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Physical Chemistry
AMS 1
1.1 Atomic structure
a)
Historical facts of our modern atomic theory:
i) Dalton's Atomic Theory (1808)
ii) Discovery of Cathode rays (Crokes - 1895)
iii) Cathode ray tube experiment (Thomson -1898)
Discovery of electrons - negatively charged particles.
Measurement of the e/m ratio of an electron.
Thomson's model of an atom - "plum pudding - a sphere of positive charges in which
negative electrons are embedded.
iv) Millikan oil drop experiment (1909)
Measurement of the electric charge of an electron.
v) Rutherford gold foil experiment (1909)
Discovery of the minute, massive and positively charged nucleus at the centre of the
atom. Rutherford put forward the existence of proton which is positively charged and
about 1800 times heavier than an electron.
Rutherford' s model of an atom : Electrons move around the minute, massive and
positively charged nucleus in circular orbit.
vi) Bohr's model of an atom (1913)
Electrons move around the positive nucleus in stable circular orbits.
Electrons can lose or gain energy by jumping from one orbit to another.
vii) Discovery of neutrons - (Chadwick 1934)
viii) Development of quantum theory - discussed in later chapters
Existence of atomic orbital - s, p, d and f orbital.
b)
The fundamental "sub-atomic" particles
Particle
Charge / C
Electron
Neutron
Electron
Relative
charge
+1
0
-1
Mass / kg
Mass / u
1.0073
1.0087
5.4858 x 10-4
Key :
C : coulomb
u : atomic mass unit
NB.
Relative size of the atom and nucleus (ratio of diameter) : 10
-10
-15
m to 10
m.:
c)
Atomic number, Z : __________________________________________________
d)
Mass number, A : ___________________________________________________
e)
Isotopes : _________________________________________________________
_________________________________________________________________
Physical Chemistry
f)
AMS 2
Nuclides : any atomic species of which the atomic number and the mass number are specified.
mass numbe r
atomic number
Notation for a nuclide :
Atomic symbol
eg.,
14
6
C
1.2 Radioactivity (Discovered by Antonie Henri Becquerel and Marie Curie - 1896 & 1898)
a)
Nuclear reaction (different from a chemical reaction)
In a nuclear reaction, there is a rearrangement of ________ and _________ in the nucleus of
the atom. New elements are formed.
b) Radioactivity
Some nuclides (with _____ ____________ratio) are unstable & radioactive, and split into
smaller ones. The nucleus divides into two. The electrons share amongst these two. Charged
particles and Radiant energy (Radioactivity) are emitted when radioactive nuclide undergoes
radioactive decay.
Curie identified 3 different types of ionizing radiation from the decay of radioactive nuclides :
Electrically charges
plates
Sheet of
aluminium
thin paper
β
+
radioactive sources
γ
α
-
Thick lead
sheet
i)
or α
2 He or He
(alpha particle),
fast moving helium nuclei, positively charged particles and highly ionizing, deflected
towards a negative electrode, less penetrating than β radiation and γ radiation.
Loss of an alpha particle : loss of 2 neutrons and loss of 2 protons from a radioactive
nuclide.
226
222
4
88 Ra → 86 Rn + 2 He
(α decay)
α radiation
As
4
2+
α particles are highly ionizing, they attract electrons
He
He2+ + 2e- →
4
2
ii)
β radiation (beta particles) less penetrating than
β
−
γ
radiation;
radiation : stream of fast moving electron travelling with speed of light. The electron
is emitted by splitting a neutron in radioactive decay. Ionize gas through which they pass.
1
0
n → 11p + -10e
Physical Chemistry
eg., radioactive decay of
NB .
iii)
γ
Th →
234
90
AMS3
234
91
0
-1
Pa + e
Deflected towards a positive electric field.
symbol for β particle : −10 e or β
radiation (gamma rays) uncharged rays, most penetrating
High energy electromagnetic radiation; comparable to X-rays but with shorter wavelength;
less ionizing than α and β radiation; not deflected by electric and magnetic field.
Emitted when a nuclide emits .α or β particles.
c)
Balancing nuclear reactions
Balancing the sum of mass numbers on both sides & the sum of atomic numbers on both sides.
16
a
Find a & b.
7 N → bO + β
-
Ex . New Way Chemistry P.19 Checkpoint 1.1
d) Uses of radioactive isotopes
i)
Radiotherapy
Cancerous tissues are treated by the use of γ rays obtained from a cobalt-60 source. The
radiation is directed at the tumour site for a few minutes each day for between two to six
weeks. The dosage must be carefully controlled. The other part of the patient body must be
well-shielded from the radiation.
ii)
Sterilization of surgical instruments
Sterilization of surgical instruments with radioactivity.
iii) Detection of leakage of underground water or fuel pipes
By introducing a short- lived radioisotope into the pipes. The level of radioactivity on the
ground surface can then be monitored. A sudden increase of surface radioactivity indicates
that there is a leakage of underground pipes.
iv) Efficiency of lubricating oils
Engine wear can be measured by using radioactive piston ring. As the ring wears away,
the lubricating oil becomes radioactive. Thus, the efficiency of different lubricating oils
can be tested.
v)
Carbon-14 dating
Used to find the age of plant and animal remains. When living organism dies, it takes in
no more radioactive carbon-14. Those carbon-14 already in the body of the dead
can be used to find the age of the remains of organisms.