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

2. 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.

3. 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.

4. 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 (+).

5. 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’.

6. 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

7. 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

8. 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’.

9. 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

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

11. 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

12. 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%

13. 1.8 Mass spectrometer (SB p.20)

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

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

16. 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+

17. 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+

18. 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.

19. 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 and respectively
Relative atomic mass of Cl =
= 35.48

20. 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.

21. 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

22. 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 , 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    x)  ( x) =
 x = 52.3
 The relative abundance of the peak at m/e 208 is 52.3.
Answer

23. 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        0.8 )  ( ) =
 The relative molecular mass of dichloromethane is
Answer

24. The END

25. The END