 20.15 amu

The average mass of a neon atom in the solar wind is 20.15 amu. (The average mass of a terrestrial neon atom is 20.1796 amu. This result demonstrates that we may find slight differences in the natural abundance of isotopes, depending on their origin.)

Check your learning

A sample of magnesium is found to contain 78.70% of 24 Mg atoms (mass 23.98 amu), 10.13% of 25 Mg atoms (mass 24.99 amu), and 11.17% of 26 Mg atoms (mass 25.98 amu). Calculate the average mass of a Mg atom.


24.31 amu

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Source:  OpenStax, Ut austin - principles of chemistry. OpenStax CNX. Mar 31, 2016 Download for free at http://legacy.cnx.org/content/col11830/1.13
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