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THE STRUCTURE
OF THE ATOM
ATOMIC MODELS
PIONEERS OF THE ATOM
• DEMOCRITUS
• LAVOISIER
• DALTON
• THOMPSON
• MILLIKEN
• RUTHERFORD
THE EARLY ATOM
IT’S ALL
GREEK
TO ME...
• DEMOCRITUS
IS THE MAN!!!
• YOU KNOW IT!
DEMOCRITUS
• WAS A GREEK PHILOSOPHER THAT
BEGAN WONDERING WHAT WAS
THE UNIVERSE MADE UP OF.
• HE PROPOSED THAT ALL THE
MATTER IN THE UNIVERSE IS
COMPOSED OF TINY PARTICLES
• THESE PARTICLES ARE INDIVISABLE
• HE CALLED THESE PARTICLES
ATOMOS (MEANS INDIVISABLE)
• BUT HE HAD AN OPPONENT…
ARISTOTLE = SCEPTIC
DEMOCRITUS IS
AN
IGNORAMUS!
ARISTOTLE WAS
MUCH MORE
POPULAR AND HE
OPPOSED THE IDEA
OF ATOMS!
ARISTOTLE = SCEPTIC
DEMOCRITUS IS
AN
IGNORAMUS!
ARISTOTLE
BELIEVED THAT
MATTER WAS
CONTINUOUS.
DEMOCRITUS’
ATOMS
SO...CHEMISTRY DIED
FOR ABOUT 1400 YEARS
UNTIL...
SO...CHEMISTRY DIED
FOR ABOUT 1400 YEARS
IN THE
1700’S
FOUNDATIONS OF
ATOMIC THEORY
• THE PREDOMINATE BELIEF BY THE
MID 1700’S WAS THE MODERN
DEFINITION OF AN ELEMENT AS A
SUBSTANCE THAT CANNOT BE
BROKEN DOWN BY ORDINARY
CHEMICAL MEANS
• IT WAS ALSO BELIEVED THAT
ELEMENTS COMBINE TO FORM
COMPOUNDS THAT HAVE
DIFFERENT PHYS AND CHEM
PROPERTIES THAN THOSE OF THE
ELEMENTS THAT FORM THEM.
FOUNDATIONS OF
ATOMIC THEORY
• HOWEVER, THERE WAS
CONTROVERSY AS TO WHETHER
ELEMENTS ALWAYS COMBINE IN
THE SAME RATIO WHEN FORMING
A PARTICULAR COMPOUND.
• IN THE 1790’S, THE STUDY OF
MATTER WAS REVOLUTIONIZED BY
A NEW EMPHASIS ON THE
QUANTITATIVE ANALYSIS
– SCIENTISTS’ INVESTIGATIONS
WERE MADE MORE ACCURATE BY
NEW IMPROVED BALANCES
ANTOINNE LAVOISIER
TOOK ADVANTAGE…
MATTER CAN BE
NEITHER CREATED
NOR DESTROYED.
HE BEGAN TO GATHER
EVIDENCE ABOUT HOW
MATTER BEHAVED
BEFORE AND AFTER A
REACTION.
ANTOINNE LAVOISIER
TOOK ADVANTAGE…
MATTER CAN BE
NEITHER CREATED
NOR DESTROYED.
HE DISCOVERED THAT THE
TOTAL MASS OF THE
INGREDIENTS OF A
CHEMICAL REACTION WAS
THE SAME AS THE TOTAL
MASS OF THE RESULTS
THE LAW OF
DEFINITE
PROPORTION
THE LAW OF
MULTIPLE
PROPORTIONS
I’M BETTER
LOOKING THAN
PROUST!
LAW OF
DEFINITE
PROPORTIONS
LAW OF
MULTIPLE
PROPORTIONS
A CHEM COMPD IS ALWAYS
COMPOSED OF THE SAME ELEM.
IN THE SAME PROPOR. BY MASS
CARBON
MONOXIDE
+
CARBON
MONOXIDE
+
=
C
O
CO
IF 2 DIFFERENT COMPDS ARE
COMPOSED OF THE SAME 2
ELEMTS, THEN THE RATIO OF OF
THE ELEMNTS IS ALWAYS A
RATIO OF SMALL WHOLE #’S
CARBON
DIOXIDE
C
O
+
+
C
=
O
CO
=
O
CO2
DALTON’S THOUGHTS
• IN 1808, JOHN DALTON PROPOSED
AN EXPLANATION FOR THE LAW OF
CONS OF MASS, THE LAW OF
DEFINITE PROPORTIONS, &
FORMULATED THE LAW OF
MULTIPLE PROPORTIONS.
