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Transcript
24/05/2017
A is
for
ATOM
+
Evidence for nuclear atom using
alpha particle scattering.
Next
The Nuclear Atom
History of the Atom - Democritus
24/05/2017
From the ancient Greeks through to the
19th centaury, there has been the question
‘what is matter is made from?’
The idea of atoms was first proposed by
Greek Philosopher Democritus in 530 B.C.
The concept was that matter could only be split in half
and half again until indivisible units were reached.
ATOMS
Back
Next
The Nuclear Atom
History of the Atom – John Dalton
24/05/2017
In 1808, John Dalton (a teacher!)
proposed the modern ATOMIC
THEORY.
It simply states that all elements are
made up of atoms and an element is
only made up from one type of atom.
Dalton's view of tiny indivisible spheres
remained unchallenged until the end of
the 19th Century.
Dalton’s
Atom
Back
Next
The Nuclear Atom
Discovering the Electron
24/05/2017
In 1897, Joseph (JJ) Thomson (British
Physicist) was experimenting with
electrical currents through gases.
The cathode rays he produced could be
deflected or moved when in
electromagnetic fields.
Cathode rays were made up of tiny
negatively charged particles –
ELECTRONS.
Back
Next
The Nuclear Atom
Thomson’s Model of the Atom
24/05/2017
From his evidence, Thomson
proposed that atoms were made up of
just tiny electrons.
He accounted for the neutrality of
atoms by the stating the electrons
existed in a ‘soup of positive charge’.
Sometimes referred to the plumpudding model.
Back
Next
The Nuclear Atom
Radioactivity
24/05/2017
Around the same time, Henri Becquerel
discovered that some unstable elements
gave off smaller particles –
RADIOACTIVITY.
Therefore atoms must be divisible and
made up of smaller parts –
SUBATOMIC PARTICLES.
Marie and Pierre Curie and Ernest
Rutherford confirmed this.
Back
Next
DIG – The Dating Game
Radiation
24/05/2017

Alpha Particle: Positively charge. Large in comparison.
Essentially a Helium Nucleus (as proved by Rutherford)

Beta Particle: Negatively charged. Light. (later to be
shown as electrons)

Gamma Rays: Neutrally charged. No mass – Energy.
Back
Next
The Nuclear Atom
Rutherford’s Experiment
24/05/2017
Rutherford and his colleagues bombarded a thin foil of gold
with a beam of alpha particles and then onto a fluorescent
screen.
Small amounts were
deflected.
Fluorescent
Screen
99.9% passed straight
through unaffected.
Thin Gold Foil
Back
Next
The Nuclear Atom
24/05/2017
Alpha Particle Scattering
Why were alpha particles scattered?
To explain back scattering Rutherford proposed the
Nuclear Model of the Atom.
+
Back
Next
The Nuclear Atom
Alpha Particle Scattering
24/05/2017
Alpha particles are positive.
High speed alpha particle bullet
travels through atom.
The electrons have little effect since
they are very light and the electrons
in the pudding model are very spread
out.
Very little deflection.
Does not support observations.
Plum-Pudding Model
Back
Next
The Nuclear Atom
Alpha Particle Scattering
24/05/2017
What is observed is that alpha
particles in some instances are
strongly deflected.
Alpha Source
With electrons practically
dismissed, the only electrostatic
force available could be a positive
charge somewhere within the
atom.
An atom
Back
Next
The Nuclear Atom
24/05/2017
Rutherford’s Model
He suggested that all of the atom’s positive
charge, together with most of its mass, is
concentrated
in the centre.
+
Alpha particles which travel close to the
nucleus are strongly deflected. The degree
of deflection depends on how close it
approaches.
Back
Next
The Nuclear Atom
Rutherford’s Model
24/05/2017
The nucleus must be very small in comparison
to the atom.
This will account for the vast majority making it
through unaffected.
Back
Next
The Nuclear Atom
24/05/2017
Rutherford’s Nuclear Model of an Atom
In summary,
He envisioned an atom that had a
positively charged nucleus in the
centre.
The atom was mostly empty
space.
An he deemed it reasonable that
electrons orbit this nucleus like
planets orbit the Sun.
