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

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.)

+--

--

-e

ee

++ +

+++

+ +

e

eeee

ee

"In science, a wrong theory can be valuable and better than no theory at all."- Sir William L. Bragg

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 EXPTGo 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

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

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.