The atom

Chap 30

Rutherford’s experiment

• He bombarded very thin pieces of gold foil (which were about 2000 atoms thick) with α particles (these are the nuclei of He atoms)

• The α-particles could be detected by flashes of light they produced on a fluorescent screen

• He found that most of the α-particles were undeflected and passed straight through the foil

• Some were deflected through a small angle

• A small no. were deflected through angles greater than 900

• He explained his observations by saying that the atom is mostly empty space (this is why most of the α-particles pass straight through).

• If an α-particle hits the positively charged central nucleus it is deflected through >900

• If an α-particle passes near the nucleus it is deflected since + charges repel

Atoms• Consist of a small positively

charged central part (nucleus containing protons and neutrons) orbited at relatively large distances by negatively charged electrons

• radius of the nucleus = 10-15m• Radius of atom = 10-10m• Thus the atom is mostly empty

space

Emission spectra

• If solids liquids or gases are given enough energy (via heating or an electric current) they may emit light.

• (e.g. filament lamp)• If this light is passed through a

prism dispersion occurs and an emission spectrum is formed. There are 2 types:

Continuous spectrum

• Emitted by a solid or a liquid

• All visible wavelengths are emitted and are seen to blend into each other.

• Look the same from all materials

Line spectrum

• Emitted by a hot gas.• Different elements emit

different wavelengths (colours)• When passed through a prism it

forms a series of bright lines on a dark background

• Each element has its own characteristic line spectrum, thus by examining the spectrum obtained from a source you can identify the elements in that source.

• By measuring the relative intensities of the lines you can work out the ratio in which the elements occur.

• This branch of study is called spectroscopy. It is a non-invasive means of studying a system as all you need is to look at the light emitted. It is the means by which we know the composition of the sun

Bohr model

• Electrons in atoms are confined to certain allowed orbits. Electrons in a particular orbit have a fixed amount of energy, E1.

• If energy is supplied to an electron it may move to an orbit of higher energy, E2. The atom is then in an excited state

• Atoms are unstable in excited states so very quickly the electron will fall down to its original state. When it does, it emits the excess energy as a photon of light with frequency given by hf = E2 – E1.

• This light appears as a line in an emission spectrum.

• Sometimes the electron will not drop down directly to its original level but may pass through a no. of intermediate steps. In this case, for each step from one level to another light is emitted such that hf = E2 – E1.

• Since every element has characteristic energy levels, every element has characteristic energy jumps or transitions, and so has characteristic frequencies of light which can be emitted. As these frequencies give the emission spectral lines, from looking at the pattern of lines you can identify the element

Laser – Light amplification by stimulated emission of radiation• In a laser many atoms have

their electrons in an excited state. If light of the same frequency as that which the atom is about to emit is then caused to strike the atom, it stimulates the atom to emit its photon. This produces an intense beam of coherent light.

Uses of lasers

• Medicine – used as a cutting or burning tool

• Industry – same uses

Nuclear structure

• Nucleus was found to consist of 2 particles, neutrons which have no charge and protons which are +.

• Atomic no. Z = no. of protons in the nucleus of an atom

• Mass no. A = total no. of protons+ neutrons in the nucleus of the atom

• No. of neutrons = A - Z

• Isotopes =atoms of an element having the same no of protons (atomic no) but different no. of neutrons (hence different mass numbers)

• All elements occur in more than one isotope

Radioactivity• Nuclei of certain isotopes are

unstable (i.e. contain excess energy) . In order to become more stable they need to emit energy. This process is known as radioactive decay.

• Radioactivity = the spontaneous disintegration or decay of the nuclei of certain atoms with the emission of one or more types of radiation

Types of radiation

• Three different types of radiation can be emitted by a nucleus

• α-radiation• β- radiation• γ-radiation

α-radiation • Consists of two protons and two

neutrons stuck together (i.e. the nucleus of a helium atom)

• Are deflected by electric and magnetic fields (i.e. are charged)

• Can be stopped by a thin sheet of paper or about 5cm of air -low penetrating power

•

• α-particles are relatively large particles, thus they have lots of collisions with atoms of the materials through which they pass. During these collisions the α-particles energy can cause ionisation of the materials. α- particles cause lots of ionisation

α-emission

• Emission of an α-particle (He24)

decreases the atomic no. by 2 and the mass no by 4

• (See book, p349)

β- radiation• β – particles are high speed

electrons ejected from the nuclei of radioactive atoms

• It occurs when a neutron in the nucleus splits to become a proton and an electron. The proton remains in the nucleus and the electron (β- particle) is emitted at high speeds

• Β – particles are more penetrating than α – particles (since they are smaller particles they have less collisions and so penetrate further).