• HE REASONED THAT ELEMENTS
WERE COMPOSED OF ATOMS &
THAT ONLY WHOLE #’S OF ATOMS
CAN COMBINE TO FORM COMPNDS
• HIS IDEAS ARE NOW CALLED THE
ATOMIC THEORY OF MATTER AND
CAN BE SUMMARIZED AS:
1.ALL MATTER IS
COMPOSED OF
EXTREMELY SMALL
PARTICLES CALLED ATOMS
2.ATOMS OF A GIVEN
ELEMENT ARE IDENTICAL
IN SIZE, MASS, AND OTHER
PROPERTIES; ATOMS OF
DIFFERENT ELEMENTS
DIFFER IN SIZE, MASS, &
OTHER PROPERTIES
ELEMENT
2
ELEMENT
3
ELEMENT
4
3.ATOMS CANNOT BE
SUBDIVIDED,
CREATED, OR
DESTROYED
4.ATOMS OF DIFFERENT
ELEMENTS COMBINE
IN SIMPLE WHOLE #
RATIOS TO FORM
CHEM COMPDS
5.IN CHEMICAL RXNS,
ATOMS ARE
COMBINED,
SEPARATED, OR
REARRANGED
+
+
DALTON AND HIS
CONTEMPORARIES
THE EARLIEST THEORISTS
THOUGHT THE ATOM WAS
HARD AND ROUND,
MUCH LIKE TINY MARBLES
OR BALL BEARINGS.
STRUCTURE OF THE ATOM
• ALTHOUGH DALTON THOUGHT
ATOMS WERE INDIVISIBLE,
INVESTIGATORS IN THE LATE
1800’S PROVED OTHERWISE
• IT SOON BECAME CLEAR THAT
ATOMS ARE ACTUALLY COMPOSED
OF SEVERAL BASIC TYPES OF
SMALLER PARTICLES
• AND IT’S THE NUMBERS AND
ARRANGEMENTS OF THESE SUBATOMIC PARTICLES THAT
DETERMINES THE IDENTITY OF
THE ATOM.
STRUCTURE OF THE ATOM
• THE FIRST DISCOVERY OF A
SUBATOMIC PARTICLE RESULTED
FROM THE INVESTIGATIONS INTO
THE RELATIONSHIP BETWEEN
ELECTRICITY AND MATTER.
• IN THE LATE 1800’S, MANY
EXPERIMENTS WERE PERFORMED
IN WHICH ELECTRIC CURRENT
WAS PASSED THROUGH VARIOUS
GASES AT LOW PRESS
– CARRIED OUT IN
TUBES CALLED
CATHODE-RAY TUBES
STRUCTURE OF THE ATOM
• INVESTIGATORS NOTICED THAT
WHEN CURRENT WAS PASSED
THROUGH A CATHODE RAY TUBE,
THE SURFACE OF THE TUBE
DIRECTLY OPPOSITE THE
CATHODE GLOWED.
STRUCTURE OF THE ATOM
• THEY HYPOTHESIZED THAT THE
GLOW WAS CAUSED BY A STREAM
OF PARTICLES
– THEY CALLED THE STREAM A
CATHODE RAY
STRUCTURE OF THE ATOM
• THE CATHODE RAY TRAVELED
FROM THE CATHODE TO THE
ANODE WHEN CURRENT WAS
PASSED THROUGH THE TUBE.
• THE CATHODE RAY TUBE OPENED
THE DOOR FOR J.J. THOMPSON
J. J. THOMPSON
I PLAY WITH
ELECTRONS
J. J. THOMPSON
• THOMPSON’S INVESTIGATIONS
SUPPLIED EVIDENCE THAT THE
CATHODE RAY COULD BE
DEFLECTED BY APPLYING A
POSITIVELY CHARGED ELECTRIC
FIELD
– THIS HELPED
THOMPSON DISCOVER THAT THE
CATHODE RAY WAS
NEGATIVELY
CHARGED.
