HISTORY OF THE ATOM
DEMOCRITUS – 400B.C.
A. He believed matter was made up of empty space and tiny
particles he called “atomos”
B. Said different things were made with different types of atoms.
ARISTOTLE – 335 B.C.
A. Matter was continuous – “what you see is what you get”
B. Identified four elements – earth, wind, fire, and water
GALILEO – LATE 1500’S
A. First person to make extensive use of the experimental method to
study natural phenomena
B. First to agree with Aristotle
C. Agreed with Copernicus that sun was center of the universe.
SIR ISAAC NEWTON – LATE
1600’S
A. English Scientist
B. When he was 23-24 years old he was stuck at home during the
Bubonic Plague. At this time he invented the following:
A.
B.
C.
D.
Calculus
Laws of Motion
Laws of Gravity
Did lots of studies of lights
C. The atom idea resurfaced- he said that there were small particles but
he offered no proof.
ANTOINE LAVOISIER -1789
A. Law of Conservation of Mass
B. Matter cannot be created or destroyed!!!!!
JOSEPH PROUST - 1799
A. Law of Definite Proportions
B. Compounds made of the same elements in the same ratios!!
JOHN DALTON - 1803
A. Published a series of papers on the findings of Proust and Lavoisier.
B. Dalton’s atomic theory
A.
B.
C.
1. Elements are made of tiny particles called atoms.
2. All atoms of a given element are identical.
3. The atoms of a given element are different from those of any other
element; the atoms of different elements can be distinguished from one
another by their respective relative weights.
D. 4. Atoms of one element can combine the atoms with atoms of other
elements to form chemical compounds; a given compound always has the
same relative number of types of atoms.
E. 5. Atoms cannot be created, divided into smaller particles, nor destroyed
in the chemical process; a chemical reaction simply changes the way atoms
are grouped together.
JOHN DALTON - 1803
 Law of Multiple Proportions
 Dalton’s model of the atoms – he used solid wooden balls to show
matter was made of indivisible particles.
EUGENE GOLDSTEIN - 1876
A. Introduce the term cathode ray and its positive counterpart the
canal ray.
WILHELM ROENTGEN 1895
A. Discovered x-rays
ANTOINE HENRY
BECQUERAL - 1896
A. Discovered Radioactivity
B. The phenomena of the unstable nuclei of certain elements
spontaneously emitting particles of energy.
ANTOINE HENRY
BECQUERAL - 1896
C. Forms of radiation
1.
Alpha particle (α)[Rutherford]
1.
2.
3.
4.
Helium nucleus (2 neutrons and 2 protons)
Do not penetrate solids
Very large, has a + charge
Because of high KE, they can damage surface tissue
A N T O I N E H E N RY
B E C QU E R A L - 1 8 9 6
2. Beta Particle (β) [Rutherford]
A.
B.
C.
D.
High speed electron is formed at the moment of decay
Hard to stop
Penetrates light materials; clothing, paper, etc.
Cannot penetrate most metals
A N T O I N E H E N RY
B E C QU E R A L - 1 8 9 6
3. Gamma Ray ( γ) [Villard]
A.
B.
C.
D.
E.
High energy x-rays
Have no (-) charge and no mass
Pure energy
Can penetrate through most materials (metals, walls, etc.)
Cannot penetrate through dense materials like lead.
JJ THOMSON - 1897
A. Did research on cathode rays and built a (CRT) or cathode ray
tube [we us these tubes in TV’s and computer screens]
A. A CRT is a glass tube filled with a gas with 2 electrodes attached
(anode = positive electrode and cathode = negative electrode)
B. Passed a voltage between the electrodes, saw a light beam which
he called a cathode ray.
C. Subjected the beam to electrical and magnetic fields, found the
beam to be negatively charged (he called the particles electrons)
JJ THOMSON - 1897
JJ THOMSON - 1897
D. HIS MODEL OF THE ATOM
1.
2.
“Plum Pudding Model”
Negative particles with a positive core.
