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2 – Atomic Structure
Leaving Certificate Chemistry
Matter is anything that occupies mass.
It is composed of atoms, molecules or ions.
Definition
Atoms
• The word atom comes from the Greek word
atomos…meaning unsplittable.
The Law of Conservation of mass
The Law of Conservation of Mass
Matter can neither be created or destroyed
but it can be changed from one form to another.
Law
Molecules
When atoms join together….molecules
are formed.
A molecule of
water (H2O)
A molecule of caffeine
Molecules of Elements
Atoms of the same element can join together to form
molecules – e.g. O2, N2, H2.
Ions
If
an
atom
loses or
gains one
or more
electrons
then it is
called an
ion.
Protons, Neutrons & Electrons
Today atoms of elements are
basically made up of three
particles – protons, neutrons and
electrons.
But how did this picture of an
atom
emerge
and
who
contributed to these ideas?
A common picture of what
an atom looks like. The
protons and neutrons are
in the centre - they make
up the nucleus. The
electrons revolve around
the nucleus in shells.
John Dalton
Dalton’s theory:
1. Every chemical element is made up
of atoms of a unique type.
2. All of the atoms in a particular
element are identical and of the same
type.
John Dalton
3. Chemical compounds are made up of
atoms joined together.
4. Atoms cannot be created or divided.
John Dalton
Dalton used symbols to
represent the formation
of
compounds
from
atoms. These symbols
are not used today but
are replaced by letters.
William Crookes
In
the
1870s
English
Scientist, William Crookes,
began
to
study
how
electricity passed through
gases in which there is very
low pressure
William Crookes
William Crookes
Rays coming from the cathode glowed when they
struck the glass of the low pressure tube.
Discovered that cathode
rays :
•Travel in straight lines
• Small objects placed in
the end of the tube cast
shadows on the tube
http://ie.youtube.com/watch?v=Xt7ZWEDZ_GI
George Johnstone Stoney
In 1891, Irish physicist
George Johnstone Stoney
suggested that cathode rays
be termed electrons instead.
J.J. Thomson
Discoverer of the fact that
cathode rays are subatomic
negatively
charged
particles
(electrons)
Cathode rays (electrons) were
deflected towards a positively
charged plate - which showed
they are negatively charged.
Sir Nobel
J.J. Thomson
Prize
for Physics (1906)
2006 – Q4 (b)
Name the scientist, shown in the photograph, who identified cathode rays as subatomic particles.
(6)
J.J. Thomson
Measured the size of the charge
to mass ration (e/m) for cathode
ray particles
Sir J.J. Thomson
Cathode ray tube experiment
Robert Millikan
In 1911 the American
Scientist
Robert
Millikan used his Oil
Drop Experiment to
measure the size of the
charge on the electron.
This therefore allowed
the
mass
of
the
electron
to
be
calculated.
Nobel Prize
for Physics (1923)
Oil drop experiment
J.J. Thomson
Thomson considered
an atom as like a
‘plum pudding’ with
the
negatively
charged ‘plums’ (the
electrons)
in
a
pudding of positive
charge
2007
Q11
(7)
2007 ––Q11
(a) (7)
Describe the model of atomic structure which existed immediately prior to Rutherford’s Gold Foil Experiment.
(7)
Rutherford’s Gold Foil Experiment
Alpha particles
consist of 2
protons and 2
neutrons. They
are
positively
charged.
2007 – Q11 (7)
2002 – Q11 (b) (7)
What are alpha particles?
(7)
Describe the experiment carried out by Rutherford that led to the discovery of the nucleus. Explain how Rutherford
interpreted the results of this experiment to conclude that the atom has a nucleus.
(18)
Higher Level
Rutherford
If the plum pudding
model was right
then he expected
that the alpha
particles would go
through the gold
atoms in the foil
Results of the golf foil experiment
When the alpha particles were fired at the foil:
• The majority (about 98%) of the particles went
straight through the foil!
• A very small percentage of particles (1-2%) bounced
off, at large angles, or straight back in the opposite
direction
What did Rutherford find out?
• Most of the alpha particles went straight through,
therefore the atom must contain a majority of
empty space.
What did Rutherford find out?
• Some alpha particles deflected off because they came very close to to the
nucleus – a dense core of positive charge
in the middle of the atom and were
repelled.
What did Rutherford find out?
• A few alpha particles rebounded back because they collided with the densely
packed nucleus
Rutherford
• Overall - Rutherford discovered that atoms
had a nucleus, (a dense core of positive
charge in the middle of the atom) using the
alpha particle scattering experiment
Higher Level
Rutherford
Higher Level
• Rutherford called the positive particles in
the nucleus “protons”
• He discovered protons in the nuclei of
various atoms using his alpha particle
method.
Bohr Model of the Atom
Later, the Danish scientist
Niels Bohr, came up with the
modern model of the atom
which we use today.
