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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.