Relative Atomic Mass

The Relative Atomic Mass of an element basically means its atomic mass (atomic number), however some elements have isotopes. This means that not all atoms of that element have the same number of Neutrons.

    • The average mass of a large number of atoms (of an ELEMENT) is its relative atomic mass
  • This considers the PERCENTAGE ABUNDANCE of all the isotopes of an element which exist
    • A recommended standard for the Ar scale was carbon-12
    • The mass of an atom of carbon-12 was 12amu
    • The Ar of Carbon is now the average mass of its isotope compared with 1/12 the mass of an atom carbon-12

To figure out the Relative Atomic Mass (RAM) of two isotopes

eg.

20 22

10Ne 90% 10 Ne 10%

1st Isotope (percentage * Atomic mass) + 2nd Isotope (percentage * Atomic mass)

1/12 of 1 atom carbon-12

in this case..

(0.9 * 20) + (0.1 * 22)

1/12 of 1 atom carbon-12

= 18.00+2.20

1

= 20.2amu

    • If you were paying attention you would understand that 1/12 of 1 atom carbon-12 is equal to 1
    • Because of this, the division step can be omitted
  • ** remember to use the percentages as decimals, or divide the answer by 100 at the end**

Atomic Structure and Periodic Table - Unit page

links to other subtopics in this section can be found here.

Chemistry-IGCSE