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03 chap 01 structure of matter
 

03 chap 01 structure of matter

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    03 chap 01 structure of matter 03 chap 01 structure of matter Presentation Transcript

    • Chapter 1STRUCTURE OF MATTER 1
    • Democritus (c460-371 BC, Abdera) Ancient Greek Atomic Theory ‘Atom’ ← ‘Atomos’: ‘a’ = not ‘tomos’ = cut, slice Leucippus (440 BC, Miletus) 1.1 The Atom 2
    • John Dalton (1766 – 1844) Modern Atomic Theory (1800’s) - John Dalton In 1803, Dalton proposed the Atomic Theory which stated that : (2) all matter was composed of small indivisible particles termed atoms, (3) atoms of a given element possess unique characteristics and weight, and (4) three types of atoms exist: simple (elements), compound (simple molecules), and complex (complex molecules).All matter is composed of individual entities called elements, distinguishablefrom the others by the physical and chemical properties of its basiccomponents-the atoms. 1.1 The Atom 3
    • Inner Structure of the Atom – ElectronsIn 1897, J.J.Thompson discovered electrons through cathode ray tubeexperiment at Cavendish Laboratory, Cambridge University.Electron is one of the basic constituents of the Atom. J.J.Thompson (1856-1940) and the cathode ray tube electrons 1.1 The Atom 4
    • Cavendish Laboratory Cambridge University River Cam 5
    • The ‘ plum pudding’ Model of the Atom (chocolate chip cookie model)In this model, the atom is composed of electrons surrounded by a soupof positive charge, like plums surrounded by pudding. The electronswere thought to be positioned uniformly throughout the atom. 1.1 The Atom 6
    • Inner Structure of the Atom –Ernest Rutherford (1871-1937) The Nucleus In 1910, Rutherford’s investigations into the scattering of alpha rays and the nature of the inner structure of the atom which caused such scattering led to the postulation of his concept of the "nucleus", 1.1 The Atom 7
    • The Bohr Model of the Atom (1913)The Bohr model depicts the atom as a small, positively charged nucleussurrounded by electrons in orbit —similar in structure to the solarsystem.Niels Bohr (1885-1962) ~10-10m ~10-14m 1.1 The Atom 8
    • James Chadwick (1891-1974) The Discovery of Neutron In 1932 Chadwick made a fundamental discovery in the domain of nuclear science: he discovered the particle in the nucleus of an atom that became known as the neutron because it has no electric charge. The neutron has almost the same weight as the proton. 1.2 The Nucleus 9
    • The properties of an atom are determinedby the constitution of its nucleus and thenumber of electrons orbiting around it. A 12An atom is specified by Z X : (e.g. 6 C )X = the chemical symbol for the element,A= the mass number, = # of nucleons (sum of neutrons and protons in the nucleus),Z = the atomic number (# of protons or electrons). Particle Symbol Charge (unit) Weight (amu) neutron N 0 1.00866 proton Z +1 1.00727 electron Z -1 0.0005481 unit charge = 1.60 × 10-19 coulombs, 121 amu (atomic mass unit) = 1/12 of the mass of a 6 C nucleus = 1.66 × 10-27 kg 1.2 The Nucleus 10
    • On the basis of different proportions of neutrons and protons in thenuclei, atoms can be classified into different categories: isotopes isotones isobars isomers Same Z N A A, Z, N energyDifferent A, N A, Z Z, N states 59 60 32 32 131mExample 27 Co , 27 Co 14 7 N , 15 8 O 15 P, 16 S 131 54 Xe , 54 Xe 1.2 The Nucleus 11
    • 160 In light atoms, stable nuclei 238tend to have the same Z and N , neutrons vs protons 92 U 12 140 in stale nucleifor example, 6 C . 206 As the atomic mass increases 120 82 Pb neutron number (n=A-p)(beyond Z=20), stable nuclei havemore N than Z, for example,206 100 82 Pb . stable nuclei 80 More than half of the stable nucleihave even numbers of neutrons and 60protons (even-even nuclei). n/p = 1 About 20% of the stable nuclei 40have even Z and odd N and aboutthe same proportion have odd Z and 20 40 20 Caeven N. 12 C 0 6 In contrast, only four stable nuclei 0 20 40 60 80 100have both odd Z and odd N. proton number (p) 1.2 The Nucleus 12
    • The mass of an atom expressed in terms of amu is known as atomicmass or atomic weight. Gram atomic weight is the mass in grams numerically equal to the atomicweight. Avogadro’s Law: every gram atomic weight of a substance contains thesame number of atoms, 6.023 × 1023 atoms per gram atomic weight (known asthe Avogadro’s number NA). For example, the atomic weight (AW) of helium is4.0026. Therefore, The number of atoms/g = NA / AW = 6.023 × 1023 / 4.0026 = 1.505 × 1023/g. Grams/atom = AW / NA = 6.646 × 10-24 g. The number of electrons/g = NA • Z / AW = 3.009 × 1023 /g. 1.3 Atomic Mass and Energy Units 13
