Atomic structure

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L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Atomic Structure, Radioactivity and Relative masses
Ch 1: p.1
PART I ATOMS, MOLECULES AND STOICHIOMETRY
Chapter 1
:
Atomic Structure, Radioactivity and Relative Masses
1.1 The Atomic Structure
Atoms – They are the smallest particles of an element that can take place in chemical changes.
Fundamental particles in an atom – Protons and neutrons make up the nucleus, which is
surrounded by the electrons.
The size of the atom – X ray work shows that the diameter of atoms are of order of 2x10-10 m,
which is 0.2 nm (1 nm = 10-9 m)
The masses of the atom – Ranges from 10-27 to 10-25 kg.
I.
Discovery
Electron –
Proposed by G.J. Stoney (1874)
Discovered by Thompson (1897)in the Cathode Ray Tube experiment.
Proton - Proposed by Goldslein (1886)
Discovered by Rutherford (1899) in the Gold Foil Experiment.
Neutron -
Discovered by Sir James Chadwick (1932)
Bombarding Be with  particles to emit neutrally electrically particles.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Atomic Structure, Radioactivity and Relative masses
II.
Ch 1: p.2
Properties of the fundamental particles of an atom
Mass
Proton
1 unit (1.6725 x 10
Charge
-24
g)
+1
Neutron
1 unit (1.6725 x 10-24 g)
0
Electron
1/1837 unit (negligible)
-1
Note:
<1> The nucleus is a dense, positively charged body.
<2> In between the electrons and nucleus is space.
<3> The number of protons should be equal to the number of electrons because the atom should
be electrically neutral.
<4> The number of protons is approximately equal to the number of neutrons.
<5> Atoms can link up to form molecules which are more stable.
Examples: O2 , N2 , CO2 , H2O
<6> Ions are produced when atoms or molecules gain or lose electrons.
Examples:
Cation: Na – e  Na+
Anion: Cl + e  ClIII.
The Nucleus of the Atom
Nuclear Structure
(A) Nuclear charge – It is the total positive charges on the protons.
e.g. He atom has +2 unit of nuclear charge.
Atomic number (Z) – It is the number of protons in the nucleus.
Mass number (A) – It is the sum of number of protons and neutrons.
Isotopes: Atoms of same element that have the same number of protons but different number
of neutrons. (or same atomic number but different mass number)
Naturally occurring Isotopes of some common elements:
16
35
1
12
O
Cl
H
C
17
37
2
13
O
Cl
H
C
18
3
14
O
H
C
Isotopic abundance:
It is the abundance of each isotope in a mixture of isotopes for an
element.
(B) Isotopic Mass and Relative Atomic Mass
It is defined that the mass of a 12C nuclide is equal to 12.000 atomic mass units (a.m.u.)
That is
1 a.m.u. = 1/12 x mass of a 12C nuclide
= 1.661 x 10-27 kg
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Atomic Structure, Radioactivity and Relative masses
Ch 1: p.3
The masses of the three fundamental particles:
Mass of proton
Mass of neutron
Mass of electron
= 1.007177 a.m.u.
= 1.008665 a.m.u.
= 0.0005486 a.m.u.
Isotopic mass of a nuclide:
e.g. 3517Cl : It has 17 protons and 18 neutrons
Isotopic mass of 3517Cl = 17 x 1.007177 + 18 x 1.008665
