Download Fundamental Particles

Fundamental Particles
Sub-atomic particles
Atoms are made from three sub-atomic particles. These are called protons, neutrons, and
electrons. The table shows the mass and electric charge of each sub-atomic particle.
It is difficult to understand the properties of sub-atomic particles if you look at their
actual masses and charges. Instead, it is often more useful to look at their masses
and charges compared to the proton. These numbers are called the relative mass
and the relative charge. The table shows the relative mass and relative charge of
each sub-atomic particle.
Fundamental Particles
The nuclear atom
The word ‘nuclear’ means anything to do with the nucleus. The nuclear atom has a
nucleus at its centre, with electrons arranged in energy levels around it. The nucleus is
made of protons and neutrons (also called nucleons). Most of the mass of the atom is
found in the nucleus. This is because the protons and neutrons in the nucleus have
much more mass than the electrons.
Any atom has an equal numbers of protons and electrons. This means that atoms are
neutral overall. For example, fluorine atoms each have nine protons and nine
electrons. The nine positive charges carried by the protons in the nucleus are
balanced by the nine negative charges carried by the electrons. Because neutrons are
neutral, there can be any number of neutrons in the nucleus without making any
difference to the overall charge of the atom.
Fundamental Particles
Atomic number and mass number
The atomic number of an atom is the number of protons in its nucleus. It is also called
the proton number. The atomic number is shown by the symbol Z. Every element has
its own unique atomic number. All the atoms of a particular element have the same
atomic number.
The mass number of an atom is the number of sub-atomic particles in its nucleus. The
mass number is shown by the symbol A. It is the number of neutrons added to the
number of protons. The full symbol for an atom is written like this:
The atomic number is shown as a subscript and the mass number as a superscript. You
can work out the number of neutrons in an atom by subtracting the atomic number
from the mass number. So for , the number of neutrons is 19 - 9 = 10. The number
of protons (and the number of electrons) in the atom is 9.
Fundamental Particles
Isotopes
Isotopes are atoms that have the same number of protons but different numbers of
neutrons in their nuclei. For example,
and
are both isotopes of chlorine.
Their atoms each contain 17 protons. But every
atom has 35 - 17 = 18 neutrons,
and every
atom has 37 - 17 = 20 neutrons. The different isotopes of an element
are chemically identical to each other because they contain the same number of
protons and electrons.
Electron Arrangements
The first shell, which is closest to the nucleus fills with electrons before the second
shell and so on.
If we know the number of protons in an atom, we would then also know the number
of electrons it has, this is because an atom is neutral. Knowing this information
allows us to draw an electron diagram for any element.
Carbon has six electrons. Four of them are in the outer shell.
Sulphur has 16 electrons. It has six in its outer shell. A good
habit to get into is to space the first four electrons evenly
around the shell and then pair up the electrons one at a
time.
Electron Arrangements
We can also draw electron diagrams of ions, as long as we know the number of
electrons.
The sodium atom has 11 electrons but a sodium ion only has
10. This means it now has a positive charge as there are still
11 protons, giving it an overall charge of +1.
Similarly an oxygen atom has 8 electrons but its ion has 10.
This means it now has a negative charge as there are 10
electrons compared to its 8 protons, giving it an overall
charge of -2.
We can write electron diagrams in shorthand. There are written in shell order, so
first shell first, then second etc. Each one is separated by a comma. So for carbon
we would write 2, 4 and for sulphur 2, 8, 6.
Mass Spectrometer
Atoms can be deflected by magnetic fields - provided the atom is first turned into an ion.
Electrically charged particles are affected by a magnetic field although electrically
neutral ones aren't. The actual process is split into four parts.
Stage 1: Ionisation
The atom is ionised by knocking one or more electrons off to give a positive ion. This is
true even for things which you would normally expect to form negative ions (chlorine,
for example) or never form ions at all (argon, for example). Mass spectrometers always
work with positive ions.
Stage 2: Acceleration
The ions are accelerated so that they all have the same kinetic energy.
Stage 3: Deflection
The ions are then deflected by a magnetic field according to their masses. The lighter
they are, the more they are deflected. The amount of deflection also depends on the
number of positive charges on the ion - in other words, on how many electrons were
knocked off in the first stage. The more the ion is charged, the more it gets deflected.
Stage 4: Detection
The beam of ions passing through the machine is detected electrically.
Mass Spectrometer Diagram
1. The need for a
vacuum
It's important that
the ions produced
in the ionisation
chamber have a
free run through
the machine
without hitting air
molecules.
2. The vaporised
sample passes into
the ionisation
chamber. The
electrically heated
metal coil gives off
electrons which are
attracted to the
electron trap which
is a positively
charged plate.
Stage 1: Ionisation
3. The particles in the
sample (atoms or
molecules) are therefore
bombarded with a
stream of electrons, and
some of the collisions
are energetic enough to
knock one or more
electrons out of the
sample particles to
make positive ions.
5. These positive
ions are
persuaded out
into the rest of
the machine by
the ion repeller
which is another
metal plate
carrying a slight
positive charge.
4. Most of the positive
ions formed will carry
a charge of +1
because it is much
more difficult to
remove further
electrons from an
already positive ion.
