Download Lecture 3 : Atoms and the Atomic Theory Early Chemical

Chem101
Lecture 3
: Atoms and the Atomic Theory
Early Chemical Discoveries and the Atomic Theory
Law of conservation of mass
The total mass of substances present after a chemical reaction is the same as the total mass of
substances before the reaction.
Fig.3-1
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Law of constant composition
All samples of a compound have the same composition- the same proportions by mass of the
constituent elements.
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Dalton’s Atomic Theory
 Each element is composed of small particles called atoms.
 Atoms are neither created nor destroyed in chemical reactions.
 All atoms of a given element are identical.
 Compounds are formed when atoms of more than one element combine.
Like all good theories, Dalton’s atomic theory led to a prediction-the law of multiple
proportions.
If two elements form more than a single compund, the masses of one element combined with
a fixed mass of the second are in the ratio of small whole numbers.
Consequences of Dalton’s theory
 Law of Definite Proportions: combinations of elements are in ratios
of small whole numbers.
Electrons and Other Discoveries in Atomic Physics
Forces between electrically charged objects
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Caption:
(a) Electrostatically charged comb. If you comb your hair on a dry day, a static charge
develops on the comb and causes bits of paper to be attracted to the comb. (b) Both objects on
the left carry a negative electric charge. Objects with like charges repel each other. The
objects in the center lack any electric charge and exert no forces on each other. The objects on
the right carry opposite charges - one positive and one negative - and attract each other. This
graphically depicted measuring device is called an electroscope as it permits one to “see” the
electrical charge.
Notes:
(a) Electrostatically charged comb. If you comb your hair on a dry day, a static charge
develops on the comb and causes bits of paper to be attracted to the comb. (b) Both objects on
the left carry a negative electric charge. Objects with like charges repel each other. The
objects in the center lack any electric charge and exert no forces on each other. The objects on
the right carry opposite charges - one positive and one negative - and attract each other. This
graphically depicted measuring device is called an electroscope as it permits one to “see” the
electrical charge.
Effect of a magnetic field on charged particles
Caption:
When charged particles travel through a magnetic field so that their path is perpendicular to
the field, they are deflected by the field. Negatively charged particles are deflected in one
direction, and positively charged particles in the opposite direction. Several phenomena
described in this section depend on this behavior.
Notes:
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When charged particles travel through a magnetic field so that their path is perpendicular to
the field, they are deflected by the field. Negatively charged particles are deflected in one
direction, and positively charged particles in the opposite direction. Several phenomena
described in this section depend on this behavior.
The Discovery of Electrons
A cathode-ray tube
Caption:
The high-voltage source of electricity creates a negative charge on the electrode at the left
(cathode) and a positive charge on the electrode at the right (anode). Cathode rays pass from
the cathode (C) to the anode (A), which is perforated to allow the passage of a narrow beam
of cathode rays. The rays are visible only through the green phosphorescence that they
produce on the zinc sulfide-coated screen at the end of the tube. They are invisible in other
parts of the tube. This is sometimes called a Crookes tube, after its developer, Sir William
Crookes (1832-1919). This technology led to the invention of the television.
Notes:
The high-voltage source of electricity creates a negative charge on the electrode at the left
(cathode) and a positive charge on the electrode at the right (anode). Cathode rays pass from
the cathode (C) to the anode (A), which is perforated to allow the passage of a narrow beam
of cathode rays. The rays are visible only through the green phosphorescence that they
produce on the zinc sulfide-coated screen at the end of the tube. They are invisible in other
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parts of the tube. This is sometimes called a Crookes tube, after its developer, Sir William
Crookes (1832-1919). This technology led to the invention of the television. Note the
naming convention of cathode/anode is based on the cathode being a source of electrons - see
the electrochemistry chapter for clarification of this sometimes confusing convention.
Properties of cathode rays
Cathode rays and their properties
Caption: (a) Deflection of cathode rays in an electric field. The beam of cathode rays is
deflected as it travels from left to right in the field of the electrically charged condenser plates
(E). The deflection corresponds to that expected of negatively charged particles - away from
the negatively charged plate. (b) Deflection of cathode rays in a magnetic field. The beam of
cathode rays is deflected as it travels from left to right in the field of the magnet (M). The
deflection here too corresponds to that expected of negatively charged particles. (c)
Determining the mass-to-charge ratio, m/e for cathode rays. The cathode-ray beam strikes the
end screen undeflected if the forces exerted on it by the electric and magnetic fields are
counterbalanced. By knowing the strengths of the electric and magnetic fields, together with
other data, a value of m/e can be obtained. Precise measurements yield a value of -5.6857 x
10-9 g/C (C is the unit Coulomb). (Because cathode rays carry a negative charge, the sign of
the mass-to-charge ratio is also negative.)
Notes: (a) Deflection of cathode rays in an electric field. The beam of cathode rays is
deflected as it travels from left to right in the field of the electrically charged condenser plates
(E). The deflection corresponds to that expected of negatively charged particles - away from
the negatively charged plate. (b) Deflection of cathode rays in a magnetic field. The beam of
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cathode rays is deflected as it travels from left to right in the field of the magnet (M). The
deflection here too corresponds to that expected of negatively charged particles. (c)
Determining the mass-to-charge ratio, m/e for cathode rays. The cathode-ray beam strikes the
end screen undeflected if the forces exerted on it by the electric and magnetic fields are
counterbalanced. By knowing the strengths of the electric and magnetic fields, together with
other data, a value of m/e can be obtained. Precise measurements yield a value of -5.6857 x
10-9 g/C (C is the unit Coulomb). (Because cathode rays carry a negative charge, the sign of
the mass-to-charge ratio is also negative.)
Charge on the electron
Millikan’s oil-drop experiment
Caption:
Ions (charged atoms or molecules) are produced by energetic radiation such as X-rays (X).
Some of these ions become attached to oil droplets, giving the droplets a net charge. The fall
of a droplet in the electric field between the condenser plates is sped up or slowed down,
depending on the magnitude and sign of the charge on the droplet. By analyzing data from a
large number of droplets, Robert Millikan (1868-1953) concluded that the magnitude of the
charge, q, on a droplet is an integral multiple of the electric charge, e. That is, q = ne (where n
= 1,2,3,...).
Notes:
Ions (charged atoms or molecules) are produced by energetic radiation such as X-rays (X).
Some of these ions become attached to oil droplets, giving the droplets a net charge. The fall
of a droplet in the electric field between the condenser plates is sped up or slowed down,
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depending on the magnitude and sign of the charge on the droplet. By analyzing data from a
large number of droplets, Robert Millikan (1868-1953) concluded that the magnitude of the
charge, q, on a droplet is an integral multiple of the electric charge, e. That is, q = ne (where n
= 1,2,3,...).
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