Download Protons, neutrons and electrons Isotopes Atomic mass units and

Nuclear Structure
Protons, neutrons and electrons
particle
proton
neutron
electron
relative charge
+1
0
-1
relative mass
1 atomic mass unit
1 atomic mass unit
negligible mass
§
Protons and neutrons make up the nucleus, at the centre of the atom, where virtually
all the mass of the atom is concentrated. A proton and a neutron have near enough
equal mass to one another.
§
Electrons orbit the nucleus in shells of various energies, and have negligible mass.
As protons in the nucleus are positively charged, they attract an equal number of electrons
into shells surrounding the nucleus. The electronic structure of an atom determines its
chemical reactions, so the number of protons determines the chemical identity of an atom.
Isotopes
Isotopes are atoms that have the same number of protons but a different number of
neutrons. Their chemical reactions are the same, and they belong to the same element.
Most elements exist naturally as a mixture of isotopes.
§
§
The mass number is the sum of protons and neutrons in the nucleus
The atomic number is the number of protons in the nucleus.
So you could say that isotopes are atoms with the same atomic number but different mass
numbers.
Atomic mass units and relative isotopic mass
It would make sense to make one atomic mass unit the mass of something simple, like a
1
H atom, but that would be too obvious! Chemists use one-twelfth the mass of a 12C atom
instead, which if you think about it, amounts to the same thing. As there are six protons
and six neutrons in this nucleus, dividing its mass by 12 gives you the mass of a proton or
neutron. However, it really is only the mass of a proton or a neutron in 12C atoms. Protons
and neutrons actually have very slightly different masses from one element’s nucleus to
another. So a proton in a 12C atom does not have exactly the same mass as one in a 1H. It
very nearly does, but its not quite exactly the same. Don’t worry about why!
So, relative isotopic mass is the mass of an atom of an individual isotope, relative to
one-twelfth the mass of a 12C atom.
In simple terms, relative isotopic mass is how many times more mass an atom has than a single
proton or neutron from a 12C atom. The relative isotopic mass of helium is almost exactly 4
because there are two protons and two neutrons in the nucleus. It is not totally exactly
four though because the protons and neutrons in helium nuclei do not have the exact same
mass as protons or neutrons in 12C atoms.
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Nuclear Structure
Relative Atomic Mass and the mole
As mentioned, most elements exist naturally as a mixture of isotopes. Because the
chemical reactions of isotopes are the same, chemists rarely have any interest in separating
them. What chemists want to know, is the average mass of an atom, taking the natural
abundances of its isotopes into account. This is so they can weigh out an accurate amount
of a substance.
So, relative atomic mass is the average mass of an atom, taking the natural abundances
of its isotopes into account, relative to one-twelfth the mass of a 12C atom.
§
Chlorine exists naturally as a mixture of 75% 35Cl and 25% 37Cl.
§
The average mass of a chlorine atom is 75/100 x 35 + 25/100 x 37 = 35.5.
One mole of substance is the amount that weighs the same in grams as the relative mass of
the substance, e.g. the number of atoms in 12 g of 12C. The actual number is 6 x 1023, and
this is Avogadro’s number.
Note that relative atomic masses have no units. For chlorine, 35.5 says that the average
chlorine atom has 35.5 times more mass than a proton or neutron in a 12C atom.
However, in reality the RAM or RMM is also the number of grams in one mole of
substance. In this case you will learn about molar mass rather than relative masses. The
molar mass of chlorine atoms is 35.5 g mol-1, whereas the RAM is simply 35.5 (no units).
Atom
1
1
4
2
12
6
35
17
37
17
H
He
C
Cl
Cl
Amount
Mass
6 x 1023 atoms
1 gram
6 x 1023 atoms
4 grams
6 x 1023 atoms
12 grams
6 x 1023 atoms
35 grams
6 x 1023 atoms
37 grams
Remember, chlorine exists naturally as a mixture of 75% 35Cl and 25% 37Cl, so:
§
§
35 grams of naturally occurring chlorine atoms would be slightly less than one mole
37 grams of naturally occurring chlorine atoms would be slightly more than one mole
The average mass of a chlorine atom is 35.5, so if you have 35.5 grams of naturally
occurring chlorine atoms, you would have one mole of chlorine atoms.
(Ideally, in the calculations above the relative isotopic masses should be used. However,
they are so similar to the mass numbers that the mass numbers can be used in their place)
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Nuclear Structure
To summarise:
§
One atomic mass unit is one-twelfth the mass of a 12C atom.
§
Relative isotopic mass is the mass of an atom of an individual isotope, relative to onetwelfth the mass of a 12C atom.
