Elements of life.

Atomic Structure

Electrons are structured 2.8.8 they have a negative charge of –1 and weigh about 1/1837ths of a proton. Protons have a charge +1 and weigh the same as neutrons, which are not charged. The atomic number is the number of protons in the nucleus which is the same as the number of electrons (making the atom neutral and of negligible weight.) Number of neutrons equal the mass number minus the atomic number. All atoms want to reach the most stable state, with a full outer shell of electrons, with enough energy they will lost electrons, this takes more and more energy to remove increasing numbers of electrons.

Isotopes

The number of neutrons varies in the same element, for example, in a sample of Carbon, there will be Carbon-12, Carbon-13 and Carbon-14, these have the same atomic number but a different mass number. These are found in different percentages. The relative atomic mass is the average mass of a large number of atoms of an element on a relative scale. This takes into account the isotopes and the percentage abundance. The scale is that of 1/12 the mass of Carbon-12.

Radioactivity

Some elements have unstable isotopes in which the nuclei break up spontaneously, issuing ionising radiation, they are radioisotopes, and are radioactive. Some are naturally occurring such as tritium from Hydrogen (  H) and Carbon-14 (   C) but many are formed in nuclear reactors. When they decay, energy is released as heat and other forms of radiation. This takes different amounts of time, it is completely random, and is not affected by temperature or pressure or whether the isotope is an element or in a compound, it is only affected by the amount of the substance. The time taken for half a sample to decay is called its half time, the time for the decay rate to fall to half its original value. There are three types of radiation which all knock electrons off atoms they collide with:

• Alpha – Has a charge of 2+ and a relative atomic mass of 4. They are 2 protons and 2 neutrons, a He2- ion. They can be stopped by a few sheets of paper, and their deflection by an electric field is small. They are from elements with a greater atomic mass than 210, often from heavier elements with atomic numbers greater than 82. This high mass makes them have too much mass to be stable, so they give out alpha particles to form more stable atoms with a higher proton : neutron ratio. A new element is formed.

U                Th +  a

• Beta - Has a charge of 1- and a relative atomic mass of 1/1837. They are an electron. They can be stopped by a few mm of plastic or aluminium, and their deflection by an electric field is high. When there is a relative atomic mass of less than 210 this occurs, often when it is a heavy isotope, with too many neutrons compared to protons. A neutron changes to a proton an electron is emitted from the nucleus as a beta particle, and the proton : neutron ratio decreases, the product has one more proton and so a new element is formed.

      Tl                 Pb +   B

• Gamma - Has a charge of 0 and a relative atomic mass of 0. They are an electromagnetic radiation of shorter wavelength than x-rays. They can be stopped by a few cm of lead, and their deflection by an electric field is nil. This is often at the same time as alpha or beta radiation.

 

Radioactive Carbon Dating makes use of the fact that Carbon-14 is present in small amounts in all living things, after death, no more is taken in, and it begins to decay, using the half life the age can be determined. Also the level of radioactivity that can pass through a container can be measured to determine how full it is. Since Cancer cells are more easily killed by radioactivity than healthy cells, gamma rays from Cobalt-60 can be used. Less penetrating rays, such as the beta rays form Strontium-90 can be used to kill skin cancer. Surgical instruments can be sterilised by gamma rays. Leaks in gas or oil pipes or in ventilation systems can be detected.

Nuclear fusion

In a nuclear fusion reaction, two light atomic nuclei fuse together to form a single heavier nucleus of a new element, this process releases enormous amounts of energy. It only happens at very high temperatures such as in the stars, because this is needed to bring the nuclei close enough together and overcome the repulsion between them. The high temperature means they are moving faster and collide with enough energy to overcome the repulsive barrier. In the sun, two hydrogen nuclei fuse to form helium, releasing large quantities of energy, causing the gas to glow.

H             H                      H

H             H                      H  +      n

According to the Big Bang theory, all elements in our bodies originate from nuclear fusion reactions. At the high temperatures in gas clouds of stars the electrons have enough energy to escape the nuclei, so the gases are ionised, in a plasma, in which the positive ionised electrons are in a ‘sea’ of delocalised negative electrons. They have very different properties to the same gases at lower temperatures. Plasma forms the stars and so is the most common form of matter in the universe. It is hoped that nuclear fusion reactions can be harnessed to produce electrical energy on earth, but safely generating the temperatures required for plasma and containing it sustainably, safely and economically, is very difficult.

