1 Atomic Structure The current model of atomic structure was not undestood until the 20th century. 1897 – J. J. Thomson discovered electron 1919 – Ernest Rutherford discovered proton 1932 – Chadwick discovered neutron 1808 - John Dalton – new idea - the matter is made of atoms (tiny indivisible spheres) Atomic structure Atomic structure 1897 - J. J. Thomson discovered that all matter contains tiny negatively-charged particles. He showed that these particles are smaller than an atom. He had found the first subatomic particle - the electron. Expected result of Thomson’s “plum-pudding” model Rutherford's experiment if the "plum pudding" model of the atom was correct. Scientists then set out to find the structure of the atom. electrons Thomson thought that the atom (the plums) Actual result: Most of the was a positive sphere of matter positive matter α-particles passed straight and the negative electrons were through the foil, some are (the pudding) embedded in it slightly deflected, as expected, but to his surprise a few were scattered back towards the Ernest Rutherford got his students Geiger and Marsden to source. fire the fast moving α-particles at very thin gold foil and observe how they were scattered. – particle 2 protons + 2 neutrons bound together Rutherford said that this was rather like firing a gun into a particle identical to a helum at tissue paper and finding that some bullets bounce 44 nucleus; hence, it can be written as 22He or back towards you! Rutherford source of α: radioactive radon Rutherford’s conclusion (1911): Calculate the distance of an alpha particle’s particle had a head-on collision with a heavier particle closest approach to a gold nucleus. heavier particle had to be very small, since very few Alpha particle (+2e) Gold nucleus particles were bounced back. r (+79e) Loss in kinetic energy must be equal to gain in potential energy of α particle in the field of heavy particles must be positive (repulsion) gold nucleus. Rutherford used an a source given to him by Madame Curie. → nuclear (planetary) model of the atom: The inital a energy was ~ 7.7MeV (KE = 7.7 x 106 x 1.6 x 10-19 J = 12 x 10-13 J). Atom contains a small but very massive positive core which he initial called nucleus, surrounded by negatively charged electrons at α paricle is brought momentarily to rest (“having climbed as far as it can up the relatively large distances from it. electrostatic hill”) when changing direction of the motion. The speed and hence the kinetic energy is zero, all the energy is now electrostatic potential energy. The most suprising thing about this model is that the atom is mainly Q KE = U = qV = 2e k gold r ~ 3x10-14 m empty space! initial final final r which is why most α particles went straight through – any electrons would -10 hardly impede the relatively massive’ high speed ). Radius of gold atoms is ~ 3 ×10 m. So a nucleus is at least 10 000 times smaller than an atom. It is important to emphasise that this calculation gives Using this model Rutherford calculated that the diameter of the an upper limit on the size of the gold nucleus; we cannot say that the gold nucleus could not be larger than 10-14 m. alpha particle touches the nucleus; a more energetic might get closer still. 2 Nuclear Atom no neutrons (AD 1911) Bohr’s Model - atomic energy levels Orbiting around nucleus in circles are the electrons. Electrons could only exist in certain orbits (“discrete states”) called (if they were not orbiting but at rest, they would move straight to the “stationary states.” nucleus; instead centripetal force is provided by the electrostatic attraction between electrons and nucleus.) Electrons in these stationary states do not emit EM waves as they orbit. Photon is emitted when an electron jumps from an excited state to a lower energy state (higher orbit to lower orbit). Energy of that photon is equal to the energy difference between two states. Ephoton = ΔE Problems with Rutherford’s model: According to Maxwell, any accelerating charge will generate Albert Einstein: relationship between energy and frequency of a photon: an EM wave E Electrons will radiate, slow down and eventually spiral into E hf f h = Planck's constant = 6.627x10-34 Js h nucleus. The end of the world as we know it. The solution was found in quantum theory. The frequency of emitted light (photon) is proportional to the change . of energy of the electron. f E h . And it was. Soon enough. These energies naturally lead to the The modern model explanation of the hydrogen atom spectrum: The model we now accept is that Bohr's model was so successful that he E5 there is a nucleus at the centre of immediately received world-wide fame. the atom and the electrons do exist Unfortunately, Bohr's model worked only in certain energy levels, but they ΔE = hf for hydrogen. Thus the final atomic model don’t simply orbit the nucleus. The E was yet to be developed. 