new way chemistry for hong kong a-level book 11 atomic structure, radioactivity and relative masses...

Atomic structure*
1.2 The Experimental Evidence of
Atomic Structure
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1.5 Radioactivity
1.8 Mass Spectrometer
New Way Chemistry for Hong Kong A-Level Book 1
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The Greek philosopher Democritus
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These are iron atoms!!
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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.
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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.
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Deflected in the electric field
Deflected in the magnetic field
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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’.
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An atom is electrically neutral
Atom
=
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An atom was a positively charged sphere
+
+
+
+
+
+
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Gold foil scattering experiment
- performed by Ernest Rutherford
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He bombarded a thin gold foil with a beam of fast-moving -particles (+ve charged)
Observation:
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Interpretation of the experimental results
- The condensed core is called ‘nucleus’.
- The positively charged particle is called ‘proton’.
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Rutherford’s atomic model
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Chadwick’s atomic model
the presence of neutrons
proved by James Chadwick
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Proton
Electron
Neutron
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- protons
neutrons
electrons
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Sub-atomic particle
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1.3 Sub-atomic particles (SB p.6)
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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?
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The mass number (A) of an atom is the sum of the number of protons and neutrons in the nucleus.
Mass number
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of some common atoms
Atom
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Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
Representation:
X
A
Z
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OR labelled as Cl-35 and Cl-37.
Cl
35
17
Cl
37
17
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Isotopes of some common elements
Element
Isotope
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1.5 Radioactivity (SB p.10)
The stability of an isotope depends on its neutron to proton ratio.
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This process is called nuclear fission.
unstable nuclei
smaller nuclei
smaller nuclei
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This phenomenon is called radioactivity.
unstable nuclei
smaller particles
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in nuclear reactions
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1.5 Radioactivity (SB p.11)
- Ernest Rutherford passed a beam of radiation from a radioactive source through electrically charged plates
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- Weak penetrating power
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- Deflected much more readily by an electric field than -particles
- Moderate penetrating power
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- Negligible mass
- No charge
- Strong penetrating power
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-particles and -radiation
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*
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Chemical reactions
Numbers of protons and neutrons in the nuclei remain unchanged
NO new elements are formed
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- New elements are formed
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- 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
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2. Atomic number 1
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- carbon undergoes -decay to form a stable isotope of nitrogen
e
0
-1
C
14
6
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2. Atomic number 1
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- uranium-238 undergoes -decay to form thorium-234
U
238
92
He
4
2
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U
238
92
Pb
206
82
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bombarding certain stable nuclei with -particles, -particles, neutrons, protons ( ) and deuterons ( ).
Be
9
4
B
10
5
H
1
1
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- 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.
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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
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Each passage of a half-life causes one half of sample remains!
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Radioactive isotope
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N: amount left
N0: initial amount
N
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Check Point 1-1
(i)
(ii)
(iii)
(iv)
(v)
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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:
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(c) Give three differences between -particle, -particle and -radiation.
Answer
-particle
-particle
-radiation
Charge
+2
-1
0
Mass
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Five main uses of radioactive isotopes:
1. Leak detection
2. As tracers
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Leak detection
- short-lived radioactive source is introduced into storage tanks and underground pipelines
located with the Geiger-Muller counter
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As tracers
- detection of the metabolic pathway of an element in a living organism
- e.g. I-131, P-32
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Radiotherapy
For those cancer cells located deep inside the body, -radiation (from Co-60 and Ce-137) is used.
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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!
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Nuclear power
- nuclear fission can be used as a source of energy
- e.g. in the disintegration of
U
235
92
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A chain reaction
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Cl has 2 isotopes: Cl-35 and Cl-37
Isotopes
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Mass spectrometer
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1.8 Mass spectrometer (SB p.20)
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m/e ratio
Corresponding ion
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m/e ratio
Corresponding ion
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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
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What is carbon-12 scale?
- use carbon-12 as the reference standard
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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)
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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
=
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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.
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What is the relative molecular mass of CH3Cl?
Relative molecular mass of CH3Cl
=
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Example 1-1
The mass spectrum of neon is given on the right. Determine the relative atomic mass of neon.
Answer
Solution:
=
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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.
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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
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new way chemistry for hong kong a-level book 11 atomic structure, radioactivity and relative masses...

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