Mass spectrometers

Inside a mass spectrometer there is a vacuum. There is also an ionisation chamber, into which a sample of the vapour of an element is injected, and then bombarded with electrons, the collisions with them cause the atoms to lose an electron and form positive ions. This beam is accelerated by an electric field, then deflected by a magnetic field. The amount of deflection varies according to the charge to mass ratio of the ions. Since the charge is the same for all electrons, the deflection varies according to the mass. Lighter ions, from lighter isotopes are more deflected than heavier ones, so different mass particles can be separated and identified. The detector counts the number of each different ion forming upon it and so a percentage of the abundancy of each isotope can be measured.

X (g)        +         e-                 X+ (g)        +        2e-

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Light

Wave model – Light is a form of electromagnetic radiation with wavelength and frequency, it always travels the same distance in the same medium in the same time, in c, e.g. c= 3 x 108 ms-1 in a vacuum. Wavelength (λ) is the distance in metres travelled in one second, frequency (v) is how many complete cycles per second there are (Hz.)

c = λv 

Particle model – Einstein believed light is made of tiny packets on energy called photons, the amount of energy is related to their spectrum position.

Atomic spectra

When the electrons in an atom gain energy such as heat or light, they become excited, when they lose this energy, it is emitted in the form of radiation from the electromagnetic spectrum and they return to their ground state. This can be split through a prism or diffraction grating, it looks like the colour spectrum seen with white light, but with black lines at the frequencies where the atom does not emit radiation from. This line sequence is unique to each element and can be seen even when the element is in a compound, its intensity shows how much of it there is. It is therefore very useful in identifying elements. In the visible spectrum, the characteristic spectrum for Hydrogen is called the Balmer series, the series in UV spectrum is called the Lyman series but there are spectrums in other areas, such as the IR.

Bohr made the first attempt to understand the spectrums in 1913, and to explain why light was only emitted in certain frequencies. He used the idea of Quantisation of Energy meaning that electrons can only posses certain quantities (quantas) of energy. This means the electrons’ energy is only allowed to change from one specific value to another. He believed that:

• The Hydrogen atom’s single electron is only allowed to exist in specific energy levels
• A photon of light is emitted or absorbed when the electron changes from one level to another
• The energy of the photon is equal to the difference between the two energy levels ΔE
• The frequency of light emitted or absorbed is related to E by:

        ΔE = hv        (h= Planck constant  v=frequency)

• Atomic spectra is caused by atoms moving between energy levels, shells, when they are excited they move up to higher energy levels and vice-versa
• The further away from the nucleus the electron is the more energy it has so the higher energy level it has
• The Lyman lines correspond with electronic changes between energy levels
• Energy levels become closer and eventually converge as the electron moves from the atom, eventually leaving the atom as an ion

X (g)                X+ (g)        +        e-

X (g)                X2+ (g)        +        e-

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The Periodic Table

This links elements in order of atomic number and groups them according to their properties. In this order they show periodicity, with physical and chemical properties repeating at regular intervals. Alkali metals, halogens and noble gases occur in regular intervals of 8 or 18 elements, this happens due to the filling of the electron shells.

Melting temperature – This depends on the strength of the bonds and the structure of the element. When metals melt, some bonds remain, but when macromolecular substances, such as Carbon melt, all bonds have to be broken, taking more energy. Thus, the Group 4 elements, bonded by strong covalent bonds, have the highest melting points. Groups 5,6 and 7 have simple structures, they are held together in simple molecules by strong covalent bonds which do not have to be broken for the substance to melt. Instead, only weak, inter-molecular forces need to be broken, requiring little energy. This is the same for boiling points.

First ionisation energy – This is the amount of energy required to remove 1mol of electrons for 1mol of gaseous ions. Along the periods this rises, peaking at the noble gases. From He to Li and Ne to Na the amount of energy drops dramatically because the additional electrons in Group 1 elements are much easier to remove than those in the noble gases. The increasing nuclear charge across the periods makes it harder to remove electrons.

