New Way Chemistry for Hong Kong A-Level Book 11 Atomic Structure, Radioactivity and Relative Masses...
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New Way Chemistry for Hong Kong A- Level Book 1 1 Atomic Structure, Atomic Structure, Radioactivity and Radioactivity and Relative Masses Relative Masses 1.1 1.1 The Atomic Nature of M The Atomic Nature of M atter atter 1.2 1.2 The Experimental Evide The Experimental Evide nce of nce of Atomic Structure Atomic Structure 1.3 1.3 Sub-atomic Particles Sub-atomic Particles Chapter 1 Chapter 1
New Way Chemistry for Hong Kong A-Level Book 11 Atomic Structure, Radioactivity and Relative Masses 1.1The Atomic Nature of Matter 1.2The Experimental
Atomic structure* 1.2 The Experimental Evidence of Atomic Structure * 1.5 Radioactivity 1.8 Mass Spectrometer New Way Chemistry for Hong Kong A-Level Book 1 * The Greek philosopher Democritus * These are iron atoms!! * New Way Chemistry for Hong Kong A-Level Book 1 * 1.1 The atomic nature of matter (SB p.2) 5. Atoms of different elements combine to form a compound. The numbers of various atoms combined bear a simple whole number ratio to each other. 1. All elements are made up of atoms. 2. Atoms can neither be created nor destroyed. 3. Atoms of the same element are identical. They have the same mass and chemical properties. 4. Atoms of different elements are different. They have different masses and chemical properties. New Way Chemistry for Hong Kong A-Level Book 1 * 1.2 The experimental evidence of atomic structure (SB p.3) A beam of rays came out from the cathode and hit the anode. He called the beam cathode rays. New Way Chemistry for Hong Kong A-Level Book 1 * Deflected in the electric field Deflected in the magnetic field New Way Chemistry for Hong Kong A-Level Book 1 * 1.2 The experimental evidence of atomic structure (SB p.4) Measure the mass to charge ratio (m/e) of the particles produced Independent of the nature of the gas inside the discharge tube The particles were constituents of all atoms!! He called the particles ‘electrons’. New Way Chemistry for Hong Kong A-Level Book 1 * An atom is electrically neutral Atom = New Way Chemistry for Hong Kong A-Level Book 1 * 1.2 The experimental evidence of atomic structure (SB p.4) An atom was a positively charged sphere + + + + + + * Gold foil scattering experiment - performed by Ernest Rutherford * 1.2 The experimental evidence of atomic structure (SB p.4) He bombarded a thin gold foil with a beam of fast-moving -particles (+ve charged) Observation: New Way Chemistry for Hong Kong A-Level Book 1 * Interpretation of the experimental results - The condensed core is called ‘nucleus’. - The positively charged particle is called ‘proton’. New Way Chemistry for Hong Kong A-Level Book 1 * Rutherford’s atomic model New Way Chemistry for Hong Kong A-Level Book 1 * Chadwick’s atomic model the presence of neutrons proved by James Chadwick * Proton Electron Neutron * - protons neutrons electrons * New Way Chemistry for Hong Kong A-Level Book 1 * Sub-atomic particle * 1.3 Sub-atomic particles (SB p.6) New Way Chemistry for Hong Kong A-Level Book 1 * 1.4 Atomic number, mass number and isotopes (SB p.7) The atomic number (Z) of an element is the number of protons contained in the nucleus of the atom. WHY? New Way Chemistry for Hong Kong A-Level Book 1 * 1.4 Atomic number, mass number and isotopes (SB p.8) The mass number (A) of an atom is the sum of the number of protons and neutrons in the nucleus. Mass number * of some common atoms Atom * 1.4 Atomic number, mass number and isotopes (SB p.8) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Representation: X A Z * OR labelled as Cl-35 and Cl-37. Cl 35 17 Cl 37 17 * Isotopes of some common elements Element Isotope * * 1.5 Radioactivity (SB p.10) The stability of an isotope depends on its neutron to proton ratio. New Way Chemistry for Hong Kong A-Level Book 1 * This process is called nuclear fission. unstable nuclei smaller nuclei smaller nuclei * This phenomenon is called radioactivity. unstable nuclei smaller particles * * in nuclear reactions * * 1.5 Radioactivity (SB p.11) - Ernest Rutherford passed a beam of radiation from a radioactive source through electrically charged plates New Way Chemistry for Hong Kong A-Level Book 1 * - Weak penetrating power * * - Deflected much more readily by an electric field than -particles - Moderate penetrating power * New Way Chemistry for Hong Kong A-Level Book 1 * - Negligible mass - No charge - Strong penetrating power * -particles and -radiation * * New Way Chemistry for Hong Kong A-Level Book 1 * Chemical reactions Numbers of protons and neutrons in the nuclei remain unchanged NO new elements are formed New Way Chemistry for Hong Kong A-Level Book 1 * - New elements are formed New Way Chemistry for Hong Kong A-Level Book 1 * - tritium undergoes -decay to form a stable isotope of helium 1.6 Nuclear reactions (SB p.14) He 3 2 H 3 1 e 0 -1 He 3 2 * 2. Atomic number 1 New Way Chemistry for Hong Kong A-Level Book 1 * - carbon undergoes -decay to form a stable isotope of nitrogen 1.6 Nuclear reactions (SB p.14) e 0 -1 C 14 6 * 2. Atomic number 1 New Way Chemistry for Hong Kong A-Level Book 1 * - uranium-238 undergoes -decay to form thorium-234 1.6 Nuclear reactions (SB p.15) U 238 92 He 4 2 * U 238 92 Pb 206 82 * bombarding certain stable nuclei with -particles, -particles, neutrons, protons ( ) and deuterons ( ). Be 9 4 B 10 5 H 1 1 * - changes of mass numbers and atomic numbers e.g. Sum of mass numbers and sum of atomic numbers on the L.H.S. Be 9 4 C 12 6 He 4 2 n 1 0
