The atom orbiting electrons Nucleus (protons and neutrons)

Nuclide notation Li 3 7 Proton number (Z) = number of protons Nucleon number (A) = number of protons and neutrons Neutron number (N) = A - Z

Isotopes Li 3 7 It is possible for the nuclei of the same element to have different numbers of neutrons in the nucleus (but it must have the same number of protons) For example, Lithium atoms occur in two forms, Lithium-6 and Lithium-7 Li neutrons 3 neutrons

How do we know the structure of the atom?

The famous Geiger-Marsden Alpha scattering experiment In 1909, Geiger and Marsden were studying how alpha particles are scattered by a thin gold foil. Alpha source Thin gold foil

Geiger-Marsden As expected, most alpha particles were detected at very small scattering angles Alpha particles Thin gold foil Small-angle scattering

Geiger-Marsden To their great surprise, they found that some alpha particles (1 in ) had very large scattering angles Alpha particles Thin gold foil Small-angle scattering Large-angle scattering

Explaining Geiger and Marsdens’ results The results suggested that the positive (repulsive) charge must be concentrated at the centre of the atom. Most alpha particles do not pass close to this so pass undisturbed, only alpha particles passing very close to this small nucleus get repelled backwards (the nucleus must also be very massive for this to happen). nucleus

Rutherford did the calculations! Rutherford (their supervisor) calculated theoretically the number of alpha particles that should be scattered at different angles. He found agreement with the experimental results if he assumed the atomic nucleus was confined to a diameter of about metres. That’s times smaller than the size of an atom (about metres).

Limitations of this model? According to the theory of electromagnetism, an accelerating charge (and the orbiting electrons ARE accelerating centripetally) should radiate energy and thus spiral into the nucleus.

Evidence for atomic energy levels When a gas is heated to a high temperature, or if an electric current is passed through the gas, it begins to glow. cathodeanode electric current Light emitted Low pressure gas

Emission spectrum If we look at the light emitted (using a spectroscope) we see a series of sharp lines of different colours. This is called an emission spectrum.

Absorption Spectrum Similarly, if light is shone through a cold gas, there are sharp dark lines in exactly the same place the bright lines appeared in the emission spectrum. Some wavelengths missing! Light source gas

Why? Scientists had known about these lines since the 19 th century, and they had been used to identify elements (including helium in the sun), but scientists could not explain them.

Niels Bohr In 1913, a Danish physicist called Niels Bohr realised that the secret of atomic structure lay in its discreteness, that energy could only be absorbed or emitted at certain values. At school they called me “Bohr the Bore”!

The Bohr Model Bohr realised that the electrons could only be at specific energy levels (or states) around the atom. We say that the energy of the electron (and thus the atom) can exist in a number of states n=1, n=2, n=3 etc. (Similar to the “shells” or electron orbitals that chemists talk about!) n = 1 n = 3 n = 2

The Bohr Model The energy level diagram of the hydrogen atom according to the Bohr model n = 1 (the ground state) n = 2 n = 3 n = 4 n = 5 High energy n levels are very close to each other Energy eV Electron can’t have less energy than this

The Bohr Model An electron in a higher state than the ground state is called an excited electron. High energy n levels are very close to each other n = 1 (the ground state) n = 2 n = 3 n = 4 n = Energy eV 0 electron

Atomic transitions If a hydrogen atom is in an excited state, it can make a transition to a lower state. Thus an atom in state n = 2 can go to n = 1 (an electron jumps from orbit n = 2 to n = 1) n = 1 (the ground state) n = 2 n = 3 n = 4 n = Energy eV 0 electron Wheeee!

Atomic transitions Every time an atom (electron in the atom) makes a transition, a single photon of light is emitted. n = 1 (the ground state) n = 2 n = 3 n = 4 n = Energy eV 0 electron

Atomic transitions The energy of the photon is equal to the difference in energy (ΔE) between the two states. It is equal to hf. ΔE = hf n = 1 (the ground state) n = 2 n = 3 n = 4 n = Energy eV 0 electron ΔE = hf

Emission Spectrum of Hydrogen Which is the emission spectrum and which is the absorption spectrum? The emission and absorption spectrum of hydrogen is thus predicted to contain a line spectrum at very specific wavelengths, a fact verified by experiment.

Pattern of lines Since the higher states are closer to one another, the wavelengths of the photons emitted tend to be close too. There is a “crowding” of wavelengths at the low wavelength part of the spectrum n = 1 (the ground state) n = 2 n = 3 n = 4 n = Energy eV 0 Spectrum produced

How do you excite an atom? 1.Heating to a high temperature 2.Bombarding with electrons 3.Having photons fall on the atom I’m excited!

