Atomic Structure & Mass Spectrometry (OCR AS Chemistry)

• 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 (A_r) 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:
  • 
  • The relative abundance of an isotope is either given or can be read off the mass spectrum

Answer

The correct answer option is A

• • A_r = 
  • A_r = 16.0044
  • A_r = 16.00

• 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 
    • 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

Answer

• • A_r = 
  • A_r = 10.801
  • A_r = 10.80

• 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

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