ROBERT MILLIKAN -1911
A. American physicist
B. Did the “Oil Drop Experiment”
A. In this he had oil drops that were charged pass through charged
plates, he offset the voltage on the plates to get the oil drop to
suspend briefly and wrote down the voltage.
B. He then found the exact charge on an electron to be -1
C. He also found the mass of the electron to be 1/1897th of a
hydrogen atom.
LORD ERNEST
RUTHERFORD
A. A New Zealand Physicist
B. In 1911-1912 worked with Bohr, Geiger, and Marsden on the
gold foil experiment.
LORD ERNEST
RUTHERFORD
C. Gold Foil Experiment Set-up
i.
ii.
iii.
iv.
Lead box with pin hole
Put a piece of radioactive material inside which gave off alpha
particles (positive particles)
Used a very thin sheet of gold foil as a target
Surrounded the foil with a screen coated in zinc sulfide that
sparked when alpha particles hit it
LORD ERNEST
RUTHERFORD
LORD ERNEST
RUTHERFORD
1. Observations he made:
1.
2.
3.
99% of the particles went straight thru the gold foil like it wasn’t
even there.
0.9% went thru but passed thru at a slight angle
0.1% hit the gold foil like a brick wall and bounced right back.
LORD ERNEST
RUTHERFORD
2. Conclusions
a. Atoms made up mostly of empty space
b. Something must repel the positive alpha particles – must be a
positively charged nucleus
c. All the mass of the atom is concentrated in one place- the
nucleus which is surrounded by empty space
d. Stated that the + charged nucleus must be (+1) to offset the (-1)
of the electron. (Atoms are neutral)
LORD ERNEST
RUTHERFORD
d. Rutherford’s model of the atom
1. Electrons travel in space around nucleus like the planets
around the sun.
NEILS BOHR MODEL -1913
a. Worked on gold foil
1914 DISCOVERY OF THE
PROTON
a. Thomson is working with CRT again and finds a positive particle
that is larger and the exact opposite charge of the electron.
b. 1800 times heavier than the electron
HENRY MOSELEY -1913
a. Studied x-rays produced by different metals (used the metals as
anodes)
b. Saw that each metal produced a different wavelength because each
had a different number of protons
c. From this development the atomic number
a. Tells the number of protons
b. # of protons = # of electrons
c. # of protons determine the identity of the element
JAMES CHADWICK - 1932
a. Found high energy particles with no charge and a mass equal to a
proton.
b. Called them neutrons
c. Nucleons – particles found in the nucleus of an atom, also known
as hadrons; these particles are protons and neutrons which are
each made up of three quarks.
MASS NUMBER
 Mass # = # of protons + # of neutrons
 # of neutrons = mass # - atomic #
Ex. Carbon
Mass #= 12
Atomic # = 6
# protons =# e- = 6
# of neutrons = 6
#protons =
86
# electrons =
86
#neutrons =
136
ISOTOPES
 JJ Thomson discovered
 Atoms of the same element that have a different mass due to the
fact that have different # of neutrons
 Disproves Dalton’s theory that atoms of the same element are the
same
AV E R A G E A T O M I C M A S S
(# ON THE PERIODIC TABLE)
Scientists found the masses of all the isotopes
and their percent abundance with a mass
spectrometer
A mass spectrometer compares each element
to Carbon-12 atom
Measured in atomic mass unit (amu)
The average atomic mass was calculated using
those numbers (get a decimal #, not a whole #,
because it’s an average)
E X . F I N D AV E R A G E A T O M I C M A S S ( F R O M
ISOTOPES)
Cu-63 62.930 amu =69.17% in the sample
Cu-65 64.928 amu=30.83% in the sample
1.
Find mass contribution of each isotope
(mass) x ( %)
Cu-63  62.930 x .6917 = 43.53
Cu–65  64.928 x .3083 = 20.02
2.
Find average mass by adding the isotopes mass contributions together
Average atomic mass =
43.53 + 20.02 = 63.55 amu