Mass number
| Nuclear physics |
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The mass number (symbol A, from the German word: Atomgewicht, "atomic weight"),
The mass number is written either after the element name or as a
C
, which has 6 protons and 6 neutrons. The full isotope symbol would also have the atomic number (Z) as a subscript to the left of the element symbol directly below the mass number: 12
6C
.[3]
Mass number changes in radioactive decay
Different types of radioactive decay are characterized by their changes in mass number as well as atomic number, according to the radioactive displacement law of Fajans and Soddy. For example,
2He2+
):[4]
238
92U
→ 234
90Th
+ 4
2He2+
On the other hand,
14
6C
→ 14
7N
+
e−
+
ν
e
Beta decay is possible because different
Another type of radioactive decay without change in mass number is emission of a
Mass number and isotopic mass
The mass number gives an estimate of the
There are two reasons for mass excess:
- The neutron is slightly heavier than the proton. This increases the mass of nuclei with more neutrons than protons relative to the atomic mass unit scale based on 12C with equal numbers of protons and neutrons.
- Nuclear binding energy varies between nuclei. A nucleus with greater binding energy has a lower total energy, and therefore a lower mass according to Einstein's mass–energy equivalence relation E = mc2. For 35Cl, the isotopic mass is less than 35, so this must be the dominant factor.
Relative atomic mass of an element
The mass number should also not be confused with the
This weighted average can be quite different from the near-integer values for individual isotopic masses. For instance, there are two main isotopes of chlorine: chlorine-35 and chlorine-37. In any given sample of chlorine that has not been subjected to mass separation there will be roughly 75% of chlorine atoms which are chlorine-35 and only 25% of chlorine atoms which are chlorine-37. This gives chlorine a relative atomic mass of 35.5 (actually 35.4527 g/mol).
Moreover, the weighted average mass can be near-integer, but at the same time not corresponding to the mass of any natural isotope. For example, bromine has only two stable isotopes, 79Br and 81Br, naturally present in approximately equal fractions, which leads to the standard atomic mass of bromine close to 80 (79.904 g/mol),[10] even though the isotope 80Br with such mass is unstable.
References
- Jensen, William B. (2005). The Origins of the Symbols A and Z for Atomic Weight and Number. J. Chem. Educ. 82: 1764. link.
- ^ "How many protons, electrons and neutrons are in an atom of krypton, carbon, oxygen, neon, silver, gold, etc...?". Thomas Jefferson National Accelerator Facility. Retrieved 2008-08-27.
- ^ "Elemental Notation and Isotopes". Science Help Online. Archived from the original on 2008-09-13. Retrieved 2008-08-27.
- ^ Suchocki, John. Conceptual Chemistry, 2007. Page 119.
- ISBN 1-56414-721-5.
- ^ Atoms with the same mass number.
- .
- .
- .
- ^ "Atomic Weights and Isotopic Compositions for All Elements". NIST.
Further reading
- Bishop, Mark. "The Structure of Matter and Chemical Elements (ch. 3)". An Introduction to Chemistry. Chiral Publishing. p. 93. ISBN 978-0-9778105-4-3. Retrieved 2008-07-08.