Stage 2: Acceleration
The positive ions are repelled away from the
very positive ionisation chamber and pass
through three slits, the final one of which is at
0 volts. The middle slit carries some
intermediate voltage. All the ions are
accelerated into a finely focused beam.
1. Different ions are
deflected by the
magnetic field by
different amounts. The
amount of deflection
depends on:
•the mass of the ion.
Lighter ions are
deflected more than
heavier ones.
•the charge on the ion.
Ions with 2 (or more)
positive charges are
deflected more than
ones with only 1 positive
charge.
Stage 3: Deflection
2. These two factors are combined into the
mass/charge ratio. Mass/charge ratio is
given the symbol m/z (or sometimes m/e).
For example, if an ion had a mass of 28 and
a charge of 1+, its mass/charge ratio would
be 28. An ion with a mass of 56 and a
charge of 2+ would also have a mass/charge
ratio of 28.
5. Assuming 1+ ions,
stream A has the
lightest ions, stream B
the next lightest and
stream C the heaviest.
Lighter ions are going
to be more deflected
than heavy ones.
3. In the last diagram, ion
stream A is most deflected - it
will contain ions with the
smallest mass/charge ratio.
Ion stream C is the least
deflected - it contains ions
with the greatest
mass/charge ratio.
4. It makes it simpler to
talk about this if we
assume that the charge
on all the ions is 1+.
Most of the ions
passing through the
mass spectrometer will
have a charge of 1+, so
that the mass/charge
ratio will be the same
as the mass of the ion.
Stage 4: Detection
Only ion stream B
makes it right
through the
machine to the
ion detector. The
other ions collide
with the walls
where they will
pick up electrons
and be
neutralised.
Eventually, they
get removed from
the mass
spectrometer by
the vacuum
pump.
A flow of electrons
in the wire is
detected as an
electric current
which can be
amplified and
recorded. The more
ions arriving, the
greater the current.
When an ion hits the metal box, its charge is
neutralised by an electron jumping from the metal on
to the ion (right hand diagram). That leaves a space
amongst the electrons in the metal, and the electrons
in the wire shuffle along to fill it.
What about streams A + C
Detecting the other ions
How might the other ions be detected - those in streams A and C which have been lost
in the machine?
Remember that stream A was most deflected - it has the smallest value of m/z (the
lightest ions if the charge is 1+). To bring them on to the detector, you would need to
deflect them less - by using a smaller magnetic field (a smaller sideways force).
To bring those with a larger m/z value (the heavier ions if the charge is +1) on to the
detector you would have to deflect them more by using a larger magnetic field.
If you vary the magnetic field, you can bring each ion stream in turn on to the detector
to produce a current which is proportional to the number of ions arriving.
The mass of each ion being detected is related to the size of the magnetic field used to
bring it on to the detector. The machine can be calibrated to record current (which is a
measure of the number of ions) against m/z directly. The mass is measured on the 12C
scale.
Mass Spectrometer Example
The output from
the chart recorder
is usually
simplified into a
"stick diagram".
This shows the
relative current
produced by ions
of varying
mass/charge ratio.
You may find diagrams in which the
vertical axis is labelled as either
"relative abundance" or "relative
intensity". Whichever is used, it
means the same thing. The vertical
scale is related to the current
received by the chart recorder - and
so to the number of ions arriving at
the detector: the greater the current,
the more abundant the ion.
That means that
molybdenum consists
of 7 different isotopes.
Assuming that the ions
all have a charge of 1+,
that means that the
masses of the 7
isotopes on the
carbon-12 scale are 92,
94, 95, 96, 97, 98 and
100.
As you will see from the
diagram, the commonest ion
has a mass/charge ratio of 98.
Other ions have mass/charge
ratios of 92, 94, 95, 96, 97 and
100.
Mass Spectrometer - Boron
The two peaks in the mass spectrum
shows that there are 2 isotopes of boron with relative isotopic masses of 10 and 11
on the 12C scale.
The abundance of the isotopes
The relative sizes of the peaks gives you a
direct measure of the relative
abundances of the isotopes. The tallest
peak is often given an arbitrary height of
100 - but you may find all sorts of other
scales used. It doesn't matter in the least.
You can find the relative abundances by
measuring the lines on the stick diagram.
In this case, the two isotopes (with their
relative abundances) are:
boron-10 23
boron-11 100
Mass Spectrometer - Boron
Working out the relative atomic mass
The relative atomic mass (RAM) of an element is given the symbol Ar and is
defined as:
The relative atomic mass of an element is the weighted
average of the masses of the isotopes on a scale on
which a carbon-12 atom has a mass of exactly 12 units.
A "weighted average" allows for the fact that there won't be equal amounts of the
various isotopes. The example coming up should make that clear.
Suppose you had 123 typical atoms of boron. 23 of these would be 10B and 100
would be 11B.
The total mass of these would be (23 x 10) + (100 x 11) = 1330
The average mass of these 123 atoms would be 1330 / 123 = 10.8 (to 3 significant
figures).
10.8 is the relative atomic mass of boron.
Notice the effect of the "weighted" average. A simple average of 10 and 11 is, of
course, 10.5. Our answer of 10.8 allows for the fact that there are a lot more of the
heavier isotope of boron - and so the "weighted" average ought to be closer to
that.