= 35.277979 a.m.u.
Relative isotopic mass = isotopic mass / (1/12 x mass of 12C )
e.g. The relative isotopic mass of 3517Cl = 35.277979 a.m.u. / 1 a.m.u.
= 35.277979
* The actual value for isotopic mass of the 3517Cl nuclide is 34.96885
Relative Atomic mass (Ar) : It is the average of the relative isotopic masses weighted to take
account of the isotopic abundances.
e.g. Relative atomic mass of chlorine:
Isotopes
Isotopic relative isotopic
mass
Abundance
35
34.97
75%
37
36.95
25%
Cl
Cl
Ar of Cl = 35.4545
(C) Mass Spectrometry
This technique has two important uses:
<1> The determination of relative isotopic masses and abundances of isotopes.
<2> The determination of relative molecular masses and structure of organic compounds.
In mass spectrometry, the device used is mass spectrometer.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Atomic Structure, Radioactivity and Relative masses
Ch 1: p.4
How a mass spectrometer works:
<1> A gaseous sample of material is allowed to enter the ionization chamber.
<2> An electron gun bombards the sample with electrons, forming positively charged ions.
(mainly single charge : +1 )
X(g) + e (fast)  X+(g) + 2e (slow)
<3> The ions are accelerated by an electric field and deflected along a circular path by a magnetic
field.
The lighter the particles, the greater degree of deflection.
<4> The intensity of the ion beam is detected electrically, amplified and finally recorded.
Mass Spectrum
charge
the line
It is produced by increasing the magnetic
field so that ions of higher mass /
ratios are gradually brought into
with the detector.
Height of the peak = relative isotopic
abundance
The peak with the highest m/e ratio will
most
‘molecular
has lost only a
to the relative
likely correspond to the
ion’, i.e. the molecule which
single electron. In most mass spectra, the values of m/e ratio can be converted
masses of the particles if the charges on the ions are taken to be one.
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Atomic Structure, Radioactivity and Relative masses
Name: ____________________ Class No.:______
Date:___________
Ch 1: p.5
Marks:_________
Exercises
1. What do you understand by the terms (a) relative atomic mass , (b) isotope ?
Outline, with the aid of a labeled diagram, the use of the mass spectrometer in the determination
of relative atomic mass.
( 10 marks )
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Atomic Structure, Radioactivity and Relative masses
Ch 1: p.6
1.2 Radioactivity
Radioactivity is the spontaneous emission of radiation by an element due to the splitting
of atomic nuclei.
(A) Discovery
<1> Henri Becquerel (1896)
- Uranium salts caused photographic plate to ‘fog’ even covered by a layer of black
paper.
<2> Mme Curie (1898)
- Radioactivity of pitchblende (mainly U3O8 ): two radioactive elements : polonium and
radium.
- She identified two radiation from radium:
 -radiation and - radiation
<3> Pierre ( 1900 )
-
- radiation
(B) Types of ionizing radiation : - ,-, - radiation
Properties
Nature
Charge
Mass
Velocity
Relative
penetration