+ + + = Sum of mass numbers and sum of atomic numbers on the R.H.S. New Way Chemistry for Hong Kong A-Level Book 1 * 1.6 Nuclear reactions (SB p.16) Half-life of a radioactive isotope is the time taken for its radioactivity to drop to half of its initial value. Not affected by: 1) no. of radioactive nuclei, 2) chemical conditions, 3) temperature New Way Chemistry for Hong Kong A-Level Book 1 * 1.6 Nuclear reactions (SB p.16) Each passage of a half-life causes one half of sample remains! New Way Chemistry for Hong Kong A-Level Book 1 * Radioactive isotope * N: amount left N0: initial amount N * Check Point 1-1 (i) (ii) (iii) (iv) (v) * 1.6 Nuclear reactions (SB p.16) Check Point 1-1 (cont’d) (b) A series of radioactive decays can be represented by the following equation. Write symbols for the elements X, Y and Z. Answer Th 232 -decay X -decay Y -decay Z 90 Ra 228 88 Ac 228 89 Th 228 90 X: Y: Z: * (c) Give three differences between -particle, -particle and -radiation. Answer -particle -particle -radiation Charge +2 -1 0 Mass * Five main uses of radioactive isotopes: 1. Leak detection 2. As tracers * Leak detection - short-lived radioactive source is introduced into storage tanks and underground pipelines located with the Geiger-Muller counter New Way Chemistry for Hong Kong A-Level Book 1 * As tracers - detection of the metabolic pathway of an element in a living organism - e.g. I-131, P-32 New Way Chemistry for Hong Kong A-Level Book 1 * Radiotherapy For those cancer cells located deep inside the body, -radiation (from Co-60 and Ce-137) is used. New Way Chemistry for Hong Kong A-Level Book 1 * Carbon-14 dating Changes to a stable isotope, N-14 How can the age of this bowl be estimated? Measure the radioactivity of C-14 in the bowl It is estimated that the bowl was made >5000 years ago! New Way Chemistry for Hong Kong A-Level Book 1 * Nuclear power - nuclear fission can be used as a source of energy - e.g. in the disintegration of U 235 92 * A chain reaction * Cl has 2 isotopes: Cl-35 and Cl-37 Isotopes * Mass spectrometer * 1.8 Mass spectrometer (SB p.20) New Way Chemistry for Hong Kong A-Level Book 1 * m/e ratio Corresponding ion * m/e ratio Corresponding ion * Relative isotopic mass The relative isotopic mass of a particular isotope of an element is the relative mass of one atom of that isotope on the carbon-12 scale. e.g. relative isotopic mass of Cl-35 = 35 relative isotopic mass of Cl-37 = 37 New Way Chemistry for Hong Kong A-Level Book 1 * What is carbon-12 scale? - use carbon-12 as the reference standard New Way Chemistry for Hong Kong A-Level Book 1 * Relative atomic mass The relative atomic mass of an element is the weighted average of the relative isotopic masses of its natural isotopes on the carbon-12 scale. 1.9 Relative isotopic, atomic and molecular masses (SB p.23) New Way Chemistry for Hong Kong A-Level Book 1 * What is the relative atomic mass of Cl? The relative abundances of Cl-35 and Cl-37 are 75.77 and 24.23 respectively Relative atomic mass of Cl = * 1.9 Relative isotopic, atomic and molecular masses (SB p.23) The relative molecular mass is the relative mass of a molecule on the carbon-12 scale. New Way Chemistry for Hong Kong A-Level Book 1 * What is the relative molecular mass of CH3Cl? Relative molecular mass of CH3Cl = * Example 1-1 The mass spectrum of neon is given on the right. Determine the relative atomic mass of neon. Answer Solution: = * Check Point 1-2 (a) The mass spectrum of lead is given on the right. Given that the relative atomic mass of lead is 207.242, calculate the relative abundance of the peak at m/e 208. Answer Let x be the relative abundance of the peak at m/e 208. (204 1.5 + 206 23.6 + 207 22.6 + 208x) (1.5 + 23.6 + 22.6 + x) = 207.242 x = 52.3 The relative abundance of the peak at m/e 208 is 52.3. New Way Chemistry for Hong Kong A-Level Book 1 * Check Point 1-2 (cont’d) (b) The mass spectrum of dichloromethane is given on the right. Calculate the relative molecular mass of dichloromethane. Answer Let y be the relative molecular mass of dichloromethane. y = (84 94 + 85 3.0 + 86 59 + 87 2.2 + 88 13 + 89 2.5 + 90 0.8 ) (90 + 3.0 + 59 + 2.2 + 13 + 2.5 + 0.8) = 85.128 New Way Chemistry for Hong Kong A-Level Book 1 *