• The fact that they have less collisions means that they cause less ionisation.

• They are deflected by electric and magnetic fields (i.e. they are charged particles)

β emission

• When a β particle is emitted the mass no. stays the same (since the mass of an electron is very small) and the atomic no. increases by one (as an extra proton is created with the β particle.

• See p350 for equation

γ-radiation

• γ radiation is high frequency electromagnetic radiation. When they are emitted from the nucleus the nuclear structure stays the same, it simply represents a loss of energy

• Do questions p350

Activity

• The activity (A) of a radioactive sample is the no. of nuclei of the substance decaying per second

• Measured in becquerels (Bq)

Law of radioactive decay• States that the no. of nuclei

decaying per second (the activity) is directly proportional to the no. of nuclei present

• dN/dt ∝ N• dN/dt = constant N• • dN/dt = λN • where λ is the decay constant

Half-life (T1/2)

• The half life of a radioactive isotope is the time taken for half of the nuclei to decay (in other words the time taken for the activity to fall to half its starting level

• T1/2 = ln 2 / λ• Do questions p356

Relative atomic mass• Rather than deal with mass in kg it

is convenient to deal with atomic masses in simpler units.

• As Hydrogen is the simplest atom, it is assigned an atomic mass of 1.

• All others are relative to this. (i.e. oxygen has 16 times the mass of hydrogen so its relative atomic mass = 16.)

• Relative atomic mass ~ mass no.

Mole

• A mole of any substance is the amount of that substance which contains as many particles as there are atoms of C12 in exactly 12g of C12

• This no. of particles is called Avogadro’s number = 6.02 x 1023 particles

• 1 mole of C12 = 12g of C12 = 6.02x1023 particles

• 1 mole of O16 = 16g of O16 = 6.02x1023 particles of O

• 1 mole of Pb207 = 207g of Pb207 = 6.02 x 1023 particles of Pb207

• Do questions p 357

Radiation detectors

• Radiation is mostly detected through the ionisation it causes when it interacts with matter.

• 2 main types of detectors• G-M tube• Solid State detector

Geiger-Müller tube

Thin mica window to allow the radiation to enter the system

Positive anode

Negative cathode

Counter

• Consists of a cylindrical tube with a thin mica window at one end to allow the radiation to pass.

• The container contains argon gas at low pressure

• There is a wire through the centre of the cylinder and a large potential difference is supplied between the case and wire, the wire being + w.r.t. the case

• When radiation enters through the window, it causes ionisation of some of the argon atoms into + ions and – electrons.

• The + ions move towards the – case and the electrons towards the wire.

• As the electrons get nearer to the wire they feel a force of attraction due to the wires electric field, and so they speed up

• These fast moving electrons collide with other gas atoms and cause further ionisation. This effect is called a Townsend avalanche and results in a large no. of electrons hitting the central wire.

• Thus a pulse of current is sent along the wire to the counter .

• The counter registers this pulse of current as one radiation event.

• The counter circuit must then reset itself before it can count another event. The time taken to reset is called the dead time. If the pulses are arriving faster than the dead time the system will not be able to count all the pulses.

Counters

• These are either• Scaler – which counts all

radiation events from the time it is switched on

• Ratemeter – which counts the no. of pulses per second

Solid state detector

• This works on the fact that when radiation strikes certain semiconducting materials it creates electron-hole pairs.

• It consists of a reverse biased pn junction with a pd across it, connected to a counter.

• When radiation strikes the depletion layer it causes electron-hole pairs to be formed.

• This causes a current across the depletion layer which is then amplified, and then registers on the counter as one radiation event

Artificial radioactivity

• Many isotopes can be made radioactive by bombarding them with neutrons

Uses of radioisotopes

• Medicine• Medical imaging – trace amounts

of short half life isotopes can be ingested and the path of the isotope traced by the radiation given off

• cancer treatment – radiation kills cancerous cells more easily than healthy cells

• Sterilisation – γ – rays can be used to kill germs and hence sterilise food and plastic equipment

• Industry – used to trace blockages in pipes, or to test the thickness of materials (by putting a source on one side of the material and detector on the other)

• Carbon dating• Once a living organism dies, it

is no longer taking in any Carbon.

• C14 is radioactive, and decays over time.

• By measuring the activity of C14 in an object and comparing it with the amount of C14 which was present initially you can estimate when the organism died

• Smoke detectors• a radioactive source ionises

the air between two electrodes. Thus current flows between them

• If smoke particles enter this space they stick to the ions and the current is reduced.

• This reduced current triggers the alarm