Atomic Structure Flashcards
What is an atomic number?
The number of protons in the nucleus of an atom.
What is the mass number?
The number of protons and neutrons in the nucleus of the atom.
What is an isotope?
Atoms of the same element so they have the same number of protons with a different number of neutrons.
Why do isotopes have the same chemical properties?
They have the same electronic configuration.
How does a time of flight mass spectrometer work (ionisation)?
1) Electron impact ionisation:
- Sample is injected into spectrometer and vaporised into a gas.
- High energy electrons from an electron gun are fired at the sample.
- The high energy electrons knock off an electron from each particle to form positive ions.
2) Electrospray ionisation:
- Sample is dissolved in polar solvent.
- Sample is injected through a fine hypodermic needle to give a fine mist.
- The tip of the needle is attached to the positive terminal of a high voltage supple.
- Particles gain a proton (H+ ion) from the solvent to form positive ions.
A volatile solvent is used.
How does a time of flight mass spectrometer work (acceleration)?
- An electric field is applied to accelerate the positive ions.
- This gives all the ions with the same charge kinetic energy.
- KE = 1/2 X m X V^2
How does a time of flight mass spectrometer work (separation)?
- The positive ions travel through the mass spectrometer to the detector.
- The time of flight of each particle depends on its velocity.
- Time of flight = distance travelled / velocity
- Lighter ions travel faster and reach the detector first.
- Heavier ions travel slower and reach the detector last.
How does a time of flight mass spectrometer work (detection)?
- A detector records the different ions as they arrive at the detector.
- At the detector, each ion gains an electron. This generates a current.
- The size of the current is proportional to the abundance of each ion. The greater the current, the greater the abundance.
What are the key principles of TOF mass spectrometry?
- The sample must be ionised so that it can be accelerated and detected.
- The whole spectrometer is kept under vacuum to prevent ions from colliding with molecules in air which would cause them to slow down.
How do you calculate the mass of one atom/ion?
Mass of one ion/atom (in kg) = relative isotopic mass X 10^-3 / avagadro’s number
What is the time of flight equation?
Velocity = Distance / Time
Therefore K = 0.5 X Mass X (Distance^2 / Time^2)
What is the relative isotopic mass?
The mass of an isotope relative to 1/12th the mass of an atom of carbon-12.
What is the relative atomic mass?
The weighted average mass of all the isotopes relative to 1/12th the mass of an atom of carbon-12.
What is the equation for relative atomic mass?
Sum of (mass X abundance of each isotope)
———————————————————
Total abundance
Why would methanol (CH3OH) have peaks at 32, 33 and below 32 in a mass spectra obtained from electron impact ionisation?
32 - Mr of the compound.
33 - Some molecules of CH3OH will contain carbon-13 and hydrogen-2.
Below 32 - Electron impact ionisation causes molecular ions to break apart, forming fragments.
Why are there no peaks below MH+ in a mass spectra obtained by electrospray?
Electrospray ionisation does not cause fragmentation of a molecular ion so mass spectras obtained by electrospray ionisation will have peaks below MH+.
What are the differences between electron impact and electrospray ionisation?
Ion:
- Electron impact makes the molecular ion (M+)
- Electrospray makes MH+
Fragmentation:
- Electron impact = yes
- Electrospray = no
Used for:
- Electron impact - elements and smaller molecules.
- Electrospray = larger molecules e.g. proteins.
What are the rules for how electrons are arranged?
- Each sub-shell fills up so that the electrons remain unpaired if possible. Once the particular sub-shell is half full, the electrons will start to pair up.
- The 1s sub-shell has the lowest energy: it is closest to the nucleus.
- The order of filling up of energy levels corresponds to the order of the elements in the periodic table.
Which 2 elements have an unusual configuration and why?
Cr: 1s2 2s2 2p6 3s2 3p6 3d5 4s1
Cu: 1s2 2s2 2p6 3s2 3p6 3d10 4s1
They are more stable with half full and full 3d sub shells respectively.
What is ionisation energy?
The energy needed to remove one mole of electrons from one mole of atoms in the gaseous state.
The first ionisation energy of an element is the energy required for which process?
M(g) —> M+(g) + e-
What factors affect the first ionisation energies?
- The distance for the electron being removed from the nucleus as more shells means greater distance.
- The number of protons in the nucleus as the more protons there are, the greater the attraction between the nucleus and the electron being removed.
- The amount of shielding as inner shells of electrons shield the outer electrons from the nuclear attraction.
What is the trend of 1st ionisation energy and why?
- 1st ionisation energy decreases down a group.
- As you go down the group, atomic radius increases as there are more shells so there is more shielding.
- Therefore the attraction between the nucleus and electron to be removed decreases, so less energy is required.
What is the trend in first ionisation energy across a period and why?
- Genera increase across a period.
- The electron to be removed is in the same shell with similar shielding. There are more protons in the nucleus so the attraction between the nucleus and outer electron increases, so more energy is required.