Electrical Conductivity – This caries across the periods as the bonding type changes. Metals are good conductors of electricity due to the delocalised electrons which move from the negative electrode, across to the positive electrode when a potential difference is applies. Metalloid elements have an increasing amount of covalent bonding which results in electrons held tighter in their structure and having a lower conductivity of electricity. Non-metallic elements, particularly liquids have no delocalised electrons and very poor electrical conductivity.

Abundance

Since isotopes are found of different substances, to find the average relative atomic mass:

= (% x atomic mass) + (% x atomic mass) …

                        100

Shape of covalent structures

Covalent bonds are directional which causes these structures to have definite shapes, this is because the electrons repel each other

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Percentage error

It is important to evaluate all experiments for errors in the procedure, and where measurements are involved, the effect the errors would have on the measurements, making them larger or smaller. Where physical measurements are made there is always some uncertainty, or experimental error. The precision errors of pieces of equipment can be used to calculate the percentage error.

Percentage error = Error X 100%

                            Reading

Finding the formula of a compound

1. Experiment carried out in closed conditions, so the products can be trapped
2. Volume or each gas measured
3. Mass of gases calculated from the volumes
4. Simplest ratio of moles and atoms found
5. Changed into empirical formula

The copper strip is cleaned with emery paper then dried on filter paper and weighed. A dry boiling tube is then weighed, and 0.3 g approximately of iodine crystals are added. The copper strip is then bent so it is about 2cm above the iodine crystals, and the copper is heated, until no more iodine vapour is seen, but it has not escaped from the test-tube, and it is not overheated. The copper strip should then be removed and weighed. Then the yellow coating of copper iodine should be removed, and the strip reweighed.

What is the percentage of Carbon Dioxide in marble?

Tie a 20cm length of cotton on the tope of the test-tube and weigh it, then add about 1cm depth of powdered marble into it and reweigh it. Add 15cm3 of Hydrochloric Acid and use the cotton and cork to suspend the tube above the acid, and reweigh it. The take the cork out, and put the test-tube into the acid , and once all the powder has reacted, reweigh it, use this to calculate the percentage of Carbon Dioxide the marble contained.

Titration

 Titration is used to find quantities. One solution of known concentration is put in a burette, the second solution is put in a conical flask, the solution in the burette is run out until there is just enough to complete the reaction. Often an indicator is used to show this, but if there is an obvious colour change this is unnecessary. For example:

5Fe2+(aq)  +  MnO4-(aq)  +  8H+(aq)           5Fe3+(aq)  +  Mn2+(aq)  +  4H20(l)

pale green    deep purple                           light brown     colourless

The weighing bottle should be weighed, then 5g iron ammonium sulphate added, and the new weight recorded. The crystals should then be washed into the 100 cm3 beaker, using sulphuric acid to make sure it all goes into the beaker. Then enough sulphuric acid should be added to dissolve all the solid, using a glass rod to stir it. Then use a funnel to transfer the beaker contents into a 250cm3 volumetric flask, using sulphuric acid to ensure it is all transferred. Then dilute sulphuric acid should be added until it is about 1cm below the graduation mark.  The beaker should then be stoppered and inverted to mix the solution. Some should then be added to the burette. Just one extra drop of MnO4- in this case, would turn the solution purple

Spectroscopy – how light and matter interact

Clean the end of a Nichrome wire (not the end with the cork on!) by heating it strongly and then dipping into concentrated Hydrochloric Acid, repeat this until when in the flame, the colour does not change. Then clamp a spectroscope horizontally inline with the Bunsen flame. Moisten the tip of the wire with distilled water, and dip it into a metal chloride and look through the spectroscope at the colour of the flame.

Chemistry of Group 1 and 2 elements

1. Observations of reactions of them with water can be made, and then pH measured with universal indicator
2. Heating them in a test tube and then bubbling the gas through limewater can be done to see the thermal stability of the carbonates of them

Lab procedure

• Before using equipment it should be cleaned with distilled water
• The meniscus should just touch the graduation marker when measuring
• A pipette should be used to add the last 1 cm of a substance so that it is accurate
• The burette should be run out initially to remove any air bubbles, into a spare beaker
• The tap should be turned with one hand, gradually, and the other hand should stir the beaker
• The volume on the buttette should be recorded before and after the titration
• The first attempt should be approximate to give an idea for the others
• It should then be repeated until the three volumes are within 0.1cm3 of each other