3.1.1 Atomic Structure Flashcards
Explain how vaporised atoms of ‘x’ are converted into ‘x+’ ions in a mass spectrometer
State and explain which of the ‘x+’ ions formed from the isotopes of ‘z’ will be deflected the most in a mass spectrometer
High energy electrons knocks out electrons to form ions
The ions with the lowest mass are deflected the most
Explain briefly how the relative abundance of an ions is measured in a mass spectrometer
Ions hit the detector and cause a current. The bigger the current, the more of that isotope
Explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer
Ions not molecules will interact with and by accelerated by an electric field. Only ions will create a current when hitting a detector
Define the term relative atomic mass
Weighted mean mass of all isotopes with 1/12 mass of 1 carbon-12 atom
Define the term relative isotopic mass
Mass of isotope of an atom compared with 1/12 mass of 1 carbon-12 atom
Calculate the RIM and the RAM
RIM: mass of isotope of atom/ 1/12 mass of 1 C-12 atom
RAM: weighted mean mass of 1 atom of element/ 1/12 mass of 1 C-12 atom
Or sum of (% abundance of isotope x mass of isotope)/ total abundance (100%)
Define the first + second ionisation energy
The energy change when one mole of gaseous atoms loses one electron per atom to produce gaseous 1+ ions. The second ionisation energy is when one mole of gaseous 2+ ions is produced from one mole of 1+ ions
Explain how ions are detected and relative abundance is measured in a TOF mass spectrometer
Ions hit the negative plate and gain an electron. The size of the current is proportional to the abundance
Describe stage 1 electron impact ionisation + give equation
Sample vaporised
High energy electrons fired at sample from electron gun
Knocks off 1 electron from each particle forming 1+ ion
X(g) + e– → X+(g) + 2e–
X(g) → X+(g) + e–
Describe stage 1 electrospray ionisation + give equation
Sample dissolved in volatile solvent
Dissolved sample injected through fine hypodermic needle
Forms fine mist
Tip of needle attached to high voltage power supply
Particles ionised by gaining a proton from solvent as they leave needle
Solvent evaporates away while XH+ are accelerated
X(g) + H+ → XH+(g)
Describe the acceleration and ion drift stages
Positive ions accelerated in electric field (by travelling through negatively charged plate in flight tube) they all have same kinetic energy
So velocity of particle depends on mass
Lighter particles- faster velocity
Describe the detection stage
The positive ions hit a negatively charged electron plate
Here, the ions gain electrons so are discharged
This generates movement of electrons hence an electric current that is measured
Size of current gives measure of number of ions hitting plate
Describe the trend in first ionisation energy down group 2
Decrease
The further down, the group, the further the outer shell electrons are from the nucleus
So the attraction is weaker attraction is weaker.
Extra shell will also shield outer electrons from nucleus’ attraction
Describe the trend in first ionisation energies across period 3
There will generally be an increase
More protons = stronger attraction
Shielding remains same
Singly-occupied orbital vs orbital containing 2 electrons first ionisation energy
Full orbital requires less due to repulsion between 2 electrons
