Chapter 1 Atomic Structure 1.1 The Atomic Nature of Matter 1.2 The Experimental Evidence of Atomic Structure

Dalton’s atomic theory (1808) 1.1 The atomic nature of matter (SB p.2) Dalton’s atomic theory (1808) John Dalton proposed his Dalton’s atomic theory.

Main points of Dalton’s atomic theory 1.1 The atomic nature of matter (SB p.2) Main points of Dalton’s atomic theory 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. 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.

Discovery of electrons 1.2 The experimental evidence of atomic structure (SB p.3) Discovery of electrons Cathode Ray Discharge Tube A beam of rays came out from the cathode (-) and hit the anode (+).

Deflected in the magnetic field Deflected in the electric field 1.2 The experimental evidence of atomic structure (SB p.4) Deflected in the magnetic field Deflected in the electric field The beam was composed of negatively charged fast-moving particles called ‘electrons’.

Location of protons? Gold foil scattering experiment 1.2 The experimental evidence of atomic structure (SB p.4) Location of protons? Gold foil scattering experiment - performed by Ernest Rutherford

most -particles passed through the foil without deflection 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: most -particles passed through the foil without deflection very few -particles were scattered or rebounded back

Interpretation of the experimental results Most of the atom is empty space. The mass concentrated at the center of an atom called ‘nucleus’. - The nucleus is positively charged. - The positively charged particle is called ‘proton’.

Chadwick’s atomic model 1.2 The experimental evidence of atomic structure (SB p.5) Mass of atom > Total mass of protons Chadwick’s atomic model the presence of neutrons proved by James Chadwick

Chadwick’s atomic model 1.2 The experimental evidence of atomic structure (SB p.5) Chadwick’s atomic model Proton Electron Neutron

Surrounding the nucleus 1.3 Sub-atomic particles (SB p.6) Characteristics of sub-atomic particles Sub-atomic particle Proton Neutron Electron Symbol p or n or e- or Location in atom Nucleus Surrounding the nucleus Actual charge (C) 1.6 x 10-9 Relative charge +1 -1 Actual mass (g) 1.7 x 10-24 9.1 x 10-28 Approximate relative mass (a.m.u.) 1 n 1 e -1 H 1

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. Cl has 2 isotopes: Cl-35 and Cl-37 Isotopes Relative abundance Cl-35 75% Cl-37 25%

1.8 Mass spectrometer (SB p.20)

A highly accurate instrument! 1.8 Mass spectrometer (SB p.20) Mass spectrometer A highly accurate instrument!

Mass spectrometer consists of 6 parts: 1.8 Mass spectrometer (SB p.20) Mass spectrometer consists of 6 parts:

Mass spectrum of Cl2: m/e ratio Corresponding ion 35 35Cl+ 37 37Cl+ 70 1.8 Mass spectrometer (SB p.21) Mass spectrum of Cl2: m/e ratio Corresponding ion 35 35Cl+ 37 37Cl+ 70 35Cl─35Cl+ 72 35Cl ─ 37Cl+ 74 37Cl ─37Cl+

Mass spectrum of CH3Cl: m/e ratio Corresponding ion 35 35Cl+ 37 37Cl+ 1.8 Mass spectrometer (SB p.21) Mass spectrum of CH3Cl: m/e ratio Corresponding ion 35 35Cl+ 37 37Cl+ 50 12CH3─35Cl+ 51 13CH3 ─ 35Cl+ 52 12CH3 ─37Cl+

1.9 Relative isotopic, atomic and molecular masses (SB p.23) 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.

What is the relative atomic mass of Cl? 1.9 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 75.77 and 24.23 respectively Relative atomic mass of Cl = = 35.48

Relative molecular mass 1.9 Relative isotopic, atomic and molecular masses (SB p.23) Relative molecular mass The relative molecular mass is the relative mass of a molecule on the carbon-12 scale.

Relative molecular mass of CH3Cl = = 50.5 1.9 Relative isotopic, atomic and molecular masses (SB p.23) What is the relative molecular mass of CH3Cl? Relative molecular mass of CH3Cl = = 50.5

1.9 Relative isotopic, atomic and molecular masses (SB p.24) Check Point 1-6 (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. 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. Answer

1.9 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. 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  The relative molecular mass of dichloromethane is 85.128. Answer

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