In RIM the I stands for isotope but you can think of it as “I for individual”.
§
Relative atomic mass is the average mass of an atom, taking the natural abundances of
its isotopes into account, relative to one-twelfth the mass of a 12C atom.
The A in RAM stands for atomic but you can think of it as “A for average”.
§
One mole is the number of atoms in 12g of 12C atoms.
§
One mole of any substance has a mass that is equal to the substance’s relative mass.
Relative Molecular Mass
In a molecule, such as CH3CH2Cl there are atoms present from different elements, in this
case carbon, hydrogen and chlorine. In addition, the chlorine atoms occur as a mixture of
35
Cl and 37Cl, the carbon occurs as 12C and about 1% 13C. The hydrogen exists almost
entirely as 1H but there are tiny amounts of both 2H and 3H.
However, the RAM values for each element already take the different isotopes of each
element into account.
Element
RAM (3 s.f.)
C
12.0
The RMM of CH3CH2Cl is therefore
H
1.00
Cl
35.5
2 x 12.0 + 5 x 1.00 + 1 x 35.5 = 64.5 (no units).
The molar mass of CH3CH2Cl therefore 64.5 g mol-1
So, relative molecular mass is the weighted sum of the individual relative atomic
masses of its atoms.
In this context the word “molecule” is used quite loosely. If the compound were MgCl2 the
method is exactly the same, even though MgCl2 is not a molecular substance, it is ionic
and consists of Mg2+ cations and Cl- anion.
The RMM of MgCl2 is
24.3 + 2 x 35.5 = 94.3 (no units)
The molar mass of MgCl2 is
94.3 g mol-1
Sometimes the term formula mass is used to avoid having to remember when to say
atomic mass and molecular mass.
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Nuclear Structure
Mass spectrometry of atomic species, such as Ne atoms
Neon is a mainly a mixture of 20 Ne and 22 Ne . If such a mixture is injected into a mass
spectrometer, the following processes would occur:
§
The atoms are ionised by bombardment with high-energy electrons. This makes
them positively charged.
20
Ne + e- è
20
Ne + + 2e-
22
Ne + e- è
22
Ne + + 2e-
§
The positively charged species are accelerated by attraction towards negatively
charged plates. This has the effect of focussing them into a narrow beam and bringing
them all up to the same velocity (see diagram later in these notes).
§
The positively charged species are deflected by a magnetic field. Because they are
travelling at the same velocity, the ease with which they can be deflected is determined
only by their mass to charge ratio. Also, because they are accelerated out of the
ionisation chamber as soon as they become positively charged, they are unlikely to
have any charge other than +1. Effectively, mass to charge ratio is just the same as
mass. The heavier particles need a stronger magnetic field to deflect them into the
detector, whereas the lighter particles need a weaker magnetic field to deflect them
into the detector.
§ The particles are detected as a small current, because they are charged. A computer
counts the number of particles detected per second and plots them against their mass
to charge ratio.
20
Ne+
Abundance
9 to 1 ratio of
peak heights
22
Ne+
16
§
18
20
22
24
m/e ratio
From the graph, you can calculate the relative atomic mass of Neon to be:
9/10 x 20 + 1/10 x 22 = 20.2
NB
The mass spectrometer is under a high vacuum; otherwise none of the process
would be possible!
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Nuclear Structure
Mass spectrometry of diatomic species, such as Cl2 molecules
Chlorine exists as Cl2 molecules, the atoms are made up of 75% 35Cl and 25% 37Cl.
35
Cl-35Cl+
Abundance
9 to 6 to 1
height ratio
35
Cl-37Cl+
3 to 1 height
ratio
35
Cl+
37
Cl+
37
Cl-37Cl+
35 37
70 72 74
m/e ratio
There are three different types of Cl2 molecule, each with its own peak in the mass
spectrum:
§
The chance of a
§
35
37
The chance of a 17
Cl - 17
Cl + species existing is 3/4 x 1/4 x 2 = 6/16
(there is double the chance because it can be made from a 35 then a 37 or from a 37 then a 35. If
you don’t get this then don’t worry, its really maths, not chemistry)
§
The chance of a
35
Cl - 17
Cl + species existing is 3/4 x 3/4 = 9/16
35
17
37
Cl - 17
Cl + species existing is 1/4 x 1/4 = 1/16
37
17
Therefore the peaks for the above species exist in a 9:6:1 ratio
In addition to these molecular species, some chlorine molecules break down into atoms, so
35
37
there are two additional peaks for 17
Cl + and 17
Cl + . Due to the isotopic abundances of Cl
atoms, these peaks exist in a 3:1 ratio.
