Download About 440 B.C. Empedocles stated that all matter was composed of

 About 440 B.C. Empedocles stated that all matter was composed of
four "elements" earth, air, water, and fire.
 Democritus (460-370 BC) Theorized that all matter is composed of
small indivisible particles called atomos (“not to cut”, meaning
indivisible). Ideas rejected by Greek thinkers of that time
 The idea that atoms are the fundamental building blocks of matter
reemerged in the 1800’s
The Dalton's atomic theory may be summed up as follows:
1. Each element is composed of minute, indivisible particles called
atoms.
2. All atoms of a given element are chemically identical to each other;
Atoms of one element are different from the atoms of all other
elements
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Atomic Theory
3. During ordinary chemical reactions, atoms of one element cannot be
changed into atoms of a different element.
4. Atoms are not created or destroyed.
5. Compounds are formed when atoms of more than one element
combine.
A given compound contains
atoms combined in definite
whole number ratios.
Fe2O3 (rust)
H2O
(water)
H2O2 (hydrogen peroxide)
These ratios are the same for all molecules of a particular pure substance.
The major points of his theory are still valid today. However, some of the
statement must be modified because investigations since Dalton's time have
shown that:
(1) Atoms are composed of subatomic particles;
(2) All the atoms of a specific element do not have the same mass; and
(3) Atoms, under special circumstances, can be decomposed.
Dalton's wooden atomic models, early
19th century. These wooden balls were
the first models made to represent atoms
and were used by John Dalton to
demonstrate atomic theory. He stated
that matter is made up of small indivisible
particles, or solid atoms. Dalton first
published, in 1808, in a small book
entitled A new system of Chemical
Philosophy, Part 1, by John Dalton.
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Thomson’s Observations
In Thomson’s model, electrons are embedded in a positive sphere of
matter just like raisins in plum pudding or blue berries in a blueberry muffin.
Electrons can be produced from all kinds of metals
Conclusion:
 All atoms must contain electrons.
 Must have a positive charge also
 Plum Pudding Model
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Atomic Theory
The discovery that positively charged particles were present in atoms
came soon after the discovery of radioactivity by Henri Becquercl in
1896.
Ernest
Rutherford
(1871-1937) established that
the positively charged alpha
particles emitted by certain
radioactive
elements
were
ions of the element helium.
Rutherford used these alpha
particles to establish the
nuclear nature of atoms. In
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Atomic Theory
these experiments performed in 1911, he directed a stream of positively
charged helium ions (alpha particles) at a very thin sheet of gold foil (about
1000 atoms thick). He observed that most of the alpha particles
passed through the foil with little or no deflection; but a few of the
particles were deflected at large angles and occasionally one even
bounced back from the foil. It was known that like charges repel each
other and that an electron with a mass of 1/1837 amu could not have
an appreciable effect upon the path of a far more massive (4 amu) alpha
particle.
Diagram representing Rutherford's
experiment on alpha particle
scattering.
Positive
alpha
particles, emanating from a
radioactive
source,
were
directed at a thin metal foil.
Diagram
illustrates
the
repulsion of the positive alpha
particles by the positive
nucleus of the metal atom.
Rutherford therefore reasoned that each gold atom must contain a
positively charged mass occupying a relatively tiny volume and that when an
alpha particle approached close enough to this positive mass, it was
deflected.
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Atomic Theory
Rutherford spoke of this positively charged mass as the nucleus of the
atom. Since alpha particles are relatively high in mass, the extent of the
deflections, remember some actually bounced back, indicated to Rutherford
that the nucleus is relatively very heavy and dense. Since most of the alpha
particles passed through the thousand or so gold atoms without any
apparent deflection, he further concluded that the bulk of an atom consists
of empty space.
When we speak of the mass of an atom, we are, for
practical purposes, referring primarily to the mass of
the nucleus. This is because the nucleus contains all
of the protons and neutrons, and these represent
more than 99.9 percent of the total mass of any atom,
the largest number of electrons known to exist in an
atom is 106. The mass of even 106 electrons
represents only about 1/17 of the mass of a single
proton or neutron. The mass of an atom, therefore, is
primarily determined by the mass of its protons and
neutrons.

Protons : positively charged subatomic particles found in the nucleus

Neutrons: neutral (uncharged) subatomic particles found in the nucleus
 Electrons: very small, negatively charged subatomic particle
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