Download Atomic Structure Ch2 powerpoint

1.2 Atomic Structure
(Time needed: 6 class periods)
Learning outcomes
• Matter is composed of particles, which may be
atoms, molecules or ions.
• Atoms. Minute size of atoms.
• Law of conservation of mass.
DIFFUSION- evidence for the existence
of small particles
• SPREADING OUT OF GASES
• COLOUR OF INK SREADING OUT WHEN MIXED
WITH WATER
• HYDROGEN CHLORIDE AND AMMONIA
SOLUTION
AMMONIUM CHLORIDE
law of conservation of mass/matter
•The law of conservation of mass/matter, also known as law
of mass/matter conservation says that the mass of a closed
system will remain constant, regardless of the processes
acting inside the system.
•Matter cannot be created/destroyed, although it may be
rearranged.
•For any chemical process in a closed system, the mass of the
reactants must equal the mass of the products.
Learning Outcomes
•Very brief outline of the historical development of atomic theory
(outline principles only; mathematical treatment not required): Dalton:
atomic theory;
•Crookes: vacuum tubes, cathode rays;
•Stoney: naming of the electron;
•Thomson: negative charge of the electron; e/m for electrons
(experimental details not required);
•Millikan: magnitude of charge of electrons as shown by oil drop
experiment (experimental details not required);
•Rutherford: discovery of the nucleus as shown by the α−particle
scattering experiment;
•discovery of protons in nuclei of various atoms;
•Bohr: model of the atom;
•Chadwick: discovery of the neutron.
Models of the Atom
"In science, a wrong theory can be valuable and better than no theo
- Sir William L. Bragg
e
e +
e +e
+e +
+
e
e
+e
+ e +e
Dalton’s
Greek model
model
(400
(1803)
B.C.)
Thomson’s plum-pudding
model (1897)
Bohr’s model
(1913)
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125
-
- +
Rutherford’s model
(1909)
Charge-cloud model
(present)
HISTORY OF THE ATOM
• GREEKS – MATTER MADE OF TINY INDIVISIBLE
PARTICLES
DALTON 1766-1844
• ALL MATTER MADE OF SMALL PARTICLES
CALLED ATOMS
• ATOMS ARE INDIVISIBLE
• ATOMS CANNOT BE CREATED OR DESTROYED
Dalton’s Symbols
John Dalton
1808
DISCOVERY OF THE ELECTRON
• CROOKES CONDUCTED EXPERIMENTS WITH A GLASS TUBE
• Go to video clip
CROOKES TUBE
CROOKES TUBES
• CATHODE CONNECTED TO NEGATIVE
ELECTRODE
• ANODE CONNECTED TO THE POSITIVE
ELECTRODE
• CNAP
VACUUM TUBES
• GAS AT LOW PRESSURE
• ELECTRIC CURRENT PASSED THROUGH
• RADIATION CAME FROM THE END OF THE
TUBE CONNECTED TO THE
NEGATIVE(CATHODE) END OF THE BATTERY
• CATHODE RAYS
TUBES
CROOKES PADDLE TUBE
CATHODE RAYS
•
•
•
•
CAST SHADOWS
CAUSE GLASS TO GLOW
TURN A PADDLE WHEEL
RAYS ARE MADE OF PARTICLES
JJ THOMPSON
• HOLE IN ANODE TO ALLOW
BEAM OF RAYS TO PASS
THROUGH.
• BEAM COULD BE
DEFLECTED BY ELECTRIC
PLATES.
• THEREFORE BEAM IS
MADE OF NEGATIVE
PARTICLES.
JJ THOMPSONS APPARATUS
JJ THOMPSON
• Used a magnetic field from an electromagnet
to deflect the electrons
• Calculated the ratio of charge to mass for
electron
GEORGE STONEY
• NAMED PARTICLES ELECTRONS
ROBERT MILLIKAN
• Famous oil-drop experiment
• It measured the charge on the electron
• X-rays ionised air molecules by striping electrons
off their atoms.
• Oil droplets picked up electrons became negative
• Increased the + charge until the droplet hovered.
• Took measurements and calculated the charge
on the electron.