• THOMPSON WAS ABLE TO MEASURE
THE RATIO OF THE CHARGE OF
CATHODE RAY PARTICLES TO
THEIR MASS
– HE FOUND THAT THE RATIO WAS
THE SAME REGARDLESS OF THE
METAL USED AS THE CATHODE
• THOMPSON CONCLUDED THAT ALL
CATHODE RAYS ARE COMPOSED OF
IDENTICAL NEGATIVELY CHARGED
PARTICLES
– WHICH WERE LATER CALLED
ELECTRONS
THOMPSON’S ATOMIC MODEL
• HE PROPOSED THAT THE ATOM
LOOKS SOMETHING LIKE A
CHOCOLATE CHIP COOKIE
THE CHIPS WOULD BE
HIS ELECTRONS
THE COOKIE PART
IS POS. MATTER TO
CANCEL OUT THE NEG.
ELECTRONS
• THOMPSON’S EXPERIMENTS
REVEALED THAT THE ELECTRON
HAS A VERY LARGE CHARGE IN
RELATION TO ITS MASS
• IN 1909, ROBERT MILLIKAN,
PERFORMED AN INGENIOUS
EXPERIMENT TO CALCULATE THE
MASS OF AN ELECTRON
– HE DISCOVERED THAT THE MASS OF
THE ELECTRON IS ABOUT 1/2000TH
THE MASS OF THE SIMPLEST ATOM
(HYDRO.)
HELLO, MILLIKAN
DAT’S RIGHT!
I SAVED THE
DAY!
• HIS OIL DROP EXPERIMENT
SHOWED THAT EVEN THOUGH THE
ATOMS OF DIFFERENT ELEMENTS
ARE VERY DIFFERENT, THEIR
ELECTRONS ARE IDENTICAL.
– AN ELECTRON IS AN ELECTRON
– THE DIFFERENCE BETWEEN TWO
DIFFERENT ATOMS IS THEIR
NUMBER OF ELECTRONS NOT THEIR
TYPE OF ELECTRON.
• HE ALSO CALCULATED THAT THE
ELECTRON’S MASS IS 9.109x10-31kg
SO FAR WE’VE LEARNED…
• THOMPSON’S & MILLIKAN’S IDEAS:
– ATOMS ARE IN FACT DIVISABLE
– ELECTRONS ARE PRESENT IN
ATOMS OF ALL ELEM.
– ONE OF THE ATOM’S FUNDMNTL
PARTICLES IS NEG. CHARGED
– ATOMS ARE ELECTRICALLY
NEUTRAL, SO THERE MUST BE A (+)
CHARGE TO BALANCE OUT THE (–)
– BECAUSE ELECTRONS ARE
BASICALLY MASSLESS THERE MUST
BE SOMETHING ELSE THAT
ACCOUNTS FOR THE ATOMS MASS.
OTHER SIGNIFICANT
DISCOVERIES…
• IN 1886, WHEN E. GOLDSTEIN
OBSERVED A CATHODE-RAY TUBE
AND FOUND RAYS TRAVELLING IN
THE OPPOSITE DIRECTION OF THAT
OF THE CATHODE RAYS
– HE CALLED THE RAYS CANAL RAYS
AND CONCLUDED THAT THEY WERE
COMPOSED OF POS. PARTICLES
• IT WAS SOON DISCOVERED THAT
THE POSITIVE PARTICLES MASS IS
ABOUT 2000 TIMES THAT OF THE
ELECTRON
OTHER SIGNIFICANT
DISCOVERIES…
• IN 1932, THE ENGLISH PHYSICIST
JAMES CHADWICK CONFIRMED THE
EXISTANCE OF YET ANOTHER
SUBATOMIC PARTICLE.
– THE NEUTRON, WHICH IS A
PARTICLE WITH NO CHARGE
– HAS A MASS NEARLY EQUAL TO THAT
OF THE PROTON
• THEREFORE THE SUBATOMIC
PARTICLES ARE THE ELECTRON,
PROTON, AND NEUTRON.
electron
e
-1
0
9.11x10-28
proton
+
p
+1
1
1.67x10-24
neutron
n0
0
1
1.67x10-24
THE ATOMIC NUCLEUS
• WHEN SUBATOMIC PARTICLES
WERE DISCOVERED, SCIENTISTS
WONDERED HOW THESE PARTICLES
WERE PUT TOGETHER IN AN ATOM.
• THIS WAS A DIFFICULT QUESTION
TO ANSWER, GIVEN HOW TINY
ATOMS ARE.
• MOST SCIENTISTS THOUGHT IT
LIKELY THAT THE ELECTRONS WERE
EVENLY DISTRIBUTED
THROUGHOUT AN ATOM FILLED
UNIFORMLY WITH POSITIVELY
CHARGED MATERIAL.
