Transcript Law of Multiple Proportions

Atomic Discovery
Early Models of the Atom
400 B.C. – Democritus proposed the existence of
fundamental particles of matter that were indivisible and
indestructible - “atomos”.
Aristotle thought all matter was continuous; he did not
believe in atoms.
Neither idea was supported by any experimental
evidence – speculation only.
Foundations of Atomic Theory
 The late 1700’s –definitions and basic laws had
been discovered and accepted by chemists.
 Element – substance that cannot be broken down by
ordinary chemical means.
 Chemical Reaction – transformation of substance or
substances into one or more new substances.
 Law of Conservation of Mass – mass cannot be created or
destroyed just changed from one form to another. Mass is
conserved in reactions. (Antoine Lavosier-Father of Chemistry
1778))
 Law of Definite Proportions – a chemical compound contains
exactly the same elements in the same proportion regardless of
sample size. (Joseph Proust from work of Gay-Lussac &
Amadeo Avogadro – 1802/1804)
 Law of Multiple Proportions – If two or more different
compounds are composed of the same two elements, then the
ratio of the masses of those elements will always exist as a ratio
of small whole numbers. (John Dalton - 1808)
Dalton’s Atomic Theory 1808
 All elements are composed of tiny indivisible particles




called atoms.
Atoms of the same element are identical. The atoms of
one element are different from the atoms of another
element.
Atoms cannot be subdivided, created or destroyed.
Atoms combine in simple whole-number ratios.
Atoms are separated, joined or rearranged in chemical
reactions. Atoms of one element are never changed into
atoms of another element as a result of a chemical
reaction.
Other Contributors to the Atomic
Theory
 Amadeo Avogadro-discovered the
relationship between volume of gases and
the number of particles in them (1811)
Discovery of Electrons
 1897 – J.J. Thomson – “Cathode Ray Tube Experiment”
 Showed existence of first know sub-atomic particle
 Determined charge to mass ratio of the electron
 1909 – Robert Millikan found the charge of the electron –
“Millikan’s Oil Drop Experiment”
Cathode Ray Tube
High Voltage
Gas at very low
pressure
Metal disk
(anode)
Metal disk
(cathode)
Cathode Ray
(electrons)
Cathode Ray Tube
High Voltage
Gas at very low
pressure
Negative plate
Metal disk
(anode)
Metal disk
(cathode)
Positive plate
Cathode Ray
(electrons)
Observations from Cathode Ray
 A glow (cathode ray) was formed between
the cathode and anode.
 A paddle wheel between the cathode and
anode rolled toward the anode. (had
enough mass)
 Cathode ray was deflected from a
magnetic field.
 Cathode ray was deflected by a negative
charge.
Discovery of Charge of Electron
 Milikan’s Oil Drop Experiment
 Negative charge
 Mass of electron
 Concluded electrons are present in all
atoms
 Concluded that atoms are divisible.
Assumptions based on Discovery
of Electron
 Because atoms are neutral, there must be
a positive charge to balance the electrons.
 Because electrons have very small mass
compared to an atom, there must be other
particles in an atom to account for the
other mass.
Rutherford’s Gold Foil Experiment
 Rutherford, Geiger & Marsden (1912) -showed that
most of the atom was empty space, but that atoms
had a solid, positive core.
Alpha Particles
Lead
shield
Radioactive
source
Discovery of Protons
 1919 -J.J. Thomson & James Chadwick–
discovered particles traveling opposite of the
cathode rays.
 Determined existence, mass and charge of protons
 Idea had actually been previously proposed by
Goldstein in 1886.
Cathode Ray Tube
High Voltage
Gas at very
low pressure
protons
Negative plate
Metal disk
(anode)
Metal disk
Positive plate
(cathode)
Cathode Ray
(electrons)
Neutrons
 James Chadwick 1932 - confirmed the existence of the
neutron. Neutrons are subatomic particles with no
charge but with a mass nearly equal to that of a proton.
 Walter Bothe had first reasoned the existence of a third
subatomic particle in 1930.
 Bothe’s work was based in part on that of Henry Mosely
who showed by X-ray analysis that not all atoms of the
same element were identical. (Isotopes – 1907)
Radioactivity
 Mosely’s X-ray analysis of atoms was an attempt to
explain radioactivity.
 1896 – Henri Becquerel – Uranium spontaneously emits
energy.
 1898 – Marie & Pierre Curie – first isolated a radioactive
element - Radium
“Planetary” Model of the Atom
 Niels Bohr (1913) – developed the “planetary” model of
the atom based upon the following:
 Rutherford’s Gold Foil Experiment
 E = mc2 – Albert Einstein (1905)
 Quantum Theory – Max Planck (1910)
Properties of Subatomic Particles
Particles
Symbol
Charge
Electron
e-
1-
Proton
p+
Neutron
nº
Relative
Mass
Mass
1/1840 amu
9.11 x 10-28 g
1+
1 amu
1.67 x 10-24 g
0
1 amu
1.67 x 10-24 g
Atom
10-13 cm
electrons
protons
neutrons
nucleus
10-8 cm
Size of the Atom
Aluminum Atom
150 m
1 mm
eOutside
edge of Al
atom
e-
e-
stands
e-
e-
goal post
e-
nucleus - size
of a marble
e-
e-
ee-
Puncher Dome