ATOMS
THE BUILDING BLOCKS OF
MATTER
I. FOUNDATIONS OF
ATOMIC THEORY
A. DEMOCRITUS- 400 B.C.
1. NAMED THE ATOM
2. FROM ATOMOS
MEANING INDIVISIBLE
3. NO EXPERIMENTAL
EVIDENCE
B. ARISTOTLE- JUST AFTER
DEMOCRITUS
1. DID NOT BELIEVE IN THE
ATOM
2. THOUGHT MATTER WAS
CONTINUOUS.
3. ACCEPTED FOR NEARLY
2000 YEARS
4. NO EXPERIMENTAL
EVIDENCE
II. DALTON’S ATOMIC
THEORY
A. JOHN DALTON- 1808
1. ENGLISH SCHOOL
TEACHER
2. USED THE LAWS OF
CONSERVATION OF MASS,
AND DEFINITE AND
MULTIPLE PROPORTIONS.
B. 5 PARTS TO DALTON’S ATOMIC
THEORY
1. ALL MATTER IS COMPOSED OF
EXTREMELY SMALL PARTICLES AND
CALLED ATOMS.
2. ATOMS OF A GIVEN ELEMENT ARE
IDENTICAL IN SIZE, MASS, AND
OTHER PROPERTIES; ATOMS OF
DIFFERENT ELEMENTS DIFFER IN
SIZE, MASS, AND OTHER PROPERTIES
3. ATOMS CANNOT BE
SUBDIVIDED, CREATED, OR
DESTROYED.
4. ATOMS OF DIFFERENT
ELEMENTS COMBINE IN SIMPLE
WHOLE NUMBER RATIOS TO
FORM CHEMICAL COMPOUNDS.
5. IN CHEMICAL REACTIONS,
ATOMS ARE COMBINED,
SEPARATED, OR REARRANGED.
C. LAW OF CONSERVATION
OF MASS- MASS IS
NEITHER CREATED NOR
DESTROYED DURING
ORDINARY CHEMICAL
REACTIONS OR PHYSICAL
CHANGES, IT MERELY
CHANGES FORM.
D. LAW OF DEFINITE
PROPORTIONS- CHEMICAL
COMPOUNDS CONTAIN THE
SAME ELEMENTS IN
EXACTLY THE SAME
PROPORTIONS BY MASS
REGARDLESS OF THE SIZE OF
THE SAMPLE OR SOURCE OF
THE COMPOUND.
E. LAW OF MULTIPLE
PROPORTIONS- IF TWO OR MORE
DIFFERENT COMPOUNDS ARE
COMPOSED OF THE SAME TWO
ELEMENTS, THEN THE RATIO OF
THE MASSES OF THE SECOND
ELEMENT COMBINED WITH A
CERTAIN MASS OF THE FIRST
ELEMENT IS ALWAYS A RATION
OF SMALL WHOLE NUMBERS.
III. MODERN ATOMIC
THEORY
A. MODERN ATOMIC THEORY
HAS BEEN MODIFIED TO
ACCOUNT FOR THE NEW
KNOWLEDGE WE HAVE
GAINED SINCE DALTON’S
TIME.
IV. DISCOVERY OF THE
ELECTRON
A. THE LATE 1800’S SAW
MANY EXPERIMENTS
INVOLVING CATHODE
RAY TUBES AND GASES
AT LOW PRESSURE.
B. CATHODE RAYS AND ELECTRONS
1. EXPERIMENTS: THE SURFACE OF
THE TUBE OPPOSITE THE CATHODE
GLOWED WHEN ELECTRIC CURRENT
WAS PRESENT.
2. HYPOTHESIS: GLOW WAS CAUSED
BY A STREAM OF PARTICLES
(CATHODE RAY)
3. OBSERVATIONS:
a. AN OBJECT BETWEEN
THE CATHODE AND ANODE
CAST A SHADOW ON THE
GLASS.
b. A PADDLE WHEEL
PLACED BETWEEN THE
CATHODE AND ANODE
ROLLS FROM THE CATHODE
TO THE ANODE.
c. RAYS DEFLECTED BY A
MAGNETIC FIELD LIKE A
NEGATIVE PARTICLE
CARRYING WIRE (i.e. AN
ELECTRICAL WIRE)
d. RAYS DEFLECTED FROM
A NEGATIVELY CHARGED
OBJECT.
