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Atomic structure - (CCEA)Calculating relative atomic mass for elements

Scientists鈥 ideas about atoms have changed over time. Today, they agree that atoms have a positively-charged nucleus made of protons and neutrons, and negatively-charged electrons that orbit the nucleus in shells.

Part of Chemistry (Single Science)Structures, trends, chemical reactions, quantitative chemistry and analysis

Calculating relative atomic mass for elements

Most elements contain a mixture of , each present in a different amount.

The term 鈥榳eighted mean mass鈥 takes into account the different contribution of each isotope to the overall mass.

The contributions of all isotopes are combined to give the 鈥 the weighted mean of the mass numbers.

\(relative~atomic~mass~(A_r)\)

\(= \frac {(mass~number~1 \times abundance)+(mass~number~2 \times abundance)}{Total~abundance}\)

are whole numbers because they are the number of protons plus the number of neutrons.

Relative atomic mass is an average mass, which takes into account all of the isotopes and their abundance. It is not always a whole number. However, on your data leaflet, apart from chlorine, the relative atomic masses have been rounded to whole numbers.

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