Mass number and isotopes

what letter is a common symbol of mass number?

A

what letter is a common symbol of atomic number?

Z

define relative atomic mass:

the average mass of an atom of an element compared to one twelfth the mass of a carbon 12 atom

why is carbon-12 important?

this isotope is used as a standard/reference to which all other atoms are measured

define relative isotopic mass:

the mass of an atom of an isotope of an element compared to one twelfth the mass of carbon-12 atom

why may isotopes have slightly varying physical properties?

because they have different masses

how do you calculate relative atomic mass?

the sum of the mass of each isotope x its % abundance / 100

define relative molecular mass:

the average mass of a molecule compared to one twelfth the mass of a carbon-12 atom

define relative formula mass:

the average mass of a formula unit compared to one twelfth the mass of carbon-12 atom

what is the symbol for relative molecular mass?

Mr

what is the symbol for relative formula mass?

RFM

when is RFM used?

for ionic or giant covalent substances that don’t exist in a ‘molecule’. There formula is the simplest ratio of the substance

what is a mass spectrometer?

It is a machine used in chemical analysis

what does a mass spectrometer give info on?

• Relative atomic mass of an element

• Relative abundance of its isotopes

• Relative molecular mass of a molecule

• identify elements

what does a mass spectrometer need to be under and why?

a vacuum otherwise air particles would ionise and register on the detector

what is the first step when using a mass spectrometer?

ionisation

what is the aim of ionisation?

to create a positive charge on all particles injected into the machine as they must be charged in order for any of the subsequent stages to work

what are the 2 methods for ionisation?

• electrospray ionisation

• electron impact ionisation

describe the process of electrospray ionisation?

• sample dissolved in a solvent and pushed through a small nozzle at high pressure

• high voltage applied that causes each particle to gain an H+ ion

• the solvent is then removed, leaving a gas made of positive ions

describe the process of electron impact ionisation?

• sample is vaporised

• an electron gun fires high energy electrons at the sample

• this knocks off at least one electron of each particle, gaining it a positive charge

what is the 2nd stage of the mass spectrometer?

acceleration

what is the aim of acceleration?

to speed up all particles with the same amount of energy

what does the speed of a particle depend on?

its mass

describe the process of acceleration?

• an electric field causes the now positively charged particles to accelerate

• the same amount of energy is given to all particles / ions in the sample

• lighter ions experience greater acceleration

• heavier ions accelerate less

what is the 3rd stage of the mass spectrometer?

ion drift

what is the aim of ion drift?

to separate the ions and record their Time Of Flight

describe the process of ion drift?

• this region has no electric field, so they travel at the speed they left the electric field

• lighter ions travel faster

• heavier ions travel slower so they take less time to reach the detector

what is the 4th stage of the mass spectrometer?

detection

what is detection?

recording what has travelled across the machine

describe the process of detection:

• when a positive charge hits the detector it generates a small current

• the detector records how long it took for that particle to generate that current

• larger current means more particles with that particular mass

• overall the data is used to calculate the mass/charge ratio for each particle

what is the 5th stage of the mass spectrometer?

data analysis

what is a mass spectrum?

a type of chart made from the info the detector records

what does the y-axis on a mass spectrum represent?

the % abundance

describe % abundance?

the more of that particle there is, the higher its % and peak on the mass spectrum will be

what does the x-axis on a mass spectrum represent?

mass(of the particle)/charge ratio

what ion is generally present when electron impact is used during the ionisation stage of mass spectometry?

+1

what happens to mass/charge ratio if electrospray ionisation is used?

it will be one greater as H+ is added

what are the extra peaks on a mass spectra due to?

fragmentation

what is fragmentation?

when a molecule is ionised it can become unstable and break into pieces. each piece could then register as a peak or a fragment

define base peak:

the highest peak/most abundant fragment

define molecular ion peak:

this is the major peak furthest to the right. the m/z of this peak will be the Mr of the molecule

define the M+1 peak:

a small peak beyond the molecular ion peak due to the naturally occurring presence of C13. this increases the molecules Mr by 1

give key info about bromine:

• diatomic

• has naturally occurring Br⁷⁹ (50%)

• has naturally occurring Br⁸¹ (50%)

give key info about chlorine:

• diatomic

• naturally occurring Cl³⁵ (75%)

• naturally occurring Cl³⁷ (25%)

what is the equation for kinetic energy?

½mv²

what is the rearranged equation for mass in kg (KE)?

2KE/v²

what is the rearranged equation for velocity (KE)?

√2KE/m

what else is the equation for velocity?

distance / time

how do u calculate the mass in g of 1 atom of an element?

RAM / 6.022 × 10²³

what is an electron gun?

a hot wire filament with a current running through it that emits electrons