Atomic Structure

• The purpose of high resolution mass spectrometry is to find the abundance and m/z ratio of each isotope of an element, allowing us to determine its relative atomic mass. It is also to find the relative molecular mass of substances made from molecules
• The first stage of high resolution mass spectrometry is ionisation
• Two methods of ionisation are electron impact and electrospray
• In electron impact, the sample is vaporised and high energy electrons are fired at it from an electron gun. This causes one electron to be knocked off for each particle, which forms a 1+ ion.
• Equation of electron impact

  X(g) --> X+(g) + e–
• In electrospray ionisation, the sample is dissolved in a volatile solvent, injected through a needle attached to the positive terminal of a high voltage power supply. The particles are ionised by gaining a proton.
• In electron impact, the molecular ion is often broken into smaller fragments due to the force of the electron gun. These can be detected in the mass spectrum.
• Equation for electrospray ionisation

  X(g) + H+ --> XH+(g)
• In electrospray ionisation, the samples form XH+ ions which they gain from the solvent.
• In electrospray ionisation, the sample that is detected will have a mass of its Mr + 1.
• Electrospray ionisation is known as a soft ionisation technique and fragmentation rarely takes place.
• The second stage of high resolution mass spectrometry is acceleration.
• In acceleration, the positive ions are accelerated by attraction to a negatively charged plate, using an electric field.
• The velocity of the particles in acceleration depends on its mass. Lighter particles have a faster velocity, and heavier particles have a slower velocity.
• The third stage of high resolution energy is flight tube (ion drift).
• In flight tube (ion drift), the positive ions travel through a hole in the negatively charged plate into a tube. The time of flight of each particle through this flight tube depends on its velocity which in turn depends on its mass.
• The time of flight along the flight tube t=d square root m divide by 2KE
• The fourth stage of high resolution mass spectrometry is detection.
• In the detection stage, the positive ions hit a negatively charged electric plate and the positive ions gain electrons from the plate. This generates a movement of electrons and an electric current is produced.
• In detection, the size of the current produced gives a measure of the number of ions hitting the plate- the abundance is proportional to the size of the current produced.
• The fifth stage of high resolution mass spectometry is data analysis.
• A computer uses the data to produce a mass spectrum. For molecules that are ionised by electron impact, the signal with the greatest m/z value is from the molecular ion and its m/z value gives the relative molecular mass. There may be other peaks due to ions with different isotopes, and due to fragmentation.
• When an element is put through high resolution mass spectometry, there will be different peaks due to its isotopes. From this, you can work out the relative atomic mass.
• 4s electron shells are filled and lost before the 3d shell is filled and lost.
• Each shell can hold 2n^2 for its level. Each shell has atomic orbitals that hold the electrons.
• Atomic orbitals can hold up to two electrons with opposite spins.
• S orbitals have spherical shapes. They can hold two electrons as there is only one S orbital in a given shell.
• P orbitals have a dumbell shape. They can hold 6 electrons as there are three P orbitals in a given shell, apart from the first shell
• D orbitals can hold ten electrons as there are five D orbitals in a given shell apart from shells 1 and 2
• A subshell is a group of the same type of orbitals in a shell.
• Different subshells have different energy levels. As we move away from the nucleus, the energy of the subshells increases.
• Three rules for filling atomic orbitals:

  1. Lowest energy orbitals are filled first
  2. We can have up to two electrons per orbital but they must have opposite spins.
  3. Electrons will fill up individual orbitals before pairing as they repel each other and don't want to be paired into one orbital.
• The energy of the 4s subshell is lower than the energy of the 3d subshell, so it is filled before it. This also means it loses electrons before the 3d subshell loses them as well.
• In chromium and copper, the 4s sub shell contains only one electron and is not filled, but there are still electrons filling the 3d sub shell. This is because the 3d subshell is more stable when it is half or completely full. Plus, electrons would rather take up individual orbitals than pairing.
• Ionisation energy is the energy required to remove one mole of electrons from one mole of atoms in the gaseous state. It is measured in kJmol-1 .
• First ionisation energy equation:

  X(g) --> X+(g) + e-
• The atomic radius is the distance between the nucleus and the outermost electron. It increases down a group as the number of shells increase. It decreases across a period as there are a greater number of protons but the same number of shells, so there is a greater pull.
• The atomic radius of an atom affects the ionisation energy. This can be seen on a graph. Ionisation energy greatly increases when moving to a new subshell that is closer to the nucleus, as the attraction is greater and requires more energy to be removed.
• First ionisation energy decreases down a group as the atomic radius increases as there are more shells, and greater shielding between the positive nucleus and negative outermost electron.
• First ionisation energy generally increases across a period as the nuclear charge increases as the amount of protons increase. There is also the same number of shells, so atomic radius decreases as the outermost electrons are more attracted to the positive nucleus the more protons it has.