ERNEST RUTHERFORD
HOW IS THE
ATOM BUILT?
IN 1911, RUTHERFORD
PERFORMED ONE OF
THE MOST BRILLIANT
INVESTIGATIONS
EVER CONCIEVED.
RUTHERFORD’S
FAMOUS EXPERIMENT
• RUTHERFORD SET OUT TO TEST THE
THOMPSON MODEL OF THE ATOM
• THE TEST USED RELATIVELY
MASSIVE RADIOACTIVE ALPHA
PARTICLES
– ALPHA PARTICLES () ARE HELIUM
ATOMS THAT HAVE LOST THEIR
2 ELECTRONS AND HAVE A DOUBLE
POSITIVE CHARGE BECAUSE OF THE
2 REMAINING PROTONS
• IN THE EXPERIMENT, RUTHERFORD
DIRECTED A NARROW BEAM OF
ALPHA PARTICLES AT A VERY THIN
SHEET OF GOLD FOIL.
– ACCORDING TO THE PREVAILING
THEORY, THE ALPHA PARTICLES
SHOULD HAVE PASSED EASILY
THROUGH THE GOLD, WITH ONLY A
SLIGHT DEFLECTION DUE TO THE
POSITIVE CHARGE THOUGHT TO BE
SPREAD OUT IN THE GOLD ATOMS.
WHAT HAPPENED?
• THE MAJORITY OF THE  PARTICLES
PASSED STRAIGHT THROUGH THE
GOLD ATOMS, WITHOUT ANY
DEFLECTION
• EVEN MORE SURPRISINGLY, A
SMALL FRACTION OF THE 
PARTICLES BOUNCED OFF THE
GOLD FOIL ATOMS AT VERY LARGE
ANGLES
• SOME EVEN BOUNCED BACK
AT THE SOURCE
• BASED ON THE RESULTS,
RUTHERFORD SUGGESTED A NEW
THEORY OF THE ATOM.
• HE PROPOSED THAT THE ATOM IS
MOSTLY EMPTY SPACE
–THIS EXPLAINED THE LACK OF
DEFLECTION OF SOME OF THE
PARTICLES
• HE CONCLUDED THAT ALL THE
POSITIVE CHARGE AND ALMOST
ALL THE MASS IS CONCENTRATED
IN A SMALL CORE
–HE CALLED THIS REGION THE
NUCLEUS
A LITTLE REVIEW!
• THERE ARE THREE FUNDAMENTAL
PARTICLES THAT TOGETHER MAKE
UP THE ATOM.
– COMPOSED OF ELECTRONS,
PROTONS, AND NEUTRONS
• THE PROTONS AND THE NEUTRONS
MAKE UP THE CENTER OF THE
ATOM
– CENTER OF THE ATOM IS CALLED
THE NUCLEUS
• THE PROTONS & NEUTRONS OCCUPY THE MASS OF THE ATOM
• THE ELECTRONS SURROUND THE
NUCLEUS AND OCCUPY MOST OF
THE ATOM’S VOLUME
• HOW, THEN, ARE ATOMS OF
HYDROGEN DIFFERENT FROM
THOSE OF OXYGEN?
– A SUBATOMIC PARTICLES FROM
ONE TYPE OF ATOM LOOKS LIKE
PARTICLES FROM OTHER ATOMS
• IT’S THE NUMBERS OF PROTONS IN
AN ATOM THAT MAKES ATOMS
DIFFERENT.
– AN OXYGEN ATOM HAS 8 PROTONS
IN ITS NUCLEUS
– A HYDROGEN ATOM HAS 1 PROTON
IN ITS NUCLEUS
• THE ATOMIC NUMBER OF AN
ELEMENT IS THE NUMBER OF
PROTONS IN THE NUCLEUS OF AN
ATOM OF THAT ELEMENT
– SINCE OXYGEN’S NUCLEUS HAS
8 PROTONS ITS ATOMIC # IS 8
• THE PROTONS IDENTIFY THE TYPE
OF ATOM, THE ONLY ATOM WITH
8 PROTONS IS OXYGEN
ATOMS OF THE 1ST TEN ATOMS
H
Helium
He
Lithium
Li
Beryllium Be
Boron
B
Carbon
C
Nitrogen
N
Oxygen
O
Fluorine
F
Neon
Ne
Hydrogen
1
2
3
4
5
6
7
8
9
10
1
2
3
4
5
6
7
8
9
10
0
2
4
5
6
6
7
8
10
10
1
4
7
9
11
12
14
16
19
20
1
2
3
4
5
6
7
8
9
10
• FOR EACH ELEMENT LISTED, THE
NUMBER OF PROTONS EQUALS THE
NUMBER OF ELECTRONS
– REMEMBER ATOMS ARE
ELECTRICALLY NEUTRAL
• IN AN ATOM, THE NUMBER OF
ELECTRONS MUST EQUAL THE
NUMBER OF PROTONS
• A HYDROGEN ATOM HAS 1 ELECTRN
• AN OXYGEN ATOM HAS 8 ELECTRNS
• THE MASS OF AN ATOM IS CONCEN.