4 probability of finding electron somewhere is given by wave equations, resulting in some E 3 interesting patterns. E2 E1 The result of this theory can be again visualised using very simple model, this time only energy level model. This model is not a picture of the atom but just represents possible energy of electrons. 1. A hot solid, liquid or gas at high pressure produces a Each element (atom/ion) continuous spectrum – all λ. Allows the identification produces a specific set of of elements across the 2. A hot, low-density / low pressure gas produces an absorption (and emission) lines. galaxy and universe. emission-line spectrum – energy only at specific λ. We call this the "spectral signature" (If we mapped it and 3. A continuous spectrum source viewed through a cool, or “fingerprints” of an atom/ion. can recognize it) low-density gas produces an absorption-line spectrum – missing λ – dark lines. Emission Spectra Thus when we see a spectrum Absorption Spectra we can tell what type of source we are seeing. 3 Nucleon The name given to the particles of the nucleus. Nuclide A particular combination of protons and neutrons that form a nucleus. It is used to distinguish isotopes among nuclei. Nucleon number (mass number) A The number of protons plus neutrons in the nucleus. Proton number Z The number of protons in the nucleus. Symbol for a nucleid Nuclear structure Isotopes Nuclei (atoms) with the same number of A 7 X 3 Li protons but different numbers of neutrons. Z Neutron number N (N = A – Z) The number of neutrons in the nucleus. Isotopes – Nuclei (atoms) of the same element differing in masses due to different numbers of neutrons (the same proton number, different nucleon number) . The existence of isotopes is evidence for the existence of neutrons, because there is no other way to explain the mass difference of two isotopes of the same element. The same number of electrons – the same bonding - the same chemical properties The strong nuclear force Different masses – different physical properties Many isotopes do not occur naturally, and the most massive 238 isotope found in nature is uranium isotope 92U About 339 nuclides occur naturally on Earth, of which 269 (about 79%) are stable the current largest atomic number element, with atomic number 118, survived for less than a thousandth of a second It is the force which attracts protons to protons, neutrons to neutrons, and What holds the nucleus together? protons and neutrons to each other. That force has a very short range, about 1.5 radii of a proton or neutron (1.5 x 10-14m) and is independent of charge and this is the reason the nucleus of an atom turns out to be so Protons are positive, neutrons are neutral (they would drift apart if put them small. together), so if the electric force was the only force involved, you couldn’t create a nucleus. If the protons can't get that close, the strong force is too weak to make them stick together, and competing electromagnetic force can influence the There has to be some other force that holds protons and neutrons together particles to move apart. and it must be stronger than the electric force. Well, in a brilliant stroke of imagination, physicists have named this force "the strong force." As long as the attractive nuclear forces between all nucleons win over the repulsive Coulomb forces between the protons the nucleus is stable. It Althoug the nuclear force is strong, nuclei do not attract each other, so that happens as long as the number of protons is not too high. Atomic nuclei are force must be very short range, unlike the electric force that extends forever. stable subject to the condition that they contain an adequate number of neutrons, in order to "dilute" the concentration of positive charges brought The strong nuclear force was first described by the Japanese physicist about by the protons. Hideki Yukawa in 1935. It is the strongest force in the universe, 1038 times 238 stronger than gravitational force and 100 times stronger than the The most massive isotope found in nature is uranium isotope 92 U electromagnetic force. For more massive nuclei strong nuclear force can’t overcome electric repulsion. 4 When nucleons bind together to form nucleus the mass of a nucleus is found to be less than the sum of the masses of the constituent protons and neutrons. Mass defect (deficit) - difference between the mass of a nucleus and the sum of the masses of its isolated nucleons A bound system has a lower potential energy than its constituent parts; this is what keeps the system together.