Mass Number
Experimental data showed that the vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons. The mass number (represented by the letter A) is defined as the total number of protons and neutrons in an atom. Consider the table below, which shows data from the first six elements of the periodic table.
Table (PageIndex{1}): Atoms of the First Six Elements Name Symbol Atomic Number (Z) Protons Neutrons Electrons Mass Number (A) (rounded to two decimals) hydrogen (ce{H}) 1 1 0 1 1.01 helium (ce{He}) 2 2 2 2 4.00 lithium (ce{Li}) 3 3 4 3 6.94 beryllium (ce{Be}) 4 4 5 4 9.01 boron (ce{B}) 5 5 6 5 10.18 carbon (ce{C}) 6 6 6 6 12.01
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Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus. Its nucleus also contains two neutrons. Since (2 + 2 = 4), we know that the mass number of the helium atom is 4. Finally, the helium atom also contains two electrons, since the number of electrons must equal the number of protons. This example may lead you to believe that atoms have the same number of protons and neutrons, but a further examination of the table above will show that this is not the case. Lithium, for example, has three protons and four neutrons, giving it a mass number of 7.
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Knowing the mass number and the atomic number of an atom allows you to determine the number of neutrons present in that atom by subtraction.
[text{Number of neutrons} = text{ rounded mass number} – text{atomic number} nonumber ]
Atoms of the element chromium (left( ce{Cr} right)) have an atomic number of 24 and a mass number of 52. How many neutrons are in the nucleus of a chromium atom? To determine this, you would subtract as shown:
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[52 – 24 = 28 : text{neutrons in a chromium atom} nonumber ]
The composition of any atom can be illustrated with a shorthand notation called A/Z format. Both the atomic number and mass are written to the left of the chemical symbol. The “A” value is written as a superscript while the “Z” value is written as a subscript. For an example of this notation, look to the chromium atom shown below:
[ce{^{52}_{24}Cr} nonumber ]
Another way to refer to a specific atom is to write the mass number of the atom after the name, separated by a hyphen. Symbol-mass format for the above atom would be written as Cr-52. In this notation, the atomic number is not included. You will need to refer to a periodic table for proton values.
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