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 Evidence of Atomic Structure Atomic Structure 1.3Sub-atomic Particles Chapter 1

New Way Chemistry for Hong Kong A-Level Book Atomic Number, Mass Number and Isotopes 1.5Radioactivity 1.6Nuclear Reactions 1.7Uses of Radioactive Isotopes 1.8Mass Spectrometer 1.9Relative Isotopic, Atomic and Molecular Masses

New Way Chemistry for Hong Kong A-Level Book 13 What is “atom”? 1.1 The atomic nature of matter (SB p.2) The Greek philosopher Democritus

New Way Chemistry for Hong Kong A-Level Book 14 Iron Continuous division 1.1 The atomic nature of matter (SB p.2) These are iron atoms!!

New Way Chemistry for Hong Kong A-Level Book 15 Dalton’s atomic theory John Dalton proposed his Dalton’s atomic theory. 1.1 The atomic nature of matter (SB p.2)

New Way Chemistry for Hong Kong A-Level Book 16 Main points of Dalton’s atomic theory 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 The experimental evidence of atomic structure (SB p.3) Discovery of electrons -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 The experimental evidence of atomic structure (SB p.4) The beam was composed of negatively charged fast- moving particles. Deflected in the electric field Deflected in the magnetic field

New Way Chemistry for Hong Kong A-Level Book 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 The experimental evidence of atomic structure (SB p.4) Thomson’s atomic model Atom An atom is electrically neutral No. of positively charged particles = No. of negatively charged particles

New Way Chemistry for Hong Kong A-Level Book The experimental evidence of atomic structure (SB p.4) How are the particles distributed in an atom? Positive charge -An atom was a positively charged sphere -Negatively charged electrons embedded in it like a ‘raisin pudding’ Electron

New Way Chemistry for Hong Kong A-Level Book The experimental evidence of atomic structure (SB p.4) Gold foil scattering experiment -performed by Ernest Rutherford

New Way Chemistry for Hong Kong A-Level Book 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: -most  -particles passed through the foil without deflection -very few  -particles were scattered or rebounded back

New Way Chemistry for Hong Kong A-Level Book The experimental evidence of atomic structure (SB p.5) 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 The experimental evidence of atomic structure (SB p.5) Rutherford’s atomic model Expectation: Mass of atom = Total mass of protons Mass of atom > Total mass of protons

New Way Chemistry for Hong Kong A-Level Book The experimental evidence of atomic structure (SB p.5) Chadwick’s atomic model -the presence of neutrons -proved by James Chadwick

New Way Chemistry for Hong Kong A-Level Book 117 Chadwick’s atomic model 1.2 The experimental evidence of atomic structure (SB p.5) Proton Electron Neutron

New Way Chemistry for Hong Kong A-Level Book 118 Sub-atomic particles 1.3 Sub-atomic particles (SB p.6) 3 kinds of sub-atomic particles: - protons - neutrons - electrons Inside the condensed nucleus Moving around the nucleus

New Way Chemistry for Hong Kong A-Level Book 119 A carbon-12 atom 1.3 Sub-atomic particles (SB p.7)

New Way Chemistry for Hong Kong A-Level Book 120 Characteristics of sub-atomic particles 1.3 Sub-atomic particles (SB p.6) Sub-atomic particle ProtonNeutronElectron Symbolp orn ore - or Location in atomNucleus Surrounding the nucleus Actual charge (C)1.6 x Relative charge+10 Actual mass (g)1.7 x x Approximate relative mass (a.m.u.) 110 H 1 1 n 1 0 e 0

New Way Chemistry for Hong Kong A-Level Book 121 Relative size of the atom and the nucleus 1.3 Sub-atomic particles (SB p.6)

New Way Chemistry for Hong Kong A-Level Book 122 Atomic number 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. Atomic number = Number of protons Number of electrons = Reason: Atoms are electrically neutral. WHY ?

New Way Chemistry for Hong Kong A-Level Book 123 Mass number 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 = Number of protons Number of neutrons +

New Way Chemistry for Hong Kong A-Level Book 124 Atomic numbers and mass numbers of some common atoms 1.4 Atomic number, mass number and isotopes (SB p.8) AtomNo. of protons No. of electrons No. of neutrons Atomic number Mass number Hydrogen 1101(1 + 0) = 1 Oxygen 8888(8 + 8) = 16 Argon (18+22) = 40

New Way Chemistry for Hong Kong A-Level Book 125 Isotopes 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 Symbol of the element Mass number Atomic number

New Way Chemistry for Hong Kong A-Level Book Atomic number, mass number and isotopes (SB p.8) e.g. the two isotopes of chlorine are written as: Cl Cl OR labelled as Cl-35 and Cl-37.

New Way Chemistry for Hong Kong A-Level Book Atomic number, mass number and isotopes (SB p.9) ElementIsotopeAtomic number No. of protons No. of neutrons Natural abundanc e (%) Hydrogen Carbon trace Isotopes of some common elements H 1 1 H 2 1 C 12 6 C 13 6 C 14 6

New Way Chemistry for Hong Kong A-Level Book Radioactivity (SB p.10) Nuclear stability n p p n Strong binding force The atom is stable binding force

New Way Chemistry for Hong Kong A-Level Book 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 Radioactivity (SB p.10) What will happen to the unstable nuclei? - split and divide to give smaller nuclei unstable nuclei smaller nuclei This process is called nuclear fission.

