C1 : 1.2-AVERAGE ATOMIC MASS

Most elements occur in nature as mixtures of isotopes. The average atomic mass, also called atomic mass, of an element is determined by using the masses of its various isotopes and their relative abundances.

ATOMIC MASS @ ATOMIC WEIGHT

- the mass of an atom is expressed in atomic mass units (amu or u)

- modern atomic mass standard, defined by the mass of one atom of carbon-12

- one atom of carbon-12 is assigned the mass of 12 u

- thus 1 u = 1/12 the mass of a C-12 atom

- all other atoms have a mass which is relative to this

  

ISOTOPIC ABUNDANCE

- the relative amount in which each isotope is present in an element

- expressed as a percent or as a decimal fraction

 -e.g magnesium is 79% Mg-24, 10% Mg-25 and 11% Mg-26

MASS SPECTROMETRY

- Technique used to determined relative atomic mass and the relative abundance of isotopes.

- Different atoms / isotopes / molecules can be identified by their characteristic pattern 

of lines (peak)

AVERAGE ATOMIC MASS                      

- it is a weighted average of the atomic masses of all of the naturally occurring isotopes of

an element according to their abundance. 

- this is the mass shown for an element on the periodic table

- a mass spectrometer is the instrument used to determine mass and abundance of the   

isotopes.

Example 1:

Example 2 :