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ghoran
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what is the relative mass of a proton
1
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whats the relative charge of a proton
+1
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whats the relative mass of a nuetron
1
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whats the relative charge of a nuetron
0
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whats the relative mass of an electron
negligible
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whats the relative charge of an electron
-1
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whats the importance of protons
- number of protons determines the type of atom
- e.g. all atoms with 6 protons are carbon
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whats the importance of nuetrons
determines the atomic mass (alond with protons) and gives rise to different isotopes , which affects physical properties
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whats the importance of electrons
number of electrons affects how an atom reacts and forms bonds and determines chemical properties
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whats the symbol for atomic numbder
Z
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whats the symbol for mass number
A
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atomic number =
number of protons in an atom
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mass number =
number of protons and nuetrons in an atom
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isotopes
atoms with the same number of protons but different numbers of neutrons
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number of neutrons =
mass number - atomic number
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realative atomic mass
- average mass of an atom
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- 1/12th mass of an atom of Carbon-12
or
- average mass of isotopes of an element
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- 1/12th mass of one atom of carbon-12
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the mass number is not shown with the symbol of an element the large number on the top is
the relative atomic mass
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how can we identify substances
use mass spectrometer
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how does a mass spectrometer work
- there are four main stages
- 1) ionistaion
- the sample is bombarded with high speed electrons from an electron gun . this knocks an electron out of the sample , forming positive ions .
- 2) acceleration
- the ions are accelerated by an electric field
- 3) deflection
- the ions are deflected by a magnetic field , the strength of the field can be varied to deflect ions more or less
- 4) detection
- the ions hit a detector where they generate a tiny current , the size of the current is proportional to the abundance of the ion
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how can ions of different masses be made to enter the detector
by varying the strength of electric and magnetic fields
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how does the desing of the mass spec help prevent the sample being analysed from colliding with particles found in the air
its in a vacuum
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what is the ion with the largest mass that is detected in the mass spec called
molecular ion or parent ion
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why do we get a range of fragements formed when we use a molecular sample in the mass spec
because as the ions are formed from the sample they may fall apart or fragment
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what physical state are the samples in when they are analysed by mass spec
they start as liquid and vapourise so by the time they are being analysed they are gases
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explain detection in more detail
- the accelerated ions are deflected using a magnetic field and the amount of deflection is greater when
- - the mass of the positive ions is less
- - the charge of the positive ions is greater
- - the velocity of the positive ion sis less
- - the strength of the magnetic field is greater
- if all the ions are travelling at the same velocity and carry the same charge , the amount of deflection in a given magentic field depends on the mass of the ion . for a given magnetic field , only ions with a particular realtive mass (m) to charge (z) ratio - the m/z value - are deflected enough to reach the detector
- the magnetic field is slowly increased during the operation of the mass spec until all of the different ions in the sample have been detected one after the other .
- ions with too small an m/z value are deflected too much and ions with a large m/z value aren't deflected enough
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explain detection in more detail
- ions that reach the detector cause electrons to be released in an ion-current detector - the number of electrons released (and hence current produced) is proportional to the number of ions striking the detector . the detector is linked to an amplifier and then to a recorder which converts the current into a peak which is shown in the mass spectrum
- in each sample , each isotope produces a peak with its own m/z value
- assuming that all the peaks in the spectrum are for unipositive ions (z=1) , the m/z value will be the same as the ion's mass number
- the height of each peak is proportional to the relative abundance of that isotope in the element
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the mass spectrum of an element can be used to
calculate the relative atomic mass of an element . each isotope of the element will produce a separate peak in the mass spectrum , so by using the m/z value and the peak height for each different isotopes , the relative atomic mass of the element can be calculated
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key ideas - the mass spec identifies
the different isotopes present in an element
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key ideas - the isotopes are ionised , and the resulting positive ions undergo
deflection in a magnetic field
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key ideas - in a mass spectrum , each isotope produces
a separate peak with its own m/z value . The peak height is proportional to the relative abundance of that isotope in the element
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why is the sample in the mass spec ionised
so that positive ions are formed so the sample can be accelerated and hence then deflected and detected too
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how to calculate Ar from a mass spectrum
- Ar = average mass of isotopes of an element
- x abundance
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- abundance of isotope
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if an element exists as diatomic molecules , then the mass spectrum will contain
- peaks for both the separate atoms and for the molecules . for example the mass spectrum of chlorine gas will have peaks
- for
- Cl2+with m/z values of
- 1) 17.5 - doubly charged ions
- 2) 18.6 - doubly charged ions
- Cl+ with m/z values of
- 1) 35
- 2) 37
- and Cl2 with m/z values of
- 1) 70 (Cl35 Cl35+)
- 2) 72 (Cl35 , Cl37+)
- 3) 74 (Cl37 Cl37+) .