New Way Chemistry for Hong Kong A-Level Book Radioactivity (SB p.10) unstable nuclei High energy radiation and smaller particles may be emitted. smaller particles high energy radiation This phenomenon is called radioactivity.

New Way Chemistry for Hong Kong A-Level Book Radioactivity (SB p.10) Radioactive isotopes: Isotopes with unstable nuclei unstable nuclei Radiation: The energy and particles emitted when a nucleus splits. smaller particles high energy radiation

New Way Chemistry for Hong Kong A-Level Book Radioactivity (SB p.11) Symbols for some common nuclides in nuclear reactions ParticleSymbol Protonp or Neutronn or Electron (beta particles) e - or or  Helium nucleus (alpha particle) or  H 1 1 n 1 0 He 4 2 e 0

New Way Chemistry for Hong Kong A-Level Book Radioactivity (SB p.11) Discovery of radioactivity

New Way Chemistry for Hong Kong A-Level Book 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 Radioactivity (SB p.12)  -particles Helium nucleus -Mass: 4 a.m.u. -Charge: +2 -Deflected by both electric and magnetic fields. -Weak penetrating power helium nucleus

New Way Chemistry for Hong Kong A-Level Book Radioactivity (SB p.12) U Th He 4 2  + + e.g. 1.Atomic number 2 2.Mass number 4

New Way Chemistry for Hong Kong A-Level Book Radioactivity (SB p.13)  -particles - fast-moving electron - Fast-moving electron - Negligible mass -Charge: -1 - Deflected much more readily by an electric field than  - particles -Moderate penetrating power

New Way Chemistry for Hong Kong A-Level Book Radioactivity (SB p.13) Th Pa e 0  + + e.g. 1.Atomic number 2 2.Mass number is not affected

New Way Chemistry for Hong Kong A-Level Book Radioactivity (SB p.13)  -radiation - Electromagnetic radiation of short wavelength - Negligible mass - No charge - NOT deflected by both electric and magnetic fields. -Strong penetrating power

New Way Chemistry for Hong Kong A-Level Book Radioactivity (SB p.13) Relative penetrating power of  -particles,  -particles and  -radiation

New Way Chemistry for Hong Kong A-Level Book Radioactivity (SB p.13) Cloud chamber experiment

New Way Chemistry for Hong Kong A-Level Book Radioactivity (SB p.13) Tracks of  -particles and  -particles

New Way Chemistry for Hong Kong A-Level Book Nuclear reactions (SB p.14) Chemical reactions -Rearrangement of electrons -Numbers of protons and neutrons in the nuclei remain unchanged -NO new elements are formed

New Way Chemistry for Hong Kong A-Level Book 145 Nuclear reactions -Rearrangement of protons and neutrons -New elements are formed 1.6 Nuclear reactions (SB p.14)

New Way Chemistry for Hong Kong A-Level Book 146 Naturally occurring radioactive decay 1.From tritium to helium - tritium undergoes  -decay to form a stable isotope of helium He 3 2 H 3 1 e 0  + + He Nuclear reactions (SB p.14)

New Way Chemistry for Hong Kong A-Level Book Mass number of the atom is not affected 2.Atomic number Nuclear reactions (SB p.14)

New Way Chemistry for Hong Kong A-Level Book From carbon to nitrogen - carbon undergoes  -decay to form a stable isotope of nitrogen C 14 6 e 0  + + N Nuclear reactions (SB p.14)

New Way Chemistry for Hong Kong A-Level Book Mass number of the atom is not affected 2.Atomic number Nuclear reactions (SB p.14)

New Way Chemistry for Hong Kong A-Level Book From uranium to lead - uranium-238 undergoes  -decay to form thorium Nuclear reactions (SB p.15) U He 4 2  + + Th

New Way Chemistry for Hong Kong A-Level Book Nuclear reactions (SB p.15) U Pb Decay series from to

New Way Chemistry for Hong Kong A-Level Book 152 Artificial nuclear reactions 1.6 Nuclear reactions (SB p.15) e.g. Be 9 4  + B H 1 1 N 14 7  + O H 2 1 n Na  + Ne n 1 0 H bombarding certain stable nuclei with  - particles,  -particles, neutrons, protons ( ) and deuterons ( ). H 2 1 H 1 1

New Way Chemistry for Hong Kong A-Level Book 153 Nuclear equations 1.6 Nuclear reactions (SB p.15) -changes of mass numbers and atomic numbers e.g. Be 9 4  + C He 4 2 n Sum of mass numbers and sum of atomic numbers on the L.H.S. = Sum of mass numbers and sum of atomic numbers on the R.H.S.

