Atomic Structure/Amount of Substance Flashcards
Relative mass of electron
0.0005 or 1/2000
Mass number
total number of nucleons (neutrons and protons)
Suggest points to consider when interpreting electron impact mass spectrum
- greatest m/z value (of relatively high abundance) is from molecular ion and its m/z value gives relative molecular mass
- exactly half m/z value could be 2+ ion, e.g. Cu+ (63 m/z) and Cu2+ (31.5 m/z)
- ions slightly greater than greatest m/z value are due to molecular ions with 2H or 13C
Suggest points to consider when interpreting electrospray ionisation mass spectrum
- greatest m/z value (of relatively high abundance) is from molecular ion and its relative molecular mass is m/z minus 1
- always MINUS 1 from m/z to account for proton
- ions slightly greater than greatest m/z value are due to molecular ions with 2H or 13C
Explain Cu and Cr electron configuration
copper - [Ar] 3d104s1
chromium - [Ar]3d54s1
- completely filled and half-filled subshells are more stable than partially filled ones
- electron from 4s orbital becomes excited and rises to 3d orbital
Describe process of electron impact/electrospray ToF mass spectrometry
- sample vaporised and fired at by high speed electrons with electron gun
- removes electrons from each particle to form ions with a single positive charge
OR - sample dissolved in volatile solvent and injected through hypodermic needle attached to positive terminal of high voltage s]power supply
- particles gain proton to form ions with a single positive charge
- acceleration by electric field so particles have same kinetic energy but different velocity depending on mass
- positive ions travel through a hole in a negatively charged plate into flight tube where time taken to hit detector is measured
- positive ions gain electrons when they hit negatively charged detector plate
- size of current gives measure of abundance of ion hitting plate
Give reasons why atoms are put into certain blocks
- last electron found in that subshell
- highest energy subshell
Reason isotopes have the same chemical properties
same electron arrangement
Atomic number
number of protons in an element
Suggest why relative atomic mass calculated differs from the value in the Periodic Table
Periodic Table takes into account other isotopes/different amounts of isotopes
Suggest what information can be obtained from a mass spectrometer
- m/z (relative isotopic mass)
- relative abundance of isotopes
Factors affecting ionisation energy
- nuclear charge
- shielding
- atomic radius
- whether electron is removed from neutral or charged species
What do electron arrows in orbitals represent
represent clockwise and anti-clockwise spin
Suggest why electrons in 2p sublevels have a higher energy than in 2s sublevels
further from nucleus so so more shielding
Explain why first ionisation energy of helium is very large
electron is not shielded from nucleus
How to calculate relative formula mass from mass spectrometer
sum of m/z multiplied by relative abundance divided by sum of relative abundances
Reasons particles must be ionised before being analysed in a mass spectrometer
- accelerated
- attracted to negative plate to be detected
Term given to .xH20
waters of crystallisation
Electron orbital
region around nucleus of an atom where electrons are most likely to be found
Give mass spectrum for diatomic molecules with two isotopes and explain why
2A-2A (25%)
1A-1A (25%)
1A-2A or 2A-1A (so 50%)
molecule has two isotopes and is a diatomic molecule so there are three possible combinations of isotopes in molecule
Suggest why there is a minimum energy for the electron gun used ionisation
so that only one electron is removed from atoms so ions with a single positive charge are formed
Suggest a benefit of electrospray mass spectrometry
soft ionisation so larger molecules do not fragment as much
Suggest which peak in mass spectrum is molecular ion peak
highest m/z value (but with relatively high abundance)
Suggest how infrared mass spectrometry is used to differentiate between bonds in a molecule
- different bonds vibrate at different discrete frequencies
- infrared radiation absorbed by bonds excites them to a higher state of vibration
- so different discrete frequencies of infrared radiation are absorbed based on bonds present
- transmittance graph show peaks where infrared radiation is absorbed by bonds
Fingerprint Region
- between 1500-400cm-1
- allows compounds with same functional groups to be distinguished
- must have exact match to spectrum of known compound
Relative atomic mass
ratio of the average mass of one atom of an element to one twelfth of the mass of an atom of carbon-12
Suggest uses of a mass spectrometer
- identify element
- find mass and abundance of each isotope in element to determine relative atomic mass
- find relative molecular mass of molecule
Compare and contrast electron impact and electrospray ionisation ToF mass spectrometry
- ionisation by electron gun vs dissolving in volatile solvent to protonate particles
- elements and low Mr vs large molecules (e.g. proteins) and high Mr
- electrospray referred to as soft ionisation since less fragmentation
- Mr = greatest m/z vs greatest m/z minus 1
- particles accelerated by an electric field for both
- time measured for particles to travel through flight tube for both
- abundance of each ion determined by size of current for both
Suggest what peaks below greatest m/z value are due to for electron impact ionisation of molecules
fragmentation of molecular ion
Explain why electrons in an orbital have opposite spin
- electrons repel strongly
- opposite spin minimises repulsion
Explain why 4s subshell is filled before 3d
lower energy level
What does Avogadro’s constant tell us
number of particles in a mole
State ideal gas equation and units for each variable
pV=nRT p - pressure in Pa V - volume in m3 n - moles R - gas constant T - temperature in K (Celsius plus 273)
Suggest what evidence there is from pattern in first ionisation energies across Period 3 to suggest 2 is maximum number of electrons in s-subshell
two elements before drop in ionisation energy
Describe how to identify group or element from successive ionisation energies
- calculate difference between each ionisation energy
- any big jumps (above 1500-2000 kJmol-1) indicate electron is removed from a different energy level
- smaller jumps refer to subshells (around 1000)
- even smaller but still evident jumps are due to orbital pairing (unlikely to be obvious so ignore)
How to use infrared spectroscopy to prove an aldehyde is definitely pentanal
- compare with known spectrum for pentanal
- fingerprint regions should match exactly
What needs to be true for molecular ions to have the same precise molecular mass
must contain same numbers of each atom not just same relative molecular mass
