Atomic structure

• State the block in the Periodic Table in which sulphur is placed and explain your answer. (1)

  P block (1)

  Highest energy electrons occupy a p-orbital (2)
• Explain how ions are detected and relative abundance is mesasured in a ToF mass spectrometer (2)

  Ions hit the detector AND gains an electron, creating a current (1)

  The relative abundance is proportional to the current (2)
• Describe how ionisation is achieved in a mass spectrometer. Explain how the principle of ToF allows ions to be distinguished from each other. (4)

  Ionisation - bombardment of HIGH ENERGY ELECTRONS (1). Causes an electron to be knocked off the ion leaving a 1+ ion(2)

  
  

  ToF - the LOWER the m/z value (or mass)(3), the FASTER the ion reaches the detector (4)
• Describe the stages of electrospray ionisation (4)

  Sample is dissolved in a volatile solvent and injected through a fine hypodermic needle (1)

  The tip of the needle is attached to the positive side of a power supply and so has a high voltage (2)

  As spray emerges from end of the needle, the particles gain a proton from the solvent(3)

  The solvent evaporates leaving 1+ ions that move towards a negative plate (4)

  Adds a hydrogen ion so the Mr increases by 1
• What does the mass spectrometer determine? (1)

  M/z value and relative abundance
• Why is the TOF mass spectrometer kept under a high vacuum? (1)

  To prevent the ions that are produced colliding with molecules from the air
• Why is electron impact only used for elements and low Mr compounds? (1)

  Can cause larger organic molecules to fragment
• What are the smaller peaks that cluster around the molecular ion peak? (2)

  Due to different isotopes (1)

  These isotopes have different masses and so different m/z values (2)
• What does the main peak on a mass spectra show? (2)

  Molecular ion peak (1)

  Peak of the greatest m/z ratio (2)

  Represents the m/z value of the molecule analysed
• What does a mass spectrum show? (2)

  M/z ratio (1)

  Abundance of each ion that reaches the detector (2)

  If ions have a 1+ charge, then the m/z value equals the mass of each ion
• What happens in flight tube/ion drift in mass spectrometry? (2)

  Positive ions travel through a hole in the negatively charged plate and travels along the flight tube (1)

  The ToF of each particle through the flight tube depends on its mass (2)
• What is the detection stage in mass spectrometry? (3)

  Ions hit a negatively charged electric plate (1)

  When hit, the positive ions are discharged by gaining electrons from the plate. This generates a movement of electrons and an electric current is produced and measured (2)

  Size of the current gives a measure of number of ions hitting the electric plate. (3)

  (The relative abundance is proportional to the current)
• What happens during the acceleration stage in mass spectrometry? (2)

  The ions are attracted to a negatively charged plate and accelerate to it

  so all ions have the same kinetic energy

  so the velocity of each particle depends on its mass
• Define orbital (1)


  a region of space around the nucleus where there is a high probability of finding an electron
• Explain why different ions take different times to travel through the flight tube (2)

  time of flight depends on mass of ions (1)

  lighter ions travel faster (2)
• Define relative atomic mass (2)

  The average mass of an atom of an element (1) compared to 1/12 of the mass of an atom of carbon 12 (2)
• Describe electron impact (3)

  Sample is vaporised (1)

  High energy electrons are fired at it from an electron gun (2)

  This knocks off one electron from each particle forming a 1+ ion (3)

  eg. Ti (g) ———> Ti+ (g) + e-
• Define atomic orbitals


  A region around the nucleus that can hold up to 2 electrons with opposite spins
• Define ionisation energy (1)

  The energy required to remove a mole of electrons from a mole of atoms in the gaseous state (1)
• What is the effect on the ionisation energy if the shielding is greater?

  - means there's more shells in the atom so there is a greater distance between the nucleus and the outermost electron

  - there's a weaker electrostatic attraction on the outermost electron

  - so less energy is needed to remove the outermost electron

  - so the ionisation energy is lower
• What is the effect on the ionisation energy if the nuclear charge is greater?

