Do now! Can you read through the syllabus whilst you are waiting? Do now! He’s going to blow! Be careful Good Neutral Excellent MOODOMETER

Radioactivity

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) Li 3 6

Isotopes Li 3 7 For example, Lithium atoms occur in two forms, Lithium-6 and Lithium-7 Li neutrons 3 neutrons

Isotopes of Hydrogen H 1 1 The three isotopes of Hydrogen even have their own names! H 1 2 H 1 3 Hi! I’m hydrogen They call me deuterium Hola! Mi nombre es tritium y yo soy de Madrid!

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.

Rutherford did the calculations! That’s times smaller than the size of an atom (about metres).

Stadium as atom YouTube - Structure of the Atom 3: The Rutherford Model YouTube - Structure of the Atom 3: The Rutherford Model If the nucleus of an atom was a ping-pong ball, the atom would be the size of a football stadium (and mostly full of nothing)! Nucleus (ping- pong ball

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.

The Bohr Model 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

The Lyman Series Transitions down to the n = 1 state give a series of spectral lines in the UV region called the Lyman series. n = 1 (the ground state) n = 2 n = 3 n = 4 n = Energy eV 0 Lyman series of spectral lines (UV)

The Balmer Series Transitions down to the n = 2 state give a series of spectral lines in the visible region called the Balmer series. n = 1 (the ground state) n = 2 n = 3 n = 4 n = Energy eV 0 UV Balmer series of spectral lines (visible)

The Pashen Series Transitions down to the n = 3 state give a series of spectral lines in the infra-red region called the Pashen series. n = 1 (the ground state) n = 2 n = 3 n = 4 n = Energy eV 0 UV visible Pashen series (IR)

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

The Strong Force acts over short distance ( m) acts only between adjacent particles in the nucleus is carried by gluons

Questions! Page 372, Questions 1, 4, 6, 8, 10, 11.