1.2 Atomic Structure (Time needed: 6 class periods)
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Transcript of 1.2 Atomic Structure (Time needed: 6 class periods)
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1.2 Atomic Structure(Time needed: 6 class periods)
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Learning outcomes
• Matter is composed of particles, which may be atoms, molecules or ions.
• Atoms. Minute size of atoms.• Law of conservation of mass.
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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
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AMMONIUM CHLORIDE
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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.
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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.
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Models of the Atom
Dalton’s model (1803)
Thomson’s plum-pudding model (1897)
Rutherford’s model (1909)
Bohr’s model (1913)
Charge-cloud model (present)
Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 125
Greek model(400 B.C.)
+--
--
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++ +
+++
+ +
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"In science, a wrong theory can be valuable and better than no theory at all."- Sir William L. Bragg
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HISTORY OF THE ATOM
• GREEKS – MATTER MADE OF TINY INDIVISIBLE PARTICLES
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DALTON 1766-1844
• ALL MATTER MADE OF SMALL PARTICLES CALLED ATOMS
• ATOMS ARE INDIVISIBLE• ATOMS CANNOT BE CREATED OR DESTROYED
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DISCOVERY OF THE ELECTRON• CROOKES CONDUCTED EXPERIMENTS WITH A GLASS TUBE• Go to video clip
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CROOKES TUBE
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CROOKES TUBES
• CATHODE CONNECTED TO NEGATIVE ELECTRODE
• ANODE CONNECTED TO THE POSITIVE ELECTRODE
• CNAP
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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
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TUBES
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CROOKES PADDLE TUBE
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CATHODE RAYS
• CAST SHADOWS• CAUSE GLASS TO GLOW• TURN A PADDLE WHEEL• RAYS ARE MADE OF PARTICLES
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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.
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JJ THOMPSONS APPARATUS
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JJ THOMPSON
• Used a magnetic field from an electromagnet to deflect the electrons
• Calculated the ratio of charge to mass for electron
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GEORGE STONEY
• NAMED PARTICLES ELECTRONS
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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.
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ROBERT MILLIKAN
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ROBERT MILLIKAN
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THOMPSON’S ATOM
• ATOM A SPHERE OF POSITIVE CHARGES WITH NEGATIVE ELECTONS EMBEDDED
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ERNEST RUTHERFORD
• Fired thin alpha particles at a tin gold foil• Thompsons plum pudding model predicted
that they would pass thru’ with little deflection
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RUTHERFORD’S EXPTGo to Atom video
•
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RUTHERFORD’S EXPT
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EXPECTED RESULT
• ALPHA PARTICLES SHOULD PASS THROUGH WITH LITTLE DEFLECTION
+ +
+
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ACTUAL RESULT• Most pass through undeflected• Some were deflected at large angles• Some bounced right back!
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EXPLANATION• Observation 1• Most pass through undeflected• Deduction • Atoms are mostly empty space.
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EXPLANATION• Observation 2• Some were deflected at large angles
• Deduction• The positive alpha particles had hit something
positive
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EXPLANATION• Observation 3• Some bounced right back!• Deduction• Hard dense core of positive matter in the
center of each atom-nucleus
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THE PROTON
• Rutherford continued to bombard different elements such as nitrogen and oxygen
• Small positive particles were given off--- PROTONS
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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.
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Bohr’s atom
• Electrons travel in orbits around the nucleus
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Learning Outcomes
• Properties of electrons, protons and neutrons (relative mass, relative charge, location within atom).
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Proton
• Protons are positively charged particles found within atomic nucleus
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Atomic number (Z ), mass number (A), isotopes; hydrogen and carbon as examples of isotopes.Relative atomic mass (A r). The12C scale for relative atomicmasses.
Learning Outcomes
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Atomic number
• Also called proton number, this is the number of protons the atom has
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Atomic number
• Also called proton number, this is the number of protons the atom has
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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.
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Mass number
•Mass Number = (Number of Protons) + (Number of Neutrons)
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Isotope
• Atoms that have the same number of protons but different numbers of neutrons are called isotopes
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Hydrogen isotopes
• The element hydrogen for example, has three commonly known isotopes: protium, deuterium and tritium
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Deuterium
•an atom of deuterium consists of one proton one neutron and one electron
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Tritium
• An atom of tritium consists of one proton two neutrons and one electrons
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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,
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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).
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Chlorine
•Chlorine-35 and Chlorine-37 are both isotopes of chlorine
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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.
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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).
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THE MASS SPECTROMETER
• Atoms can be deflected by magnetic fields - provided the atom is first turned into an ion.
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Stage 1: Ionisation
• The atom is ionised by knocking one or more electrons off to give a positive ion.
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Stage 2: Acceleration
• The ions are accelerated so that they all have the same kinetic energy.
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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.
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Stage 4: Detection
• The beam of ions passing through the machine is detected electrically.
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