1. Atomic Structure Flashcards
Relative mass and charge of proton,neutron,electron
proton - mass= 1, charge= +1
neutron - mass = 1, charge = 0
electron - mass = 1/1840, charge =-1
What is relative atomic mass, Ar ?
the average mass of an atom of an element on a scale where an atom of carbon-12 is 12
What is relative isotopic mass ?
The mass of an atom of an isotope of an element on a scale where an atom of carbon-12 is 12.
what is the relative molecular ( formula ) mass, Mr ?
sum of all relative atomic masses of the atom in one formula unit
What does a mass spectrometer do ?
tells you
- relative atomic mass
- relative molecular mass
- relative isotopic abundance
- and used to identify elements
What are the 4 key stages in time of flight mass spectrometry?
- ionisation
- acceleration
- Ion drift
- detection
What are the two key ways a sample can be ionised?
- electron impact ( or electromagnetic ionisation )
- electrospray ionisation
When is electron impact ionisation used instead of electrospray ionisation ?
It’s used for elements and substances which have a lower molecular mass
what happens during electron impact ionisation?
- sample is vaporised to a gaseous state
- electron gun is fired with high energy electrons
- this knocks electrons off the sample to form 1+ ions as they lose an electron
X (g) ——> X+ (g) + e-
what happens in electrospray ionisation?
- sample dissolved and in a volatile state
- sample goes into a fine hypodermic needle with a positive terminal at high voltage
- sample gains a proton as it leaves terminal
X (g) + H+ ——> XH+ (g)
Why is electrospray ionisation used instead of electron impact ionisation?
It’s used for substances which have a higher molecular mass
- called a soft ionisation technique as fragmentation is unlikely
What happens during stage 2, acceleration ?
- the 1+ ions formed from either method are accelerated through by an electric field
- they are all accelerated to have the same kinetic energy
What happens in stage 3, ion drift/flight tube?
- the ions enter a region with no electric field so they just drift through it
- lighter ions will travel faster so have a shorter time of flight
what happens in stage 4, detection ?
- once passed through the positive 1+ ions will hit a negatively charged detector plate
- as they hit this they gain an electron
- this gaining of electron discharges ion and a current is produced
- the size of the current is proportional to the abundance of the ions hitting the plate
- detector is connected to computer that produces mass spectrum
Key equation in time of flight mass spectrometry?
KE = 1/2 mv^2
Key equation for finding time of flight
t = d/v
t= time of flight (s)
d= length of flight tube (m)
v= velocity (m s-1)
What’s an isotope?
Atoms with the same number of protons but different number of neutrons
How many electrons can each shell hold?
Shell 1 - 2 electrons
Shell 2 - 8 electrons
Shell 3 - 18 electrons
Shell 4 - 32 electrons
In shell 1 what sub-shells are there?
Shell 1 = 2 electrons - S
In shell 2 what sub-shells are there?
Shell 2 = 8 electrons
S - 2 electrons
P - 6 electrons
In Shell 3 what sub-shells are there?
Shell 3 = 18 electrons
s - 2 electrons
p - 8 electrons
d - 10 electrons
In shell 4 how many sub-shells are there?
Shell 4 = 32 electrons
s - 2 electrons
p - 6 electrons
d - 10 electrons
f - 14 electrons
When is 4s added into electron configuration?
Always after 3p and before 3d
e.g. 1s2s2p3s3p4s3d4p
electron configuration for Mg
Mg = 12 electrons
1s^2 2s^2 2p^6 3s^2