Bromine atoms exist as 50% 79 Br and 50% 81 Br , and bromine molecules are also diatomic
like those of chlorine. A mass spectrum of bromine is shown below:
Abundance
1 to 1 height
ratio
79
Br+
79
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79
81
81
Br+
81
+
Br- Br
79
Br-79Br+
81
158 160 162
1 to 2 to 1
height ratio
Br-81Br+
m/e ratio
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Nuclear Structure
Mass spectrometry of simple molecules, such as CH3CH2F
When the mass spectrum a molecular substance is obtained, some of the molecules break
up into fragments that, if charged, can be detected.
For example, a mass spectrum of CH3CH2F is shown below:
Abundance
CH3+
F+
CH3CH2+
15 19
29
CH2F+
33
CH3CH2F+
48
m/e ratio
The peak with the highest mass to charge ratio (furthest to the right) is called the
molecular ion or parent ion. Because this peak is the whole molecule, its mass to charge
ratio is also the relative molecular mass of the substance. Mass spectrometry is therefore a
useful way to quickly find out the RMM of an unknown substance.
The different heights of the peaks in the above spectrum are not important, you are not
expected to guess at how likely the various fragments are to occur compared to each other.
The scheme below shows an example of fragmentation:
CH3CH2F
è
CH3CH2F+
è
CH3+ and CH2F, or
CH3CH2F
è
CH3CH2F+
è
CH3 and CH2F+
For example, if the molecule lost an electron from the C-C bond when bombarded, this
would make the bond weak. When it breaks, the remaining electron in it goes either with
the CH2F fragment, or with the CH3 fragment. Whichever fragment doesn’t get the electron
is the fragment that is charged, and therefore the fragment that gets detected by the mass
spectrometer.
§
Usually, the electron goes one way some of the time, the other the rest of the time, so
usually both fragments are actually detected.
§
Sometimes, the fragmentation occurs in a certain way only, so sometimes only one
fragment is detected. This happens if one of the fragments is much more stable, or less
stable, than the other.
Usually you should assume that the peak furthest to the right gives the RMM. If this is not
the case, the question will make it fairly obvious.
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Nuclear Structure
Other points about mass spectrometry
§
When suggesting fragments, you are not allowed to re-combine pieces of the
molecule. Only suggest fragments that you could make by breaking up the molecule
bit by bit.
•
If you break two bonds to get a fragment, you do not need to put a higher charge on it.
§
Always give structural formula for your fragments, e.g. CH3CH2O+ not C2H5O+.
C2H5O+ could also mean CH3OCH2+ for example.
•
Always put a plus charge on anything you suggest for a peak. Nothing can be
detected in mass spectrometer without a plus-charge!
§
It is possible, but unlikely, that the molecular ion might be absent altogether from the
spectrum. If so, this would be because the entire molecule fragmented, perhaps
because the resulting fragments are more stable. This always occurs with polymers, so
you cannot use mass spectrometry to measure the RMM of polyethene, for example.
§
Many peaks in a mass spectrum have satellite peaks associated with them with lower
mass to charge ratios. This is due to further loss of H atoms from the fragment.
§
Any peak due to a species with carbon atoms in it will also have very low intensity
satellite peaks with one unit greater mass to charge ratio. These are due to the small
amount of the 136 C isotope that is present in naturally occurring carbon.
For example, a peak at 15 for a CH3+ fragment might actually look more like as shown
below (peak at 16 exaggerated for clarity)
Abundance
15
•
§
Peak at 16 due to
13
CH3+
§
Peak at 15 due to
12
CH3+
§
Peak at 14 due to
12
CH2+
§
Peak at 13 due to
12
CH+
mass/charge
Be able to label a diagram of a mass spectrometer (see next page). If you have enough
time, memorise how to draw one as well.
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Nuclear Structure
The mass spectrometer
Positive plate
Sample in
Negative plates
Electromagn
et
Positive
plate
Hot tungsten
filament
All under high
vacuum
Detector
Think about the following questions:
§
Why do many samples not need to be heated, even if they are liquids or solids?
The apparatus is under a high vacuum, and liquids boil more easily when the external
pressure is low.
§
Which parts of the spectrometer cause the particles to become charged?
The tungsten filament and positive plate constitute the “electron gun” and there is a stream
of fast moving electrons between them, which knock electrons off the sample atoms.
§
Why to the particles need to become charged?
Without a charge they cannot be accelerated, deflected or detected.
§
Why are there small holes in the accelerating negative plates?
To focus the moving ions into a narrow beam.
§
Why is there a small hole in the detector?
So that the magnetic field can deflect particles with a given mass to charge ratio into the
detector.
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