The mass of an atom is determined largely by the number of protons and neutrons in its nucleus. Atoms of different elements have different masses.
Atoms of the same element may also have different masses if they exist as isotopes. Isotopes have the same number of protons, but the number of neutrons in their nucleus is different.
mass spec C
are extremely
tiny. The mass of a carbon atom is 2 × 10
–23 g. It is much more convenient to
use the masses of atoms relative to one another. The relative atomic masses of selected elements are
shown in the portion of the
periodic table.
There are
two numbers given
with the symbol for each element
The
relative atomic mass is the
larger of the two numbers.
Relative atomic masses for atoms which exist as more than one isotope are not integers.
These relative atomic masses can be determined by a technique known as mass spectrometry. The mass spectrum of a sample of carbon is shown above.
The
relative mass of the formula unit for any substance is the
sum of the
relative atomic masses of the component atoms, with each of these being
multiplied by the subscript in the formula. Note that
Mr( ) is the relative mass of the atom or formula unit shown in the brackets.
Example: For HNO3
Mr(HNO3) = Mr(H) + Mr(N) + 3 × Mr(O) = 1 + 14 + 3 × 16 = 63