2.3 Atomic structure and symbolism (Page 4/18)

Chemistry Page 4 / 18

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	$_{1}^{1} H$ (protium)	1	1	0	1.0078	99.989
$_{1}^{2} H$ (deuterium)	1	1	1	2.0141	0.0115
$_{1}^{3} H$ (tritium)	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

Use this Build an Atom simulator to build atoms of the first 10 elements, see which isotopes exist, check nuclear stability, and gain experience with isotope symbols.

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 = \sum_{i} {(fractional abundance \times 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:

\begin{matrix} boron average mass & = (0.199 \times 10.0129 amu) + (0.801 \times 11.0093 amu) \\ = 1.99 amu + 8.82 amu \\ = 10.81 amu \end{matrix}

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?

Solution

\begin{matrix} average mass & = (0.9184 \times 19.9924 amu) + (0.0047 \times 20.9940 amu) + (0.0769 \times 21.9914 amu) \\ = (18.36 + 0.099 + 1.69) amu \\ = 20.15 amu \end{matrix}

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.

Answer:

24.31 amu

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Source: OpenStax, Chemistry. OpenStax CNX. May 20, 2015 Download for free at http://legacy.cnx.org/content/col11760/1.9

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