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how to calculate the Ar of chlorine if this data were shown on a mass spectrum
- the peak at mass 35 has 3 times the height of the one of mass 37 because there are three times as many of these . So we have 75% Cl-35 and 35% Cl-37 so
- (35 x 75) + (37 x 20
- ---------------------
- 100
- = 35.5 = Ar of chlorine
- or (35 x 3) + (37 x 1)
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- 4
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how to know which peak to use for diatomic molecules to calculate Ar
- don't use the ones with small peak - doubly charged ions
- dont use ones with massive peak - molecule
- use middle one that is 1/2 of one and 2 times the other
- if there's only two use smaller one
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during ionisation , a small number of ions with a 2+ charge are formed . Because of their double charge how do you expect them to be accelerated and deflected
they will be accelerated and deflected more
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doubly charged ions behave like ions with half the mass of a singly charged ion . However since double ionisation is rare , these peaks have
very small abundances
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all elements have a characteristic pattern that shows the relative abundances of their isotopes . this can be used identify any particular element . High resolution mass specs can
measure the masses of atoms to several dp . this allows us to identify the elements by exact masses of their atoms that , with the exception of carbon-12 , aren't exactly whole numbers
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mass spectrometry can be used in :
- space probes to determine the composition of the atmosphere on distant moons and planets
- forensic science to identify compounds found in trace evidence
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high resolution mass spectrometry enables us to
- distinguish compounds with the same mr to to 1 dp
- for example
- hexane = 86.1 or 86.1106 to 4dp
- pentanal - 86.1 or 86.0739 to 4dp
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when electrons are fired at molecules during mass spec
bonds can be broken . this cause fragmentation of the molecular ion , leading to peaks with a lower m/z value than the Mr . these fragments can help us identify the molecule
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both propanone and propanol have the same Mr , even using high resolution mass spec because they contain the same number and type of atoms . therefore we have to use
the fragmentation pattern to tell them apart
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how can propanol be fragmented
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- oxygen gas has the following peaks
- O+ with m/z values
- 1) 15 - doubly charged
- 2) 16 - doubly charged
- O2 with m/z values
- 1) 30 - 15 , 15
- 2) 31- 15 , 15
- 3) 32 - 16 , 16
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the measured relative atomic mass of oxygen is 16.5 in a particular sample . which one of the following is false
each atom of oxygen contains 8 protons
the relative molecular mass of this sample is 33
there are at least two isotopes of oxygen in this sample
the mass spectrum of this sample shows a peak for the 0+ ion at m/z value 16.5
the mass spectrum of this sample shows a peak for the O+ ion at m/z value 16.5
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the number of neutrons in the nucleus of a cation influences
- the m/z ration of the ion
- the deflection of an ion when moving in a magnetic field
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in mass spec under the same conditions N-14+ and N-15+ ions follow different paths . 1) state the property of these ions that causes them to follow different paths 2) state one change in the operation of the mass spec that will change the path of an ion
- 1) they have different m/z values
- 2) change the strength of the magnetic field
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do isotopes of the same element have different chemical properties
no because they have the same number of electrons and electrons are responsible for chemical properties
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