atomic structure 3.1.1 in papers 1 & 3 Flashcards
list the different atomic models over the years.
- Dalton
atoms are indivisible, atoms of each element have different masses. - Thomson
atom is positively charged sphere embedded with negative electrons. - Rutherford
the mass of the atom is concentrated is a tiny positively charged center. - Bohr
electrons have fixed energy levels - Shrondinger
electrons are found in orbitals.
what are isotopes?
- atoms with the same number of protons, but different numbers of neutrons.
- they have similar chemical properties because they have the same shell structure.
- they may have slightly varying physical properties due to the different masses.
time of flight mass spectrometry
what is mass spectrometry?
A powerful instrumental method of analysis that can be used to:
- find the abundance and mass of each isotope in an element allowing us to determine its relative atomic mass.
- find the molecular atomic mass of substances made of molecules.
time of flight mass spectrometry
what are the 2 ways a sample can be ionised?
- electron impact ionisation (gun method)
- electrospray ionisation
time of flight mass spectrometry
what is electron imapct ionisation?
- the sample is vapourised and fired with high energy electrons from an electron gun which knocks of 1 electron from each particle.
- the ions formed are attracted towards a negative plate where they are accelerated.
time of flight mass spectrometry
what is electrospray ionisation?
- the sample is dissolved in a volatile solvent and injected through a fine needle to give a fine mist.
- the tip of the needle is attached to the positive terminal of high voltage power supply.
- the particles are ionised by gaining a proton (H+) from solvent as they leave the needle.
time of flight mass spectrometry
what is electron imapct ionisation used for?
- X (g) + e -> X+ (g) + 2e
- elements
- substances made of molecules with a low molecular mass.
- however molecules often fragment.
time of flight mass spectrometry
what is electron spray ionisation used for?
- X(g) + H+ -> XH+(g)
- substances made of molecules with a high molecular mass, including biological ones (proteins).
- molecules rarely fragment.
- resulting ions have a Mr one unit higher due to gain of protons.
- cant be used if particles arent capable o gaining a proton.
time of flight mass spectrometry
what happens in stage 2?
- acceleration
- positive ions are accelerated using an electron field so that they all have the same kentetic energy.
- KE = joules
- mass = KG
- given particles have the same kenetic energy, the velocity of each one depends on its mass, lighter ones are faster.
time of flight mass spectrometry
what happens in stage 3?
- flight tube
- the positive ions travel through a hole in the negatively charged plate into a tube.
- the time of flight depends on velocity which depends on mass.
- v=d divided by t
time of flight mass spectrometry
what happens in stage 4?
- detection
- the postive ions hit a negatively charged electric plate.
- the ions reach the detector and generate a small current, which is fed to a computer for analysis.
- the current is produced by electrons transferring from the detector to the positive ion.
- the size of the current is proportional to the abundance of the species.
- the size of the current gives a measure of the number of ions hitting the plate.
time of flight mass spectrometry
what happens in stage 5?
- data analysis
- a computer uses the data to produce a mass spectrum.
- shows mass of the ion.
time of flight mass spectrometry
how do you work out the Ar?
total mass of each isotope
—————————-.
total abundance
mass spectrometry
how do you work out the relative atomic mass?
R.A.M = (isotopic mass x relative abundance)
—————————-.
total relative abundance
mass spectrum
explain the mass spectrum graph.
- furthest to the right of the significant peaks = molecular ion peak.
- mass/charge = Mr of the molecule.
- other peaks are fragments you ignore.
