# EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记：1.3.2 Calculate Relative Atomic Mass

### Calculate Relative Atomic Mass

• The relative atomic mass of each element is calculated from the mass number and relative abundances of all the isotopes of a particular element
• The steps below are to calculate the relative atomic mass
• Start by finding out the mass of 100 atoms, then divide the result by 100 to get the Ar

• The top line of the equation can be extended to include the number of different isotopes of a particular element present, so, if there were 3 isotopes present then the top line of the equation would read:

<!--MathML:

(mass of isotope 1 x % of isotope 1) + (mass of isotope 2 x % of isotope 2) + (mass of isotope 3 x % of isotope 3)" alt="begin mathsize 14px style left parenthesis mass space of space isotope space 1 space straight x space percent sign space of space isotope space 1 right parenthesis space plus space left parenthesis mass space of space isotope space 2 space straight x space percent sign space of space isotope space 2 right parenthesis space plus space left parenthesis mass space of space isotope space 3 space straight x space percent sign space of space isotope space 3 right parenthesis comma space etc end style" data-mathml="<math style="font-family:Arial" class="wrs_chemistry" xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mstyle mathsize="14px"><mo>(</mo><mi>mass</mi><mo> </mo><mi>of</mi><mo> </mo><mi>isotope</mi><mo> </mo><mn>1</mn><mo> </mo><mi mathvariant="normal">x</mi><mo> </mo><mo>%</mo><mo> </mo><mi>of</mi><mo> </mo><mi>isotope</mi><mo> </mo><mn>1</mn><mo>)</mo><mo> </mo><mo>+</mo><mo> </mo><mo>(</mo><mi>mass</mi><mo> </mo><mi>of</mi><mo> </mo><mi>isotope</mi><mo> </mo><mn>2</mn><mo> </mo><mi mathvariant="normal">x</mi><mo> </mo><mo>%</mo><mo> </mo><mi>of</mi><mo> </mo><mi>isotope</mi><mo> </mo><mn>2</mn><mo>)</mo><mo> </mo><mo>+</mo><mo> </mo><mo>(</mo><mi>mass</mi><mo> </mo><mi>of</mi><mo> </mo><mi>isotope</mi><mo> </mo><mn>3</mn><mo> </mo><mi mathvariant="normal">x</mi><mo> </mo><mo>%</mo><mo> </mo><mi>of</mi><mo> </mo><mi>isotope</mi><mo> </mo><mn>3</mn><mo>)</mo><mo>,</mo><mo> </mo><mi>etc</mi></mstyle><annotation encoding="application/vnd.wiris.mtweb-params+json">{"language":"en","fontFamily":"Times New Roman","fontSize":"18"}</annotation></semantics>[/itex]" />

#### Worked Example

The table shows information about the Isotopes in a sample of rubidium with 72% 85Rb and 28% 87Rb

Use information from the table to calculate the relative atomic mass of this sample of Rubidium. Give your answer to one decimal place:

Relative Atomic Mass = 85.6

#### Exam Tip

Isotopes are easy to recognize from their notation as they have the same symbol but different mass numbers. For example, the two stable isotopes of copper are 63Cu and 65Cu