    •  The mass of an electron = 0.000548 amu; proton = 1.00727 amu,neutron = 1.00866 amu. The mass of an electron is approximately 1/2000of a proton or a neutron. The mass of an atom is less than the sum of the masses of itsconstituents, because certain mass is converted into energy which ‘glues’the nucleons together. The reduction in mass is called the mass defect orthe binding energy. 1.3 Atomic Mass and Energy Units 14
    •  Energy unit in atomic and nuclear physics: 1 eV = the kinetic energyacquired by an electron in passing through a potential difference of 1 Volt. eV = 1 V × 1.602×10-19 C = 1.602×10-19 J. Energy unit: 1 joule (J) is the work done when a force of 1 newton (kg-m/sec2) acts through a distance of 1 m. The mass of an electron at rest (so-called rest mass) is 9.1×10-31 kg. Itsenergy equivalent, according to E = mc2 (c = 3×108 m/sec), is 0.511 MeV.It can be shown that 1 amu = 931 MeV. 15
    •  The maximum number of electrons in an orbit is 2n2, where n is the orbit number. That is, the maximum number of electrons that can exist is 2, 8, and 18 in theorbit K,L,M respectively. For example, in an oxygen atom (Z=8), there are 2 electrons in the K-shell and 6electrons in the L-shell. O N M L K 1.4 Distribution of Orbital Electrons 16
    •  Electrons are bound to the (0) L seriesnucleus by the coulomb force of t e d stathe positive changes in the n gr ounucleus. Electrons in inner orbits Ks(shells) are more tightly bound eri es -2,500 eVthan those in the outer orbits. Each orbit has its own energy -11,000 eVlevel (potential energy, or -69,500 eVbinding energy). Moreover, higher Z atomshave greater binding energies. K L When an electron falls from an Mouter shell (higher potentialenergy) into an inner shell (lower Npotential energy), the energydifference of the two levels isemitted as radiation (called Tungsten atomcharacteristic X-rays). 1.5 Atomic Energy Levels 17
    • There are 4 different forces in nature, in the order of their strengths:(1) strong nuclear force,(2) electromagnetic force,(3) weak nuclear force, and(4) gravitational force.Strong nuclear force is a short range, attractive force that overcomes the repulsive electrostatic force of the protons and keeps the nucleons together.Weak nuclear force is a force involved in nuclear β–decay. particle with charged no charge particle 1.6 Nuclear Forces 18
    •  The shell model of the nucleus is similar to that of the atomic model witheach shell characterized by a distinct energy level. When energy is imparted to a stable nucleus, it may be raised to an excitedstate. When it returns to a lower energy state, it gives off energy equal to the 60energy difference of the two states. For example, the radioactive 27 Co was 59created in a nuclear reactor by bombarding the stable 27 Co with neutrons. The 60 60 27 Co is transformed to 28 Ni by a β–decay and 2 successive release of γ–rays(1.17-MeV and 1.33-MeV). 60 27 Co (5.26 y) - β (Emax=0.32 MeV), 99+% - 2.50 β (Emax=1.48 MeV), (1.17 MeV) 0.1% 1.33The Co-60 is produced in a nuclear (1.33 MeV)reactor from Co-59 by 59Co(n, γ)60Co 60 28 Ni 1.7 Nuclear Energy Levels 19
    •  Energy propagated by traveling corpuscles that have a non-zero definitiverest mass. Examples are electrons, protons, neutrons. The distinction between particle radiation and electromagnetic wavebecame less sharp when de Broglie hypothesized the wave-particle dualitynature of matter. Elementary particles have either zero or unit charge. Particle Symbol Charge Mass Electron e-, β- -1 0.000548 amu Positron e+, β+ +1 0.000548 amu Proton p, 1 1 H+ +1 1.00727 amu 1 Neutron n, 0 n 0 1.00866 amu neutrino ν 0 ~ zero 1.9 Particle Radiation 20
    • Electromagnetic Radiation - Wave Model Electromagnetic wave has twocomponents: magnetic and electricfields oscillating at right angle to eachother. Examples are light waves, heatwaves, radio waves, microwaves,ultraviolet rays, x-rays, and γ–rays. Energy is propagated with thespeed of light (3×108 m/sec).The relationship betweenwavelength (λ), frequency (ν), andthe speed of propagation (c) is givenby: c = νλ. 1.9 Electromagnetic Radiation 21
    • 1.9 Electromagnetic Radiation 22
    • Electromagnetic Radiation - Quantum Model In order to explain certain experimental results such as Compton scattering,electromagnetic radiation needs to be treated as particles (with zero mass). The relation between the energy and its frequency is given by: E = hν, where h is the Planck’s constant (6.62×10-34 J-sec), E is the energy in joule (J). By combining with the previous equation: E = hc/λ. If E is expressed in eV and λ in meters (m), then E (eV) = 1.24 × 10-6/λ (m). 1.9 Electromagnetic Radiation 23