  - more protons in the nucleus

  - bigger nuclear charge

  - stronger electrostatic attraction between outermost electron and nucleus

  - more energy is needed to remove the outermost electron

  - higher first ionisation energy
• Trends in first ionisation energy across a period and down the group

  - across the period - ionisation energy increases

  - down a group - ionisation energy decreases
• Explain the general trend of ionisation energy across a period

  - outermost electrons are in the same shell so the shielding remains similar

  - more protons in the nucleus so the nuclear charge increases

  - electrostatic attraction on the outermost electron increases

  - more energy is needed to remove the outermost electron

  - so first ionisation energy increases
• Explain the general trend of ionisation energy down a group

  - more shells so outermost electron is further form the nucleus

  - so more shielding from inner electrons

  - although nuclear charge increases, this is outweighed by greater distance and shielding

  - so weaker electrostatic attraction on outermost electron

  - less energy needed to remove outermost electron

  - so first ionisation energy decreases
• Explain the drop in 1st ionisation energy between group 2 and 3

  Eg. Mg - ends in 3s2

  Al ends in 3p1

  - The electron in the 3p sub-shell is of a higher energy than 3s

  - the electron in the 3p sub-shell is slightly further from the nucleus and experiences slightly more shielding than the 3s electrons

  - so slightly less energy is required to remove it
• Explain the drop in 1st ionisation energy between group 5 and 6

  Eg. Phosphorus - ends in 3p3

  Sulphur ends in 3p4

  - P has no paired electrons while S has a pair of electrons in one of of the 3p orbitals

  - these paired electrons repel each other so less energy is needed to remove the paired electron than the unpaired
• Explain the general trend in ionisation energy graphs

  - general trend increases

  - successive ionisation energies increase, as electrons are being removed from an increasingly positive ion

  - so electrostatic attraction between the nucleus and remaining outermost electron increases
• Explain the big jump in successive ionisation energy graphs

  - a large increase in ionisation energy represents a change of shell

  - so when an electron is being removed from a lower shell, which is closer to the nucleus, the electrostatic attraction is much stronger so ionisation energy suddenly becomes much larger

  - the shell loser to the nucleus experiences less shielding
• Explain why the ionisation energy of every element is endothermic (1)

  - energy is needed to overcome the attraction between the negative electron and positive nucleus (1)
• Explain how the abundance of an isotope in a sample of Rb is determined (2)

  - detector (1)

  - current is proportional to the relative abundance (2)
• State how and explain why the values of the first ionisation energies of Al and S deviate from the general trend (5)

  - both are lower and show a decrease (1)

  - in aluminium, the outer electron is in a 3p orbital (2)

  - 3p is higher in energy and more shielded (3)

  - in sulfur, electron is paired in a 3p orbital (4)

  - paired electrons repel so less energy is needed to remove it (5)
• State and explain the general trend in the values of first ionisation energy of Na to Ar

  - general increase (1)

  - greater nuclear charge (2)

  - same shielding (3)

  - stronger attraction for outer electrons (4)
• Explain why lithium has a higher first ionisation energy than potassium

  - the outermost electron in potassium is further from the nucleus

  And more shielding from the inner electrons

  - although nuclear charge is higher in potassium, this is far outweighed by greater distance and shielding

  - so potassium has a weaker electrostatic attraction on the outermost electron

  - so less energy is needed to remove the outermost electron

  - so lithium has a higher first ionisation energy
• State 2 differences between the plum pudding model and the model of atomic structure used today

  - nucleus contains protons AND neutrons (1)

  - electrons are now arranged in energy levels/shells (2)
• Explain the existence of a a peak which is smaller than the others (2)

  - 2+ ion is formed (1)

  - so it is 128/64=2 (2)

  Or 2 straight marks for 128 Te 2+
• Define isotopes

  Atoms with the same number of protons but different number of neutrons
• What is the mass spectrometer used for?

  To determine all the isotopes present in a sample of an element

  Therefore to identify elements
• Why is electrospray ionisation used for larger organic molecules?

  Because the softer conditions of this technique means that fragmentation won‘t occur
• Define successive ionisation energy

  The removal of more than one electron from the same atom
• A decrease in first ionisation energy for aluminium is evidence for what?

  For atoms having sub shells

  The outermost electron is aluminium sits in a higher energy sub shell slightly further from the nucleus

  The atomic model Niels Bohr came up with didn’t explain sub shells