New Way Chemistry for Hong Kong A-Level Book 154 Half-life of radioactive isotopes 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 155 Decay of a 16 g sample of phosphorus 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 156 Half-lives of some radioactive isotopes 1.6 Nuclear reactions (SB p.16) Radioactive isotopeHalf-life Oxygen x seconds Radon days Iodine days Phosphorus days Cobalt years Hydrogen years Carbon years Plutonium years Uranium billion years Radioactive isotopeHalf-life Oxygen x seconds Radon days Iodine days Phosphorus days Cobalt years Hydrogen years Carbon years Plutonium years Uranium billion years A wide range!

New Way Chemistry for Hong Kong A-Level Book Nuclear reactions (SB p.16) N=N 0 e -kt N: amount left N 0 : initial amount k: constant t: time taken for the decay

New Way Chemistry for Hong Kong A-Level Book Nuclear reactions (SB p.16) Check Point 1-1 (a)Balance the following equation: (i) (ii) (iii) (iv) (v) N He 4 2 O  Li H  O n 1 0 C  U n  U  Np H 1 1 Answer He U e 0

New Way Chemistry for Hong Kong A-Level Book 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. 90 Th 232  -decay X  -decay Y Z X: Y: Z: Ra Ac Th Answer

New Way Chemistry for Hong Kong A-Level Book Nuclear reactions (SB p.16) Check Point 1-1 (cont’d) (c) Give three differences between  -particle,  -particle and  -radiation.  -particle  -particle  -radiation Charge+20 Mass4 a.m.u.0 a.m.u. Relative penetrating power lowmoderatehigh Answer

New Way Chemistry for Hong Kong A-Level Book 161 Uses of radioactive isotopes 1.7 Uses of radioactive isotopes (SB p.17) Five main uses of radioactive isotopes: 1. Leak detection 2. As tracers 3. Radiotherapy 4. Carbon-14 dating 5. Nuclear power

New Way Chemistry for Hong Kong A-Level Book Uses of radioactive isotopes (SB p.17) 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 Uses of radioactive isotopes (SB p.17) As tracers -detection of the metabolic pathway of an element in a living organism -e.g. I-131, P-32 I-131 is used for diagnosing thyroid disease

New Way Chemistry for Hong Kong A-Level Book Uses of radioactive isotopes (SB p.18) 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 Uses of radioactive isotopes (SB p.18) Carbon-14 dating Half-life of C-14: 5730 yrs 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 Uses of radioactive isotopes (SB p.18) Nuclear power - nuclear fission can be used as a source of energy - e.g. in the disintegration of U

New Way Chemistry for Hong Kong A-Level Book Uses of radioactive isotopes (SB p.18) U Initial neutron A chain reaction

New Way Chemistry for Hong Kong A-Level Book Mass spectrometer (SB p.20) Cl has 2 isotopes: Cl-35 and Cl-37 IsotopesRelative abundance Cl-3575% Cl-3725% How can we know?

New Way Chemistry for Hong Kong A-Level Book Mass spectrometer (SB p.20) Mass spectrometer A highly accurate instrument!

New Way Chemistry for Hong Kong A-Level Book 170 Mass spectrometer consists of 6 parts: 1.8 Mass spectrometer (SB p.20)

New Way Chemistry for Hong Kong A-Level Book 171 Mass spectrum of Cl 2 : 1.8 Mass spectrometer (SB p.21) m/e ratioCorresponding ion Cl Cl Cl─ 35 Cl Cl ─ 37 Cl Cl ─ 37 Cl +

New Way Chemistry for Hong Kong A-Level Book 172 Mass spectrum of CH 3 Cl: m/e ratioCorresponding ion Cl Cl CH 3 ─ 35 Cl CH 3 ─ 37 Cl CH 3 ─ 37 Cl Mass spectrometer (SB p.21)

New Way Chemistry for Hong Kong A-Level Book Relative isotopic, atomic and molecular masses (SB p.22) 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 Relative isotopic, atomic and molecular masses (SB p.22) What is carbon-12 scale? Mg has the same mass as two C-12 atoms - use carbon-12 as the reference standard

New Way Chemistry for Hong Kong A-Level Book 175 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 Relative isotopic, atomic and molecular masses (SB p.23) What is the relative atomic mass of Cl? The relative abundances of Cl- 35 and Cl-37 are and respectively Relative atomic mass of Cl = = 35.48

New Way Chemistry for Hong Kong A-Level Book 177 Relative molecular mass 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 Relative isotopic, atomic and molecular masses (SB p.23) What is the relative molecular mass of CH 3 Cl? Relative molecular mass of CH 3 Cl = = 35.48

New Way Chemistry for Hong Kong A-Level Book Relative isotopic, atomic and molecular masses (SB p.23) Example 1-1 The mass spectrum of neon is given on the right. Determine the relative atomic mass of neon. Solution: Relative atomic mass of neon = = Answer

New Way Chemistry for Hong Kong A-Level Book Relative isotopic, atomic and molecular masses (SB p.24) Check Point 1-2 (a)The mass spectrum of lead is given on the right. Given that the relative atomic mass of lead is , 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    x)  ( x) =  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 Relative isotopic, atomic and molecular masses (SB p.24) 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        0.8 )  ( ) =  The relative molecular mass of dichloromethane is

New Way Chemistry for Hong Kong A-Level Book 182 The END