+
Nuclear Model of an
Atom
Back
Next
The Nuclear Atom
24/05/2017
Rutherford’s Nuclear Model of an Atom
The model appeared flawless and
convinced most of the scientific
community.
Rutherford and his colleagues (Hans
Geiger and Ernest Marsden) were able
to precisely predict the effects of:
Alpha particle energy
Thickness of sample
Different metals
However there was a problem…
+
Nuclear Model of an
Atom
Back
Next
The Nuclear Atom
The Problem…
24/05/2017
As the electrons move in circles, they would lose energy.
Losing energy would slow them down.
Therefore they would be pulled into the positively charged
nucleus.
It has been calculated that a Rutherford atom would only
exist for about 1 billionth of a second!
The answer lies within QUANTUM MECHANICS –
when things get really small!!
Back
Next
Rutherford’s model could not
explain:
• Why the electrons did not
lose energy as they
orbited.
• What held the protons
together in the nucleus.
• The origins of emission
spectra of gases could not
be explained.
+
The Nuclear Atom
24/05/2017
James Chadwick’s Protons
The number of protons in a nucleus did not match the atomic
weight of the atom.
Therefore a third neutrally charged particle must exist!
Alpha Radiation
Neutron Released
Beryllium Foil
These he named NEUTRONS.
Back
Next
Line
Spectra
Gases absorb certain
frequencies of light.
Each gas absorbs a
unique combination of
frequencies – each
frequency corresponding
to a unique colour.
So each gas has a
unique set colours which
is known at its “line
spectra” – because they
are unique they can be
used to identify a gas –
similar to fingerprints.
Bohr’s Atom
Bohr - brought the concept of
quantization into atomic
theory.
Electrons could only move in
certain specific orbits
corresponding to specific
amounts of energy.
These ENERGY LEVELS radiated
out from the nucleus with
higher energies being further
away.
Electrons do not radiate energy
in these orbits.
Energy is gained or lost when
they move between orbits.
This model enabled Bohr to
explain the hydrogen
spectrum.
Atomic Spectra
A glass prism can be used
to generate a colour
…………………..
If this the light generated
by a hot (glowing) gas is
viewed through a prism
specific colour lines are
seen as AN ………………
………….SPECTRUM.
If light is shone through a
cold sample of the same
gas, the same specific
colour lines are absent
and appear as an
………………………..
LINE SPECTRUM.
Atomic Spectra
A glass prisim can be
used to generate a colour
spectrum.
If this the light generated
by a hot (glowing) gas is
viewed through a prism
specific colour lines are
seen as AN EMISSION
LINE SPECTRUM.
If light is shone through a
cold sample of the same
gas, the same specific
colour lines are absent
and appear as an
ABSORPTION LINE
SPECTRUM.
Absorption & Emission spectrum
………………………
-
…………………………
• In absorption spectrum radiation is again absorbed
by electrons being …………… to higher energy
levels.
• The same frequencies (colours) are again emitted
when the excited electrons ……………………. to the
ground state in an ………………………spectrum.
Absorption & Emission spectrum
ABSORBED LIGHT
-
EMITTED LIGHT
• In absorption spectrum radiation is again absorbed
by electrons being excited to higher energy levels.
• The same frequencies (colours) are again emitted
when the excited electrons drop down to the ground
state in an emission spectrum.
Emission Spectrum
Excited electrons dropping
down from unstable energy
levels ……………………..in the
form of light.
The frequency (colour) of the
radiation is directly related to
the ………………………..
between the energy levels.
Since each element has its own
……………………series of
energy levels, each element
also has its own unique series
of ……………………………
lines.
The line spectrum can therefore
be used to …………………each
element much like a fingerprint.
Emission Spectrum
Excited electrons dropping
down from unstable energy
levels radiate energy in the
form of light.
The frequency (colour) of the
radiation is directly related to
the energy gap between the
energy levels.
Since each element has its own
unique series of energy levels,
each element also has its own
unique series of
emission/absorption lines.
The line spectrum can therefore
be used to identify each
element much like a fingerprint.
THE NEUTRAL ATOM
• The atom consists of a _____________________
______________________________ surrounded by a
__________________________.
• Atomic Number Z: ___________________ in the
Nucleus = _________________ in a ______ atom.