4. ALL OBSERVATIONS
WERE SUPPORTED BY
JOSEPH JOHN THOMSON IN
1897 (READ P71) WHO
REVEALED THE LARGE
CHARGE ON AN
ELECTRON
C. CHARGE AND MASS OF THE
ELECTRON
1. ROBERT MILLIKAN (OIL DROP
EXPERIMENT) 1909 DISCOVERED
THE MASS OF THE ELECTRON
2. e- HAS A MASS OF 9.109x10-31 kg.
a. MILLIKAN CONFIRMED THE
ELECTRON’S NEGATIVE
CHARGE.
b. ATOMS ARE
ELECTRICALLY NEUTRAL,
THEY MUST CONTAIN THE
SAME NUMBER OF + AND –
CHARGES
c. ELECTRONS ARE LESS
MASSIVE THAN AN ATOM, SO
AN ATOM MUST CONTAIN
OTHER PARTICLES.
V. DISCOVERY OF THE
ATOMIC NUCLEUS
A. ERNEST RUTHERFORD, HANS
GEIGER, ERNEST MARSDEN
(GOLD FOIL EXPERIMENT)- 1911
1. EXPERIMENT: BOMBARDED
GOLD FOIL WITH ALPHA
PARTICLES.
2. HYPOTHESIS:
a. MASS AND CHARGE
UNIFORMLY DISTRIBUTED
b. ALPHA PARTICLES
SHOULD PASS THROUGH.
3. RESULTS: 1 IN 8000 ALPHA
PARTICLES BOUNCED
DIRECTLY BACK
4. CONCLUSION: RUTHERFORD
DECIDED THAT THERE MUST BE A
DENSELY PACKED BUNDLE WITH A
POSITIVE CHARGE
5. CALLED THE BUNDLE THE
NUCLEUS.
6. SUGGESTED NUCLEUS
SURROUNDED BY ELECTRONS
7. MODEL SOMETIMES CALLED THE
PLANETARY MODEL.
VI. COMPOSITION OF THE
ATOMIC NUCLEUS
A. NEUTRONS AND PROTONS
B. SIMPLEST H NUCLEUS ONLY HAS A
PROTON (1.673 x 10-27 kg)
C. ALL OTHER ATOMS HAVE
NEUTRONS (MASS 1.675 x 10-27 kg)
D. PROTONS DETERMINE AN ATOM’S
IDENTITY
E. ATOMIC NUMBER IS # OF PROTONS
F. SEE P 74 TABLE 3-1
G. FORCES IN THE NUCLEUS
1. OPPOSITES ATTRACT, LIKES REPEL
2. PROTONS HAVE STRONG
ATTRACTION WHEN REALLY CLOSE.
THE SAME FOR PROTON-NEUTRON
AND NEUTRON-NEUTRON
INTERACTIONS
3. KNOWN AS NUCLEAR FORCES!
VII. SIZES OF ATOMS
A. ELECTRONS- ELECTRON CLOUD
B. RADIUS- DISTANCE FROM CENTER
OF THE NUCLEUS TO THE OUTER
PORTION OF THE ELECTRON
CLOUD
C. RADII EXPRESSED IN PICOMETERS
D. 1pm=10-12m=10-10cm
E. RADII RANGE FROM 40 TO 270 pm
VIII. ATOMIC NUMBER
A. SYMBOL IS Z
B. NUMBER OF PROTONS IN THE
NUCLEUS OF EACH ATOM OF A
PARTICULAR ELEMENT
C. PERIODIC TABLE ARRANGE IN
ORDER OF INCREASING ATOMIC
NUMBER
D. ATOMIC NUMBER IDENTIFIES THE
ELEMENT.
