Topic 1 - Atomic Structure & Periodic Table (P1) Flashcards
How to calculate the maximum number of orbiting electrons per shell:
- Use 2n^2, where n is the number of the shell
- For example, the second shell can hold 2(2)^2 = 2x4 = 8 electrons
Define relative atomic mass (Ar):
- The mean mass of an atom of an element, relative to one twelfth of the mean mass of an atom of the Carbon-12 isotope.
How to calculate relative atomic mass
- = Mean mass of an atom of an element / (1/12 x mean mass of C-12 isotope)
Define relative isotopic mass:
- The isotopic mass of an isotope relative to one twelfth of the mean mass of an atom of the carbon-12 isotope.
Define relative molecular mass (Mr):
- The mean mass of a molecule of a compound, relative to one twelfth of the mean mass of an atom of the carbon-12 isotope.
State the five stages involved in TOF mass spectrometry:
- Ionisation
- Acceleration
- Ion Drift (deflection)
- Detection
- Analysis
Describe the ionisation stage of the TOF mass spectrometry (5 points):
- A sample of an element is vaporised.
- This sample is injected into the mass spectrometer.
- Here, a high voltage is passed over the chamber.
- This causes electrons to be removed from the atoms (as they are ionised).
- +1 charged ions are left in the chamber.
Describe the acceleration stage of the TOF mass spectrometry (1 point):
- The positively charged ions are then accelerated towards a negatively charged detection plate.
Describe the ion drift (deflection) stage of the TOF mass spectrometry (2 points):
- The ions are then deflected by a magnetic field into a curved path.
- The radius of their path is dependent on the charge and mass of the ion.
Describe the detection stage of the TOF mass spectrometry (3 points):
- When the positive ions hit the negatively charged detection plate, they gain an electron.
- This produces a flow of charge.
- The greater the abundance, the greater the current produced.
Describe the analysis stage of the TOF mass spectrometry:
- The obtained current values are then used in combination with the flight times to produce a spectra print-out with the relative abundance of each isotope displayed.
What happens if a 2+ charged ion is produced during TOF mass spectrometry? (3 points)
- Its mass to charge ratio (m/z) is halved.
- This is because it is affected more by the magnetic field producing a curved path of smaller radius.
- This can be seen on spectra as a trace at half the expected m/z value.
Calculating Ar from mass spectrometry:
- (M/Z x Abundance) / Total Abundance
What can Ar be used for in mass spectrometry? (3 points)
- The element can be identified by referring to the Periodic Table.
- The tallest peak on a mass electrum corresponds to the relative molecular mass of the molecule.
- This peak is known as the molecular ion peak and is formed from the M+ species.
What is the tallest peak on a mass spectrum known as?
- This peak is known as the molecular ion peak and is formed from the M+ species.
Define ionisation energy:
- The minimum amount of energy to remove one mole of electrons from one mole of atoms in a gaseous state.
What is ionisation energy measured in?
- kJmol^-1
Why do successive ionisation energies require more energy?
- As electrons are removed, the electrostatic force of attraction between the positive nucleus and the negative outer electron increases.
- More energy is therefore needed to overcome this attraction so ionisation energy increases.
First ionisation energy across a period:
- Increases due to a decreasing atomic radius
- Plus, greater electrostatic forces of attraction.
First ionisation energy down a group:
- Decreases due to an increasing atomic radius.
- Plus, increased electron shielding which reduces the effect of the electrostatic forces of attraction.
Why is the first ionisation energy for Aluminium lower than expected?
- Single pair of electrons
- Have opposite spin
- Natural repulsion
- Reduces energy needed to put in to remove the outer electron