Nobel Prize
for Physics (1922)
We will study Bohr’s theories
in greater detail in 6th Year
James Chadwick (1932)
The English physicist, James
Chadwick, discovered that
neutral
particles
called
neutrons are also to be found
in the nucleus of atoms of
elements.
Nobel Prize
for Physics (1935)
Today’s objectives
•
•
•
•
Learning about:
Atomic numbers
Mass numbers
Isotopes
Discovery of atomic
structure
Atomic Number & Mass Number
The atomic number of an element
is the number of protons in an atom of that element
Definition
The mass number of an element is the number
of protons and neutrons in an atom of that element
Definition
Interpreting the number of protons, neutrons
and electrons in an atom or ion
Take a neutral sodium atom
The larger number
is
the
mass
number
- the
number of protons
& neutrons
23
Na
11
The
smaller
number
is
the
atomic number the
number
of
protons only.
P = 11, E = 11 and N = 12
Interpreting the number of protons, neutrons
and electrons in an atom or ion
Take a negative chloride ion
The larger number
is
the
mass
number
- the
number of protons
& neutrons
The
negative
charge tells me
there is one extra
electron present.
37
Cl
17
Higher
2003
– Q4Level
(a) (6)
The
smaller
number
is
the
atomic number the
number
of
protons only.
2003 - Q.4 (a) (6)
P = 17, E = 18 and N = 20
Isotopes of chlorine
What do you notice about these atoms of chlorine?
17 protons
17 protons
18 neutrons
20 neutrons
17 electrons
17 electrons
Isotopes of carbon
What do you notice about these three carbon atoms?
6 protons
6 protons
6 protons
6 neutrons
7 neutrons
8 neutrons
6 electrons
6 electrons
6 electrons
Isotopes
Isotopes are atoms of the same element
that have the same atomic numbers but different
mass numbers as they have different amounts
of neutrons in their nuclei.
Definition
Higher Level
Higher Level
2002 - Q.4 (a) (6) 2005 - Q.5 (a) (5)
Higher Level
2006 – Q10 (a) (4)
Isotopes of Carbon
Relative Atomic Mass
Atomic weight of an element is now referred to as its
relative atomic mass
The relative atomic mass of an
element is the average mass of an
atom of that element relative to
one-twelfth the mass of an atom
of carbon-12.
Definition
Higher Level
2004 - Q.4 (a) (6)
Higher Level
2006 – Q10 (a) (6)
Calculating the relative atomic mass of an
element from isotopic abundance
Calculating relative atomic masses
of elements from isotopic abundance
1. Find out the percentage abundance
of each isotope of the element.
You are usually given this information.
2. Pretend you have 100 atoms of
that element.
3. Calculate the mass of 100 atoms
from their isotopic abundance.
4. Divide by 100 to get the
relative atomic mass.
Calculation
Calculating the relative atomic mass of an
element from isotopic abundance
Calculate the relative atomic mass of a
sample of lithium given that a mass
spectrometer shows that it consists of 7.4 %
of 6Li and 92.6 % of 7Li
7.4 of mass 6
7.4 x 6 = 44.40
100 Atoms
92.6 of mass 7
Higher Level
2006 – Q10 (a) (6)
92.6 x 7 = 648.20
Mass of 100 Atoms = 44.40 + 648.20 = 692.60
Mass of 1 Atoms = Relative Atomic Mass = 692.60/100 = 6.926
Mass spectrometer in determining
relative atomic masses
The mass spectrometer can be
used to measure relative
atomic masses.
It is also commonly used to determine
concentrations of drugs in urine
samples
Higher Level
Fundamental processes that occur
in the mass spectrometer
1.
2.
3.
4.
5.
Vaporisation
Production of positive ions
Acceleration
Separation
Detection
Higher Level
Stage 1 - Vaporisation
The liquid sample is
injected into the
instrument.
There is a vacuum
inside the chamber so
the liquid turns into a
gas – becomes
vaporised.
Higher Level
Stage 2 – Ionisation
The vaporised sample
passes into the ionisation
chamber.
The particles in the sample
are bombarded with a stream
of electrons.
The collisions will knock one
or more electrons out of the
sample particles to make
positive ions.
Higher Level
3 - Acceleration
Negatively charged plates in
the accelerator attract the
positively charged ions out of
the ionisation chamber and
into the accelerator
As the ions pass through the
plates they accelerate and an
ion beam passes into the
separator.
Negatively
charged plates
Higher Level
4 - Separation
A single beam of mixed ions
passes into the electromagnet.
Different ions are deflected by the
magnetic field by different
amounts.
The amount of deflection
depends on the mass of the ion
(lighter ions are deflected more
than heavier ones).
Higher Level
5. Detection
By changing the strength of the
magnetic field ions of different
masses are focused on the
detector
The signal is amplified and
can be viewed on a computer
screen
The signal is sent to a
recorder which traces out a
mass spectrum.
The mass spectrum is
interpreted by the scientist.
Higher Level
A mass spectrum
Mass spec video – Go to 3mins
0 4 mins