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J.J. Thomson Webquest
By: Ritesh Modi
Democritus(460 BC-371 BC )
Co-originator of the belief that all matter is made
up of various imperishable indivisible elements
which he called “atoms."
Atoms were completely solid, homogeneous,
indestructible objects and imperceptible to the
senses, composed of exactly the same matter but
different in size, shape, and weight.
Democritus postulated the constant motion of
atoms and, on this basis, explained the creation
of worlds.
The whirling motion caused by the falling of
atoms resulted in aggregations–the heavier
atoms forming the earth and the lighter ones the
heavenly bodies.
Antoine Laurent Lavoisier(1783)
The father of modern chemistry
He stated the first version of the law of
conservation of matter, which states that mass
is neither created nor destroyed in any ordinary
chemical reaction
Recognized and named oxygen
Helped to reform chemical nomenclature
Lavoisier believed that weight was conserved
through the course of chemical reactions —
even those involving gases.
He explained combustion (and respiration) in terms
of chemical reactions that involve a component of air
which he called oxygen.
He explained combustion and thereby discredited
the phlogiston theory
Joseph Louis Proust(1799)
The Law of Constant Composition
established by Joseph Louis Proust
States that in a pure compound, the elements
are always present in the same definite
proportion by mass
 Ratio
of H:0 is always 2:16 or 1:8
 2Na + Cl2 -----> 2NaCl
 2.3 g + 3.55 g -----> 5.85 g
 4.6 g + 7.1 g -----> 11.7 g
John Dalton(1803)
Formalized the discovery of Lavoisier into the "Law of
Definite Proportions (when atoms combine to form a
particular compound, they always combine in the same
ratios by weight) and Proust’s Law of Constant
Compostion (States that in a pure compound, the
elements are always present in the same definite
proportion by mass).”
Proposed an "atomic theory" with spherical solid
atoms based upon measurable properties of mass.
All matter was composed of small indivisible particles
termed atoms
Atoms of a given element possess unique
characteristics and weight
Three types of atoms exist: simple (elements),
compound (simple molecules), and complex (complex
The work identified chemical elements as a specific
type of atom, therefore rejecting Newton's theory of
chemical affinities.
Dalton inferred proportions of elements in compounds
by taking ratios of the weights of reactants, setting the
atomic weight of hydrogen to be identically one.
He proposed that chemical elements combine in
integral ratios.
Despite the importance of the work as the first view of
atoms as physically real entities and introduction of a
system of chemical symbols, New System of Chemical
Philosophy devoted almost as much space to the
Caloric Theory as to Atomism.
The Law of Dalton's Multiple
One of the strongest arguments for Dalton's atomic
theory of chemistry was the Law of Multiple
He found that when carbon combined with oxygen
to form a gas, there were two possible outcomes,
depending on the conditions - and in one outcome
each gram of carbon combined with precisely twice
as much oxygen as in the other.
He correctly interpreted this as the formation of
CO2 and CO respectively.
Dmitri Mendeleev(1869)
Mendeleev discovered that by sorting the
elements ascending of their atomic weight, their
chemical properties show characteristic
periodicity and this later become known as the
Periodic Law.
Later this table was named the periodic table of
He Arranged elements into 7 groups with similar
Mendeleev determined the holes,. the spaces for the
missing elements. For example, germanium was
discovered upon his guidance. Moreover, he had
foretell all the important chemical properties of this
J.J. Thomson(1897)
Subjected cathode rays to magnetic and electric
fields and showed that the beam was deflected as
would be expected for negatively charged particles.
He calculated the ratio of the electron's charge to its
Thomson announced that the cathode rays consisted of
negatively charged particles, which represented
fundamental particles of matter.
Discovered the electron.
Thomson’s Plum Pudding Model
In 1898 Thomson proposed that the atom was a
sphere of positive electricity containing most of
the mass
Small negative electrons were embedded in the
 He
calculated the charge to mass ratio
e/m for the electron by careful
observations of the curvature of an
electron beam in cathode ray tubes in
a magnetic field.
Ernest Rutherford(1898)
Published his atomic theory describing the atom as
having a central positive nucleus surrounded by
negative orbiting electrons.
This model suggested that most of the mass of the
atom was contained in the small nucleus, and that the
rest of the atom was mostly empty space.
Rutherford came to this conclusion following the
results of his famous gold foil experiment.
This experiment involved the firing of radioactive
particles through minutely thin metal foils (notably
gold) and detecting them using screens coated with
zinc sulfide.
Rutherford found that although the vast majority of
particles passed straight through the foil approximately
1 in 8000 were deflected leading him to his theory that
most of the atom was made up of 'empty space'.