ATOMIC STRUCTURE

• This question is about atomic structure.

  The figure below is a model proposed by Rutherford to show the structure of an atom.

  State two features of the current model that are not shown in the Rutherford model.

  Nucleus contains protons and neutrons

  Electrons on different orbitals around nucleus
• What is the electron configuration of V(2+) in the ground state?

  A 1s2 2s2 2p6 3s2 3p6 3d3

  B 1s2 2s2 2p6 3s2 3p6 3d1 4s2

  C 1s2 2s2 2p6 3s2 3p6 3d3 4s2

  D 1s2 2s2 2p6 3s2 3p6 3d5 4s2

  A
• Define the term relative atomic mass.

  Average mass of one atom of an element / 1/12 mass of one 12C atom
• State in terms of the numbers of fundamental particles, one similarity and one difference between atoms of 50Cr and 53Cr.

  SIMILARITY

  same number of electrons

  DIFFERENCE

  different number of neutrons
• Give two reasons why it is necessary to ionise the isotopes of chromium before they can be analysed in a TOF mass spectrometer.

  So ions can be accelerated by an electric field

  To make it charged and allow it to produce a current at the detector
• Describe how the molecules are ionised using electrospray ionisation.

  Sample dissolved in volatile solvent (water/ methanol)

  Injected through needle at high voltage

  Each molecule gains a proton
• The first ionisation energies of the elements in Period 2 change as the atomic number increases.

  Explain the pattern in first ionisation energies of the elements from lithium to neon.

  GENERAL TREND (Li -> Ne)

  First ionisation energy increases

  Higher proton number

  Electrons in same energy level

  Similar shielding

  Stronger electrostatic attraction between nucleus and outermost electron

  DEVIATION (Be -> B)

  B lower than Be

  Outer electron in 2p

  Higher in energy than 2s

  DEVIATION (N -> O)

  O lower than N

  2 electrons in 2p need to pair

  Causes repulsion
• Describe the process of electrospray ionisation.

  P added to solvent

  Injected through needle at high voltage

  Gains a proton
• State two differences between plum pudding model and model of atomic structure used today.

  No protons and neutrons in plum pudding model

  No orbitals in plum pudding model (electrons throughout)
• State which of the elements magnesium and aluminium has the lower first ionisation energy.

  Al

  Outer electron in 3p orbital

  Higher in energy so easier to remove
• Describe how ions are formed in a time of flight (TOF) mass spectrometer.

  High voltage applied, substance gains proton

  OR

  Sample bombarded by high energy electrons, loses an electron
• Explain why the first ionisation energy of the Group 2 elements decreases down the group.

  Outer electrons in higher energy level

  Less electrostatic attraction between nucleus and outer electrons
• Explain why the third ionisation energy of magnesium is much higher than the second ionisation energy of magnesium.

  Electron removed from 2p which is closer to nucleus

  Less shielded than 3s
• A student makes a solution of citric acid by dissolving some solid citric acid in water. Describe a method to add an accurately known mass of solid to a beaker to make a solution.

  Measure mass of weighing boat and solid

  Add solid to beaker

  Reweigh measuring boat
• For each of these three steps

  • identify
  • explain
  • suggest overcome

  Using a measuring cylinder

  -> large uncertainty

  -> use a volumetric pipette

  Too much indicator

  -> react and affect the endpoint reading

  -> use a smaller volume

  Rinsing the burette with distilled/ deionised water

  -> slightly dilutes alkali solution

  -> rinse burette with alkali solution

  Adding solution until indicator “just” changes colour

  -> not have fully reacted (mixture not swirled)

  -> add until a permanent colour change
• Explain why CBr4 is not a polar molecule.

  Symmetrical molecule

  Dipoles cancel out