3.1.1 Atomic Structure

• What are the three sub-atomic particles of an atom?


  Proton, neutron, and electron
• Where are protons and neutrons located in an atom?

  In the nucleus
• What is the relative mass of a proton?

  1
• What is the relative charge of a neutron?

  0
• What is the relative mass of an electron?

  1/1840
• What is the atomic number of Lithium (Li)?

  3
• What does the mass number represent?

  The total number of protons and neutrons in the atom
• How do you calculate the number of neutrons in an atom?

  Number of neutrons = A - Z
• What are isotopes?

  Atoms with the same number of protons but different numbers of neutrons
• What happens to positive ions in an electric field?

  They are accelerated to a constant kinetic energy
• What is the formula for kinetic energy?

  KE = \frac{1}{2} mv^2
• How can you rearrange the kinetic energy formula to find velocity?

  v = \sqrt{\frac{2KE}{m}}
• What determines the velocity of ions in a flight tube?

  The mass of the ions
• How do lighter and heavier particles behave in a flight tube?

  Lighter particles move faster than heavier particles
• What is the relationship between time of flight and mass in a flight tube?

  Time of flight is proportional to the square root of mass
• What is the formula for time of flight in a flight tube?

  t = \frac{d}{v} = \sqrt{\frac{m}{2KE}}
• What happens when ions reach the detector in a mass spectrometer?

  They generate a small current for analysis
• What does the size of the current generated by ions indicate?

  It is proportional to the abundance of the species
• What is measured by the mass spectrometer for each isotope?

  The m/z (mass/charge ratio) and abundance
• What is the effect of removing two electrons from a particle?

  It forms a 2+ ion
• What is the m/z of a particle with a mass of 46 and a 2+ charge?

  23
• How long would one ion of ⁵⁹Ni take to travel along a flight tube of 0.8000 m?

  1. 771 × 10⁻⁵ s
• How do you calculate the mass of one ion of ⁵⁹Ni?

  Mass = \frac{59}{6.022 \times 10^{23}} \text{ kg}
• What is the kinetic energy of the ions in the example given?

  1. 600 × 10⁻¹⁹ J
• How long would one ion of ⁵⁸Ni take to travel along a flight tube of 0.8000 m?

  27210¹¹ s
• How do you calculate the time of flight for an ion?

  t = l \sqrt{\frac{m}{2KE}}
• What are the key steps in the mass spectrometry process?

  1. Acceleration of positive ions by an electric field
  2. Ions travel through a flight tube
  3. Detection of ions generating a current
  4. Analysis of current to determine m/z and abundance
• What is the relative atomic mass quoted on the periodic table?

  A weighted average of all the isotopes
• How is the relative atomic mass (RAM) calculated?

  RAM = Σ(isotope mass x % abundance) / total relative abundance
• If asked to give the species for a peak in a mass spectrum, what format should you use?

  Provide the charge and mass number, e.g., "24Mg"
• How do you calculate the relative atomic mass of magnesium using its isotopes?

  RAM = (24 x 78.7) + (25 x 10) + (26 x 11.17) / 100 = 24.3
• How do you calculate the relative atomic mass of tellurium from its abundance data?

  RAM = (120 x 2) + (122 x 6) + (128 x 7) + (130 x 1) = 127.8
• What are the isotopes of copper and their relative atomic mass?

  Copper has two isotopes: 63-Cu and 65-Cu, with a relative atomic mass of 63.5
• How do you calculate the percentage abundances of copper isotopes given the relative atomic mass?

  63.5 = 63y + 65(1-y) leads to y = 0.75, so 63-Cu = 72.5% and 65-Cu = 27.5%
• What are the isotopes and their abundances for Cl₂ and Br₂?

  • Cl₂: ³⁵Cl (75%), ³⁷Cl (25%)
  • Br₂: ⁷⁹Br (50%), ⁸¹Br (50%)
• Why does the 160 peak in the mass spectrum of Br₂ have double the abundance of the other two peaks?


  Because there is double the probability of ⁷⁹Br⁷⁹Br and ⁸¹Br⁸¹Br compared to ⁷⁹Br⁸¹Br and ⁸¹Br⁷⁹Br
• What is the purpose of mass spectrometers in planetary space probes?

  To identify elements on other planets which may have different compositions of isotopes
• What happens to a molecule in a mass spectrometer with electron impact ionization?

  It often breaks up, giving a series of peaks, but the peak with the largest m/z is the parent ion
• What is the relative molecular mass (Mr) of a molecule in a mass spectrometer with electro spray ionization?

  The peak equals the mass of the Mr⁺ ion, and you subtract 1 to get the Mr of the molecule
• How are elements categorized in the periodic table?

  • s block: outer electron filling an s-sub shell
  • p block: outer electron filling a p-sub shell
  • d block: outer electron filling a d-sub shell