Atomic Structure & Mass Spectrometry (OCR A Level Chemistry)

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Isotopes & Relative Atomic Mass

  • Isotopes are different atoms of the same element that contain the same number of protons and electrons but a different number of neutrons.
    • Isotopes can also be described as atoms of the same elements but with different mass numbers

  • Therefore, the mass of an element is given as relative atomic mass (Ar) by using the average mass of the isotopes
  • The relative atomic mass of an element can be calculated by using the relative abundance values by using the following equation:
    • fraction numerator open parentheses r e l a t i v e space a b u n d a n c e subscript i s o t o p e space 1 end subscript space cross times m a s s subscript i s o t o p e space 1 end subscript close parentheses space plus open parentheses r e l a t i v e space a b u n d a n c e subscript i s o t o p e space 2 end subscript space cross times m a s s subscript i s o t o p e space 2 end subscript close parentheses space e t c over denominator 100 end fraction
    • The relative abundance of an isotope is either given or can be read off the mass spectrum

Worked example

Calculating the relative atomic mass of oxygen

A sample of oxygen contains the following isotopes:

2-1-3-calculating-relative-atomic-mass-of-oxygen

What is the relative atomic mass of oxygen in this sample, to 2 decimal places?

  1. 16.00
  2. 17.18
  3. 16.09
  4. 17.00

Answer

The correct answer option is A

    • Arbegin mathsize 16px style fraction numerator open parentheses 99.76 space cross times space 16 close parentheses space plus open parentheses 0.04 space cross times space 17 close parentheses space plus open parentheses 0.20 space cross times 18 close parentheses over denominator 100 end fraction end style
    • Ar = 16.0044
    • Ar = 16.00

Mr from Mass Spectra

  • The percentage abundance of the isotopes in an element can be found by the use of a mass spectrometer
  • The basic processes of mass spectrometry are:
    • The sample is vapourised
    • The sample is ionised to form positive ions
    • The ions are accelerated
      • Heavy ions move slower / are less deflected
    • The ions are detected as a mass-to-charge ratio, written as bevelled m over z
      • Each ion produces a signal, so the larger the signal, the greater the abundance
  • The mass spectra produced can be used to calculate the relative atomic mass of an element and its isotopes

Worked example

Calculating the relative atomic mass of boron

Calculate the relative atomic mass of boron using its mass spectrum, to 2dp:

Analytical Techniques Mass Spectrum Boron, downloadable AS & A Level Chemistry revision notes

Answer

    • Arbegin mathsize 16px style fraction numerator open parentheses 19.9 space cross times space 10 close parentheses space plus space open parentheses 80.1 space cross times space 11 close parentheses over denominator 100 end fraction end style
    • Ar = 10.801
    • Ar = 10.80
Relative molecular and formula mass

  • These are essentially the same ideas 
    • Relative molecular mass is applied to chemicals that have a fixed formula in terms of the number of atoms involved, e.g. ethanol, C2H5OH, contains two carbon atoms, six hydrogen atoms and one oxygen
    • Relative formula mass is applied to chemicals that use an empirical formula to represent them, e.g. calcium chloride, CaCl2, is an ionic lattice containing a ratio of 1 calcium ion : 2 chloride ions throughout the structure
  • These terms are often mis-used, with relative molecular mass being applied to any compound regardless of its composition
  • The relative molecular and formula mass are both calculated by adding up the relative atomic masses of all the component atoms

Worked example

Calculate:
  1. The relative molecular mass of ethanol, C2H5OH
  2. The relative formula mass of calcium chloride, CaCl2

(Ar values: C = 12.0, H = 1.0, O = 16.0, Ca = 40.1, Cl = 35.5)

Answers

Answer 1: (2 x C) + (5 x H) + O + H → (2 x 12.0) + (5 x 1.0) + 16.0 + 1.0 = 46.0

Answer 2: Ca + (2 x Cl) → 40.1 + (2 x 35.5) = 111.1

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.