• Mass number A - Number of ______ + ________
Notation
___________ Number
(bigger)
______________ Number
(smaller)
A
Z
X
symbol
THE NEUTRAL ATOM
• The atom consists of a nucleus containing
protons and neutrons surrounded by a cloud
of electrons.
• Atomic Number Z - Number of protons in
the Nucleus = number of electrons in a
neutral atom.
• Mass number A - Number of protons +
neutrons.
Notation
Mass Number
(bigger)
Atomic Number
(smaller)
A
Z
X
symbol
Relative Masses
• Relative atomic(Ar): The mass of the atom relative to
________________________________________.
(Number of times heavier than…)
Eg: O - 16 one atom of oxygen is ________________ than 1/12 of the mass
of a C12 atom,
Formula mass (Mr) - The _______________________ of
the atoms in a molecule.
Water H2O one molecule of water has a relative mass of
_____________________________ that is the molecular
or formula mass of water.
Mr(H2O) = 18 (Times heavier than…)
Relative Masses
• Relative atomic mass (Ar): The average mass of an
atom of an element relative to 1/12 of the mass of a C12
atom. (Number of times heavier than…)
Eg: O - 16 one atom of oxygen is 16 times heavier than 1/12 of the mass of
a C12 atom.
• Formula mass (Mr) - The sum of all the atomic masses
of the atoms in a molecule.
Water H2O one molecule of water has a relative mass of
(2x(1)+16) = 18 - that is the molecular or formula mass
of water.
Mr(H2O) = 18 (Times heavier than…)
Relative Masses - examples
Calculate the Formula masses of:
• O2 (oxygen gas) Mr(O2) =
• Cl2 (chlorine gas)
• NaCl (sodium chloride - table salt)
• CaCO3 (calcium carbonate)
• (NH4)2Cr2O7 (ammonium dichromate)
Relative Masses - examples
Calculate the Formula masses of:
• O2 (oxygen gas) Mr (O2) = 2x16 = 32
• Cl2 (chlorine gas) Mr (Cl2) = 2x35.5 = 71.0
• NaCl (sodium chloride - table salt)
Mr (NaCl) = 23+35.5 = 58.5
• CaCO3 (calcium carbonate)
Mr (CaCO3) = 40.1+12+(3x16) = 100.1
• (NH4)2Cr2O7 (ammonium dichromate)
Mr ((NH4)2Cr2O7 ) = 2(14+4)+2(52)+7(16) = 252
Isotopes
The two atoms below both belong to carbon but they are not identical – can
you spot what is different?
e-
e
e-
e-
-
e-
e-
6 C
13
e
e-
-
e-
e-
e-
e-
6 C
12
Isotopes
Atoms of the same element which have different
numbers of neutrons. Others – Boron 10 & 11,
Hydrogen 1 & 2, Chlorine 35 & 37. Write notation and
work out numbers of neutrons.
Isotopes
• Isotopes - Atoms of the same element which
have different numbers of neutrons. Eg: 613C
& 612C
•
37Cl
(25%) & 35Cl (75%) - ratio 1:3
Av Ar(Cl) = (37x25)+(35x75) = 35.50
100
Or
Av Ar(Cl) = (37x1)+(35x3) = 35.50
4
Relative atomic mass is (actually) the average mass of
an atom of an element relative to 1/12 of the mass of a
carbon-twelve atom.
Bohr’s Atom - problems
• Only explain hydrogen
spectrum.
• Could not explain
molecules (bonding of
atoms) - formation or
properties.
• Why fixed orbits and no
energy radiation in orbits.
• At variance with
Heisenberg’s uncertainty
principle.
Heisenberg: Not possible to know both the position and velocity of an electron at
the same time with the same amount of accuracy.
A Wave Model
• De Broglie - light - ‘matter
wave’ - theory.
• Davisson & Germer - electron
diffraction -proof of ‘matter
waves’.
• Shroedinger, Heisenberg et.
al. - wave & quantum
mechanical model.
• Orbits ---> Orbitals standing electron waves - a
region or space defining the
standing wave pattern.
Orbital: - a region where there is a high probability of finding an electron.
Ionisation Energy (Ei)
The ENERGY REQUIRED to REMOVE AN ELECTRON
completely from an atom.
