Nuclear Structure. particle relative charge relative mass proton +1 1 atomic mass unit neutron 0 1 atomic mass unit electron -1 negligible mass

Protons, neutrons and electrons

particle relative charge relative mass

proton +1 1 atomic mass unit

neutron 0 1 atomic mass unit

electron -1 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

C 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

C atoms. Protons and neutrons actually have very slightly different masses from one element’s nucleus to another. So a proton in a

C atom does not have exactly the same mass as one in a

H. 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

C atom.

In simple terms, relative isotopic mass is how many times more mass an atom has than a single

proton or neutron from a

C 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

C atoms.

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

C atom.

§ Chlorine exists naturally as a mixture of 75%

Cl and 25%

Cl.

§ 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

C. The actual number is 6 x 10

, 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

C 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

, whereas the RAM is simply 35.5 (no units).

Atom Amount Mass

1

1

H ^{6 x 10}

^{atoms 1 gram}

2

4

He ^{6 x 10}

^{atoms 4 grams}

6

12

C ^{6 x 10}

^{atoms 12 grams}

17

35

Cl ^{6 x 10}

^{atoms 35 grams}

17

37

Cl ^{6 x 10}

^{atoms 37 grams}

Remember, chlorine exists naturally as a mixture of 75%

Cl and 25%

Cl, 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)

To summarise:

§ One atomic mass unit is one-twelfth the mass of a

C atom.

§ Relative isotopic mass is the mass of an atom of an individual isotope, relative to one- twelfth the mass of a

C 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

C 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

C 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 CH

CH

Cl 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

Cl, the carbon occurs as

C and about 1%

C. The hydrogen exists almost entirely as

H but there are tiny amounts of both

H and

H.

However, the RAM values for each element already take the different isotopes of each element into account.

Element C H Cl

RAM (3 s.f.) 12.0 1.00 35.5

The RMM of CH

CH

Cl is therefore 2 x 12.0 + 5 x 1.00 + 1 x 35.5 = 64.5 (no units).

The molar mass of CH

CH

Cl therefore 64.5 g mol

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 MgCl

the method is exactly the same, even though MgCl

is not a molecular substance, it is ionic and consists of Mg

cations and Cl

anion.

The RMM of MgCl

is 24.3 + 2 x 35.5 = 94.3 (no units) The molar mass of MgCl

is 94.3 g mol

Sometimes the term formula mass is used to avoid having to remember when to say

atomic mass and molecular mass.

Mass spectrometry of atomic species, such as Ne atoms

Neon is a mainly a mixture of

Ne and

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

Ne

+ 2e

22

Ne + e

è

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.

Abundance

Ne

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

Mass spectrometry of diatomic species, such as Cl

molecules

Chlorine exists as Cl

molecules, the atoms are made up of 75%

Cl and 25%

Cl.

Abundance

Cl-

Cl

35

Cl-

Cl

9 to 6 to 1 height ratio 3 to 1 height

ratio

35

Cl

37

Cl

Cl-

Cl

35 37 70 72 74 m/e ratio There are three different types of Cl

molecule, each with its own peak in the mass

spectrum:

§ The chance of a

Cl -

Cl

species existing is 3/4 x 3/4 = 9/16

§ The chance of a

Cl -

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

Cl -

Cl

species existing is 1/4 x 1/4 = 1/16 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 there are two additional peaks for

Cl

and

Cl

. Due to the isotopic abundances of Cl atoms, these peaks exist in a 3:1 ratio.

Bromine atoms exist as 50%

Br and 50%

Br , and bromine molecules are also diatomic like those of chlorine. A mass spectrum of bromine is shown below:

Abundance

79

Br-

Br

1 to 2 to 1 height ratio 1 to 1 height

ratio

79

Br

Br

Br-

Br

Br-

Br

79 81 158 160 162 m/e ratio

Mass spectrometry of simple molecules, such as CH

CH

F

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 CH

CH

F is shown below:

Abundance

CH

CH

CH

F

F

CH

CH

F

CH

15 19 29 33 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:

CH

CH

F è CH

CH

F

è CH

and CH

F, or CH

CH

F è CH

CH

F

è CH

and CH

F

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 CH

F fragment, or with the CH

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.

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. CH

CH

O

not C

H

O

. C

H

O

could also mean CH

OCH

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

C isotope that is present in naturally occurring carbon.

For example, a peak at 15 for a CH

fragment might actually look more like as shown below (peak at 16 exaggerated for clarity)

Abundance

15 mass/charge

§ Peak at 16 due to

CH

§ Peak at 15 due to

CH

§ Peak at 14 due to

CH

§ Peak at 13 due to

CH

• 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.

The mass spectrometer

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.

Detector

Electromagn et

Negative plates Positive

plate

Hot tungsten filament Sample in

All under high vacuum

Positive plate