Limitations of the Bohr Model 1.Can only treat atoms or ions with one electron 2.Does not predict the intensities of the spectral lines 3.Inconsistent with the uncertainty principle (see later!) 4.Does not predict the observed splitting of the spectral lines

Forces in the nucleus

The Coulomb Force The repulsive force between protons in the nucleus + +

The Strong Force The nucleons (protons and neutrons) in the nucleus are bound together by the strong nuclear force acts over short distance ( m) acts only between adjacent particles in the nucleus is carried by gluons

Unstable nuclei Some nuclei are unstable, for example Uranium 235 (it’s to do with the relative numbers of protons and neutrons) Hi! I’m uranium-235 and I’m unstable. I really need to lose some particles from my nucleus to become more stable.

Unstable nuclei To become stable, an unstable nuclei emits a particle Weeeeeeeeeeeeee!

Unstable nuclei We say the atom has decayed Weeeeeeeeeeeeee!

Unstable nuclei The decay of an unstable nucleus is random. We know it’s going to happen, but we can’t say when! It is spontaneous. It cannot be affected by temperature/pressure etc. Weeeeeeeeeeeeee!

Becquerels (Bq) The amount of radioactivity given out by a substance is measured in Becquerels. One becquerel is one particle emitted per second.

Detection Particles can be detected by photographic film Particles can also be detected (and counted) by a Geiger-Müller tube (GM tube) connected to a counter

Alpha particles 2 protons and 2 neutrons joined together The same as the nucleus of a helium atom Stopped by paper or a few cm of air Highly ionising Deflected by electric and strong magnetic fields He

Alpha Decay U Th Atomic number goes down by 2 Atomic mass goes down by 4 He

Beta particles Fast moving electrons Stopped by about 3 mm of aluminium Weakly ionising Deflected by electric and magnetic fields e 0

Beta decay In the nucleus a neutron changes into an electron (the beta particle which is ejected) and a proton (which stays in the nucleus) During beta decay the mass number stays the same but the proton number goes up by 1. e + ע e 0 Th Pa antineutrino

Gamma rays High frequency electromagnetic radiation Stopped by several cm of lead Very weakly ionising NOT affected by electric or magnetic fields

Gamma rays Associated with alpha decay U Th α

½ - life This is the time it takes half the nuclei to decay half-life (t ½ ) Number of nuclei undecayed time

½ - life This is the time it takes half the nuclei to decay half-life (t ½ ) Number of nuclei undecayed time A graph of the count rate against time will be the same shape

Different ½ - lives Different isotopes have different half-lives The ½-life could be a few milliseconds or 5000 million years! half-life (t ½ ) Number of nuclei undecayed time

Nuclear reactions N O p 1 1 He

Unified mass unit (u) Defined as 1/12 of the mass of an atom of Carbon-12 u = x kg

Energy mass equivalence E = mc 2 E = x x ( x 10 8 ) 2 E = x J Remembering 1 eV = x J 1 u = MeV

Mass defect For helium, the mass of the nucleus = u But, the mass of two protons and two nuetrons = u!!!! Where is the missing mass?

Mass defect The missing mass (mass defect) has been stored as energy in the nucleus. It is called the binding energy of the nucleus. It can be found from E = mc 2

Mass defect calculation Find the mass defect of the nucleus of gold, Au

Mass defect calculation The mass of this isotope is u Since it has 79 electrons its nuclear mass is u – 79x u = u This nucleus has 79 protons and 118 neutrons, individually these have a mass of 79x u + 118x u = u The difference in mass (mass defect) is therefore 1.156u

Mass defect calculation The difference in mass (mass defect) is therefore 1.156u This “missing mass” is stored as energy in the nucleus (binding energy). 1u is equivalent to MeV

Binding energy This is the work required to completely separate the nucleons of the nucleus.

Binding energy per nucleon This is the work required to completely separate the nucleons of the nucleus divided by the number of nucleons. It is a measure of how stable the nucleus is.

The binding energy curve

Nuclear Fission

Uranium Uranium 235 has a large unstable nucleus.

Capture A lone neutron hitting the nucleus can be captured by the nucleus, forming Uranium 236.

Capture A lone neutron hitting the nucleus can be captured by the nucleus, forming Uranium 236.

Fission The Uranium 236 is very unstable and splits into two smaller nuclei (this is called nuclear fission)

Free neutrons As well as the two smaller nuclei (called daughter nuclei), three neutrons are released (with lots of kinetic energy)

Fission These free neutrons can strike more uranium nuclei, causing them to split.

Chain Reaction If there is enough uranium (critical mass) a chain reaction occurs. Huge amounts of energy are released very quickly.

Bang! This can result in a nuclear explosion! YouTube - nuclear bomb 4 YouTube - nuclear bomb 4

Nuclear fusion – Star power!

The binding energy curve