3.1 Atomic Structure

define relative atomic mass

the mean mass of an atom of an element, divided by one twelfth of the mean mass of an atom of the carbon-12 isotope

What is mass spectrometry

an analytical technique used to identify different isotopes and find the overall relative atomic mass of an element

What is time of flight mass spectrometry

a recording of the time it takes for ions of each isotope to reach a detector. Using this, spectra can be produced showing each isotope present.

what are the 5 stages of TOF mass spectrometry

ionisation, acceleration, ion drift, detection, analysis

what conditions must TOF mass spectrometry be done under and why?

a high vacuum to prevent ions produced from colliding with molecules present in the air

what are the 5 stages of TOF mass spec?

ionisation, acceleration, ion drift, ion detection, data analysis

with which TOF mass spec is the sample dissolved?

electrospray ionisation

in electrospray ionisation, what is the sample dissolved into?

a volatile and polar solvent

what is the sample injected through for electrospray ionisation?

at high pressure through a fine, hollow needle connected to the positive terminal of a high voltage supply

how is the sample sprayed out of the needle?

at a high pressure with high voltage, causing the sample molecule to gain a H+ ion from the solvent

what is the equation for ionisation for electrospray ionisation

X + H+ → XH+

In what state is the sample injected for electron impact ionisation?

vapourised and at a low pressure

how is the sample ionised for electron impact ionisation?

the electron gun fires high energy electrons at the sample, knocking an outer electron away

why is electron impact ionisation only used for elements and substances with a low mr?

larger molecules would be fragmented by high energy electrons

what is the formula for electron impact ionisation?

X → (X+) + e-

what happens to the positive ions in the acceleration section (either method)?

they are accelerated to a constant kinetic energy by an electric field towards a negatively charged plate

what would happen in the acceleration section if the sample wasn’t ionised?

nothing as if the particles were not charged, they would experience no force of attraction to the negatively charged plate

what is the ions velocity towards the negative plate dependent on and why?

the mass of the ion as all ions have contant kinetic energy

how does a beam of electrons reach the detection plate?

the positive ions pass through a hole in a negatively charged plate, forming a beam, and are attracted to the negatively charged detection plate

which ions reach the detection plate first?

the lighter ones

what happens at the detection plate?

the ions gain an electron from the detection plate and this movement produces a current

why is a current produced at the detection plate?

there is a movement of electrons from the negatively charged plate to the ion

what does a greater current signify

a greater abundance of that species

what is analysed from a TOF mass spectrometer

amount of current produced and flight tube times

how can Ar be calculated from spectra?

(m/z x abundance)/total abundance

define first ionisation energy

the energy required to remove one electron from each atom in one mole gaseous atoms producing one mole of 1+ gaseous ions

what is the formula for first ionisation energy

X(g) → X+(g) + e-

define second ionisation energy

the energy required to remove one electron from each ion in one mole of gaseous ions with a charge of 1+ to produce one mole of gaseous ions with a 2+ charge

what is the formula for second ionisation energy?

X+(g) → X2+(g) + e-

what are 3 factors affecting ionisation energy?

attraction of the nucelus, distance of the electrons from the nucleus, shielding of the attraction of the nucleus

why is there a general increase in IE across a period

nuclear charge increases, atomic radius decreases, shielding remains the same

why are there small drops that look like exceptions for IE, from Be to B or Mg to Al?

a new subshell is starting to fill, electrons in that shell are slightly easier to remove as they are higher in energy

why are there small drops that look like exceptions for IE, from N to O, or P to S?

two electrons of opposite spin are double-filling the same orbital, so electrons experience repulsion making them easier to remove

what happens to the electrons in a sample during a flame tested while they are in the flame?

thermal energy is absorbed by the electrons so they are promoted to a higher energy level. they move from their ground state to an excited state.

what happens after the electrons reach ana excited state during a flame test?

the electrons in the excited state lose their gained energy in a process called relaxation, and move back to their ground state

what happens during the process of relaxation in flame tests?

light is emitted

why is HCl used to clean nichrome wires?

it cleans them effectively and the chlorides of the metals are especially volatile (so will produce positive flame tests)