IN ITS NUCLEUS AND DEPENDS ON
THE NUMER OF PROTONS AND
NEUTRONS
• THE TOTAL NUMBER OF PROTONS
AND NEUTRONS IN AN ATOM IS
CALLED THE MASS NUMBER
– A CARBON ATOM, WHICH HAS 6
PROTONS AND 6 NEUTRONS, HAS A
MASS NUMBER OF 12
• IF YOU KNOW THE ATOMIC NUMBER
AND MASS NUMBER OF AN ATOM OF
ANY ELEMENT, YOU CAN
DETERMINE THE ATOM’S
COMPOSITION
• THE COMPOSITION OF ANY ATOM
CAN BE REPRESENTED IN
SHORTHAND NOTATION:
CHEMICAL COMPOSITION
SHORTHAND
# OF PROTONS
+
# OF NEUTRONS
MASS
NUMBER
35
ATOMIC
NUMBER
17
Cl
NUMBER OF
PROTONS
ISOTOPES
• EVERY CHLORINE ATOM HAS 17
PROTONS, WITHOUT EXCEPTION,
– HOWEVER, NOT EVERY CHLORINE
ATOM HAS
18
NEUTRONS.
– ATOMS WITH THE SAME NUMBER OF
PROTONS BUT CONTAIN DIFFERENT
NUMBERS OF NEUTRONS ARE
CALLED ISOTOPES.
• BECAUSE ISOTOPES OF AN
ELEMENT HAVE DIFFERENT
NUMBERS OF NEUTRONS THEY HAVE
DIFFERENT MASS NUMBERS.
• ISOTOPES ARE CHEMICALLY ALIKE
BECAUSE THEY HAVE IDENTICAL
NUMBERS OF PROTONS AND
ELECTRONS
– IT’S THE ELECTRONS AND PROTONS
THAT ARE RESPONSIBLE FOR
CHEMICAL BEHAVIOR
BERYLLIUM
ISOTOPES
proton
neutron
electron
EXAMPLE OF AN
ISOTOPE
ATOMIC MASS
Cl
Cl
35
37
17
17
18
NEUTRONS
20 NEUTRONS
ATOMIC NUMBER
IONS
• AN ELEMENT’S ATOMS ARE NOT
ALWAYS NEUTRAL IN CHARGE.
– WHEN AN ATOM LOSES OR GAINS
ONE OR MORE OF ITS ELECTRONS
IT BECOMES ION.
• AN ION THAT HAS MORE
ELECTRONS THAN PROTONS HAS A
NEGATIVE ELECTRICAL CHARGE
• AN ION THAT HAS FEWER
ELECTRONS THAN PROTONS HAS A
POSITIVE ELECTRICAL CHARGE
NOTE: IT’S THE PROTONS THAT DEFINE
THE TYPE OF ATOM IT IS, BUT THE
ELECTRONS DEFINE THE ATOM’S
CHARGE.
SOME ATOMS
GAIN ELECTRONS
ATOM’S IONIC CHARGE =
# PROTONS - # ELECTRONS
-
-
-
-
-
-
-
O
-
-
-
-
-
-
-
O-2
-
ATOMS, IONS, AND ISOTOPES
ATOMS
NEUTRAL AND ARE DEFINED BY
THE # OF PROTONS IN THEIR
NUCLEUS
3 p+ = Li ATOM, ETC.
IONS
HAVE AN ELECTRICAL CHARGE
DETERMINED BY
# PROTONS - # ELECTRONS
N-2 = 7 p+ - 9 e- ; ETC.
ISOTOPES
TWO ATOMS WITH THE SAME # OF
PROTONS, BUT DIFFERENT #’S OF
NEUTRONS OR MASSES
CALCIUM-40 & CALCIUM-44