Sodium atom
Electronic structure Na: 1s22s22p63s1
Sodium
ion
Na+: 1s22s22p6
FIRST ionisation energy (Ei1): Energy required to remove OUTERMOST
electron.
M  M+ + 1eSECOND ionisation energy (Ei2): Energy required to remove SECOND
OUTERMOST electron.
M+  M2+ + 1e-
Successive Ionization Energies
Analyse this graph in light of your knowledge of atomic & electronic structure. This
graph can be used to provide ‘evidence’ for some of the features of modern atomic
theory.
What can be inferred about the electronic structure of the atom?
Successive Ionization Energies
FIRST ionisation energy (Ei1): Energy required to remove OUTERMOST
electron.
M  M+ + 1eSECOND ionisation energy (Ei2): Energy required to remove SECOND
OUTERMOST electron.
M+  M2+ + 1eHard to
remove
close to
Inner
nucleus
level
Second energy
level
Easy to
remove far
from the
nucleus
Outer
(valence)
level
What can be inferred about the electronic structure of the atom?
This graph
provides
EVIDENCE
for energy
levels.
Patterns in Energies
FIRST ionisation energy (Ei1): Energy required to remove OUTERMOST
electron.
M  M+ + 1eWhat does this graph tell us about the electronic structure of the atom?
Patterns in Energies
FIRST ionisation energy (Ei1): Energy required to remove OUTERMOST
electron.
M  M+ + 1e-
First Ionisation Energies
Which of the following electrons would be easier to remove?
H’s electron
would be
removed.
1s
1 proton
H
Electron Structure
Bohr Orbits energy levels
N=4
N=3
3s
2s
N=2
N=1
Electron Structure
Bohr Orbits energy levels
N=4
Energy sub levels and orbitals
4p
4s
3p orbitals
N=3
3s
2p orbitals
N=2
N=1
2s
1s orbital
(3d
orbitals)
Electron distribution
Pauli:
– Electrons occupy
lowest vacant
energy levels.
– Max two electrons
per orbital
– spin paring occurs
when two electrons
sharing the same
orbital. 
N
S
S
N
Energy levels (1,2,3 etc) are divided up
into sub-levels (s, p, d, f) each of which
has a specific number of orbitals
(s-1, p-3, d-5).
N=2 second energy leve
N=1 first energy level
Electron Structure
4s
4s
3p
3p
3s
3s
2p
2p
2s
1s
2s
1s
Electron Structure
4s
4s
3p
3p
3s
3s
2p
2p
2s
1s
2s
1s
Electron Structure
4s
4s
3p
3p
3s
3s
2p
2p
2s
1s
2s
1s
Electron Structure
Afbau Diagrams
4s
3p
3s
2p
2s
1s
1s
H
1s1
He 1s2
Electron Structure
4s
4s
3p
3p
3s
3s
2p
2p
2s
2s
1s
1s
Li 1s2 2s1
Be 1s2 2s2
Electron Structure
4s
4s
3p
3p
3s
3s
2p
2p
2s
1s
B
2s
1s
C 1s2 2s2 2p2
Electron Structure
4s
4s
3p
3p
3s
3s
2p
2p
2s
1s
2s
1s
Electron Structure
4s
4s
3p
3p
3s
3s
2p
2p
2s
1s
2s
1s
Patterns in Energies
FIRST ionisation energy (Ei1): Energy required to remove OUTERMOST
electron.
M  M+ + 1e-
The Periodic Table
Atomic Radius Trends
The Periodic Table
Rows
Periods
VIII
II
1
2
Columns-Groups
I
1
<-- Group Numbers --> III
2
3
3
4
5
S
Block
IV
V
VI
4
5
O
6
VII
O
P block
d Block
6
7
Energy Levels
F block
Oxygen is in group 6 and period 2. It therefore
has an outer electronic structure of: 2s22p4
The Periodic Table - Blocks
D-block – TRANSITION
METALS
METALS
F – Block – Lanthanides and Actinides
Noble (Inert gasses)
– NON
Halogens
Metals
SAlkali-earth
- Block
Alkali Metals
P Block
References
• Simon Ball
• Nelson 12
• Mcgraw-Hill Ryerson