He nuclei
+2
4 units
0.05c
1

Stream of fast
moving e
-1
1/1836 units
3-99% c
100

Electromagnetic
radiation
No charge
Nil
c
10000
<1> - radiation: Stream of high velocity of He nuclei
When there is a loss of an - particle of an element,
Atomic number: decrease by ______ (loss of ____ protons )
Mass number :
decrease by _______ ( loss of ____ protons and _____ neutrons )
e.g. The nuclide radium-226 loses an - particles to form the nuclide radon –222
Nuclear equation :
<2> - radiation – Stream of electrons at high speed.
The electrons are emitted from unstable nuclei as a result of the splitting of a neutron.
1
1
0
0n  1p + -1e
When there is a loss of - particle,
atomic number : increase by _______ mass number: __________
e.g. The nuclide thorium-234 emits an - particle to form a nuclide of protactinium-234
Nuclear equation:
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Atomic Structure, Radioactivity and Relative masses
Ch 1: p.7
<3> - radiation: High energy electromagnetic radiation
It has high penetrating power and low ionization power. -rays are emitted when a nuclide
emits - or - particles.
Note: - rays are also emitted in a process called electron capture.
e.g. 3718Ar + 0-1e  3717Cl
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Atomic Structure, Radioactivity and Relative masses
Name: ____________________ Class No.:______
Date:___________
Ch 1: p.8
Marks:_________
Exercise
1.
In 1909, Geiger and Marsden reported the results of experiments in which a beam of -particles
was directed at very thin metallic foils. The large majority of the -particles went through the foils
without deflection, but some were deflected from their straight-line parts and a very small number
were deflected or scattered backwards.
(a)
(b)
(c)
(d)
What is an -particles? State its mass number and its charge.
Why were most -particles not deflected?
Why were some -particles deflected or scattered backwards?
What do these experiments illustrate about the structure of the metal atoms?
(e) State one other method of deflecting -particles.
( 6 marks )
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Atomic Structure, Radioactivity and Relative masses
Ch 1: p.9
(C) The detection and measurement of radioactivity
The radioactivity can be detected and measured by use of a Geiger-Muller tube. It consists of a
chamber filled with argon. The radiation passes through a thin mica window and ionize the argon.
The positive argon ions move to a cathode and the electrons to the anode. This creates an electrical
pulse, which is amplified. The pulse registers as clicks, light flashes or meter readings on the
counter.
(D) Stable and Unstable isotopes
A stable isotope do not under radioactive decays whereas an unstable isotope can
undergo radioactive decays.
Note:
<1> Naturally occurring elements consist of one stable isotope in higher abundance.
<2> Unstable isotopes are radioactive and known as radioactive isotope or radioisotope.
<3> The stability of an isotope depends on the neutron/proton ratio.
Stable isotopes (elements) of low atomic number (<20) should be equal to 1:1
For isotopes with higher atomic numbers, the ratio increases up to 1.6 to maintain
stability. (see fig 1.15)
(E) Decay series
Decay Series is a sequence of radioactive decay reactions leading to the
formation of a stable isotope.
A decay series can be represent graphically by plotting number of neutrons against number of
protons or by plotting mass number against atomic number.
Examples
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Atomic Structure, Radioactivity and Relative masses
Ch 1: p.10
(F) Natural Radioisotopes
Uranium minerals 238U are the greatest sources of natural radioisotopes.
Example: Carbon-14 emits -radiation.
(G) Artificial Nuclear Transformations
Artificial nuclear transformations involves the conversion of one element to another
artificially by nuclear reactions. These involve the bombardment of nuclei with
particles such as protons and -particles.
Example: The bombardment of aluminium nuclei with -particles to form a phosphorus-30
nuclei as well as a neutron.
Note:
<1> Neutrons are particular useful in such nuclear reaction because it is neutral.
<2> New elements are made after the transformation.
Such nuclear reactions can be expressed by nuclear equations.
The symbol AZX is used to represent the nuclide X with atomic number Z and mass number A.
The symbols used to represent the various types of radiation or particles are listed below.
Particles/ Radiation
Symbols
Alpha particle
 or 42He
Beta particle
 or 0-1e
1
Neutron
0n
Examples:
9
4
Be
4
+
2He

______ +
1
0n
23
11Na
+
1
0n

______
+
1
1H
14
7N
+
2
1H

______
+
1
0n
<3> The sums of the atomic number and mass number must always be balanced on the left and
right hand side of such nuclear equation.
Mass number:
Left: _________
right : _________
9
1
6
4
+

+
4Be
1H
3Li
2He
Atomic number:
Left: __________
right: _________
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Atomic Structure, Radioactivity and Relative masses
Name: ____________________ Class No.:______
Date:___________
Ch 1: p.11
Marks:_________
Exercise
1. State what type of emission occurs at each of the following stages in the decay chain for:
 22889Ac  22890Th  22488Ra
(a)
(b)
(c)
Indicate by nuclear equations (balanced atomic and mass number) the nuclear reaction when:
228
88Ra
(d)
59
27Co absorbs
(e)
(f)
14
a neutron.
N reacts with an -particle and the proton is emitted.
decays with the emission of a -particles.
15
6C
( 6 marks )
L.S.T. Leung Chik Wai Memorial School
F.6 Chemistry
Atomic Structure, Radioactivity and Relative masses
Ch 1: p.12
(H) Uses of Isotopes (Radioisotopes)
1.
Leak detection:
position of
A radioactive source is introduced into systems like storage tanks
and buried pipelines, and a detector is used to locate the
leakage.
Leakage checking
2.
Radiotherapy:
 radiation is used in cancer treatment.
3.
Nuclear power:
Nuclear reactions result in the release of a vast quantity of energy,
which is used to generate electricity.
4.
As tracers:
Radioactive isotopes can be used as tracers to study metabolism in living
organism.
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