IX. ISOTOPES
A. ATOMS OF THE SAME ELEMENT
THAT HAVE DIFFERENT MASSES
B. EXAMPLE: HYDROGEN
1. PROTIUM (HYDROGEN-1)
2. DEUTERIUM (HYDROGEN-2)
3. TRITIUM (HYDROGEN-3)
C. MOST ELEMENTS CONSIST OF
MIXTURES OF ISOTOPES
X. MASS NUMBER
A. TO IDENTIFY AN ISOTOPE ONE
MUST HAVE THE MASS NUMBER
AND THE NAME OF THE ELEMENT.
B. THE MASS NUMBER IS THE TOTAL
NUMBER OF PROTONS AND
NEUTRONS.
XI. DESIGNATING ISOTOPES
A. HYPHEN NOTATION
1. HYDROGEN-3
2. URANIUM-235
B. SYMBOL NOTATION
1. 3
H
1
2. 235U
92
C. NUCLIDE- A GENERAL TERM FOR
AN ISOTOPE OF ANY ELEMENT.
XII. RELATIVE ATOMIC
MASSES
A. ONE ATOM WAS ARBITRARILY
CHOSEN AS A STANDARD AND
ASSIGNED A RELATIVE MASS
VALUE.
B. THIS ATOM IS CARBON-12.
1. HAS AN ATOMIC MASS OF 12
ATOMIC MASS UNITS (AMU).
C. ALTHOUGH ISOTOPES HAVE
DIFFERENT MASSES, THEY DO NOT
DIFFER SIGNIFICANTLY IN THEIR
CHEMICAL BEHAVIOR.
D. SUBATOMIC PARTICLES’ MASSES
CAN BE EXPRESSED IN ATOMIC
MASS UNITS AS WELL.
1. ELECTRON- 0.0005486 AMU
2. PROTON- 1.007276 AMU
3. NEUTRON- 1.008665 AMU
XIII. AVERAGE ATOMIC
MASSES OF ELEMENTS
A. MOST ELEMENTS OCCUR
NATURALLY AS A MIXTURE OF
ISOTOPES.
B. WE CAN AVERAGE THE ATOMIC
MASSES OF THESE ELEMENTS TO
GET AN AVERAGE ATOMIC MASS
FOR EACH ELEMENT.
C. LIKE YOUR GRADES THIS IS A
WEIGHTED AVERAGE DEPENDING
ON THE PERCENTAGE OF EACH
ISOTOPE.
D. EXAMPLE:
HYDROGEN-1 x (99.985%) = 1.007825
HYDROGEN-2 x (0.015 %) = 2.014102
HYDROGEN-3 (% NEGLIGIBLE)
AVERAGE: 1.00794 amu
ROUND ALL ATOMIC
MASSES TO TWO
DECIMAL PLACES FOR
USE IN CALCULATIONS,
EXCEPT OXYGEN WHICH
IS ALWAYS 16.00 amu!!!
XIV. RELATING MASS TO
NUMBERS OF ATOMS
A. THE MOLE-SI UNIT FOR AMOUNT
OF A SUBSTANCE.
B. THE AMOUNT OF SUBSTANCE THAT
CONTAINS AS MANY PARTICLES AS
THERE ARE ATOMS IN EXACTLY
12g OF CARBON-12
C. COUNTING UNIT JUST LIKE A
DOZEN!!!
D. AVOGADRO’S NUMBER-THE
NUMBER OF PARTICLES IN A MOLE
E. 1 mol = 6.0221367 x 1023 PARTICLES
F. ROUND TO 6.02 x 1023
G. MOLAR MASS
1. ALTERNATIVE DEFINITION OF THE
MOLE- THE AMOUNT OF A
SUBSTANCE THAT CONTAINS
AVOGADRO’S NUMBER OF
PARTICLES.
2. MOLAR MASS IS THE MASS OF ONE
MOLE OF A PURE SUBSTANCE.
3. UNITS: g/mol
4. = TO ATOMIC MASS
H. GRAM/MOLE CONVERSIONS
SEE THE EXAMPLES I WILL GIVE
YOU!!
I. CONVERSIONS WITH AVOGADRO’S
NUMBER
SEE THE EXAMPLES I WILL GIVE
YOU!!