ROBERT MILLIKAN
ROBERT MILLIKAN
THOMPSON’S ATOM
• ATOM A SPHERE OF POSITIVE CHARGES WITH
NEGATIVE ELECTONS EMBEDDED
ERNEST RUTHERFORD
• Fired thin alpha particles at a tin gold foil
• Thompsons plum pudding model predicted
that they would pass thru’ with little
deflection
RUTHERFORD’S EXPT
Go to Atom video
•
RUTHERFORD’S EXPT
EXPECTED RESULT
• ALPHA PARTICLES SHOULD PASS THROUGH
WITH LITTLE DEFLECTION
+
++
ACTUAL RESULT
• Most pass through undeflected
• Some were deflected at large angles
• Some bounced right back!
EXPLANATION
•
•
•
•
Observation 1
Most pass through undeflected
Deduction
Atoms are mostly empty space.
EXPLANATION
• Observation 2
• Some were deflected at large angles
• Deduction
• The positive alpha particles had hit something
positive
EXPLANATION
•
•
•
•
Observation 3
Some bounced right back!
Deduction
Hard dense core of positive matter in the
center of each atom-nucleus
THE PROTON
• Rutherford continued to bombard different
elements such as nitrogen and oxygen
• Small positive particles were given off--PROTONS
THE NEUTRON
• James Chadwick bombarded beryllium with
alpha particles.
• Small particles were given off which were
neutral and had the same mass as the
proton—the neutron.
Bohr’s atom
• Electrons
travel in
orbits
around the
nucleus
Learning Outcomes
• Properties of electrons,
protons and neutrons
(relative mass, relative
charge, location within
atom).
Proton
• Protons are
positively
charged
particles
found within
atomic
nucleus
Learning Outcomes
Atomic number (Z ), mass number (A),
isotopes; hydrogen and carbon as
examples of isotopes.
Relative atomic mass (A r). The
12C scale for relative atomic
masses.
Atomic number
• Also called
proton number,
this is the
number of
protons the
atom has
Atomic number
• Also called
proton number,
this is the
number of
protons the
atom has
The Number of Electrons
• Atoms must have equal numbers
of protons and electrons. In our
example, an atom of krypton
must contain 36 electrons since it
contains 36 protons.
Mass number
•Mass Number =
(Number of Protons) +
(Number of Neutrons)
Isotope
• Atoms that have the same
number of protons but different
numbers of neutrons are called
isotopes
Hydrogen isotopes
• The element hydrogen for
example, has three commonly
known isotopes: protium,
deuterium and tritium
Deuterium
•an atom of deuterium
consists of one proton
one neutron and one
electron
Tritium
• An atom of tritium consists
of one proton two
neutrons and one electrons
Relative Atomic Mass
• The relative atomic mass of an
element the mass of one of
the element's atoms -- relative
to the mass of an atom of
Carbon 12,
Learning Outcomes
• Calculation of approximate relative atomic
masses from abundance of isotopes of given
mass number (e.g. Calculation of approximate
relative atomic mass of chlorine).
Chlorine
•Chlorine-35 and
Chlorine-37 are both
isotopes of chlorine
Relative mass of chlorine
• Chlorine consists of roughly 75%
Chlorine-35 and roughly 25%
Chlorine-37. We take an average
of the two figures The relative
atomic mass of chlorine is usually
quoted as 35.5.
Learning outcomes
• Use of the mass spectrometer in determining
relative atomic mass.
• Fundamental processes that occur in a mass
spectrometer:
• vaporisation of substance,
• production of positive ions,
• acceleration, separation,
• detection (mathematical
• treatment excluded).
THE MASS SPECTROMETER
• Atoms can be deflected by
magnetic fields - provided the
atom is first turned into an ion.
Stage 1: Ionisation
• The atom is ionised by
knocking one or more
electrons off to give a positive
ion.
Stage 2: Acceleration
• The ions are accelerated so
that they all have the same
kinetic energy.
Stage 3: Deflection
• The ions are then deflected by a
magnetic field according to their
masses. The lighter they are, the
more they are deflected.
Stage 4: Detection
• The beam of ions passing
through the machine is
detected electrically.