CHM 211 Chapter Notes - Chapter 2: Oil Drop Experiment, Cathode Ray Tube, Pierre Curie
Basic Concepts
The Atomic Theory of Matter
The Discovery of Atomic Structure
Chemists make their observations in the macroscopic world and seek to understand the
fundamental properties of matter at the level of the microscopic world (i.e. molecules and
atoms). The reason why certain chemicals react the way they do is a direct consequence of their
atomic structure.
2.1 The Atomic Theory of Matter
The word "atom" is derived from the Greek word "atomos", meaning indivisible. The
philosopher Democritus (460-370 B.C.) believed that matter was composed of fundamentally
indivisible particles, called "atomos".
Dalton's atomic theory of 1803:
1. Each element is composed of extremely small particles called atoms
2. All atoms of a given element are identical; the atoms of different elements are different
and have different properties (including different masses)
3. Atoms of an element are not changed into different types of atoms by chemical reactions;
atoms are neither created nor destroyed in chemical reactions
4. Compounds are formed when atoms of more than one element combine; a given
compound always has the same relative number and kind of atoms.
Atoms are the basic building blocks of matter; they are the smallest units of an element:
• An element is composed of only one kind of atom
• In compounds the atoms of two or more elements combine in definite arrangements
• Mixtures do not involve the specific interactions between elements found in compounds,
and the elements which comprise the mixture can be of varying ratios
Simple "laws" (i.e. theories) of chemical combination which were known at the time of Dalton:
1. The law of constant composition (in a given compound the relative number and kind of
atoms are constant)
2. The law of conservation of mass (the total mass of materials present after a chemical
reaction is the same as the total mass before the reaction)
Dalton used these "laws" to derive another "law" - the law of multiple proportions (if two
elements, A and B, can combine to form more than one compound, then the ratios of the relative
masses of each element which can combine can be represented by characteristically small whole
numbers).
2.2 The Discovery of Atomic Structure
• 1803 Dalton - the atom is a indivisible, indestructible, tiny ball
• 1850 Evidence is accumulating that the atom is itself composed of smaller particles
• The current modelÖ
The behavior of electrically charged particles
Like charges repel each other, unlike charges attract
Cathode rays and electrons
Electrical discharge through partially evacuated tubes produced radiation. This radiation
originated from the negative electrode, known as the cathode (thus, these rays were termed
cathode rays).
• The "rays" traveled towards, or were attracted to the positive electrode (anode)
• Not directly visible but could be detected by their ability to cause other materials to glow,
or fluoresce
• Traveled in a straight line
• Their path could be "bent" by the influence of magnetic or electrical fields
• A metal plate in the path of the "cathode rays" aquired a negative charge
• The "cathode rays" produced by cathodes of different materials appeared to have the
same properties
These observations indicated that the cathode ray radiation was composed of negatively charged
particles (now known as electrons).
J.J. Thompson (1897) measured the charge to mass ratio for a stream of electrons (using a
cathode ray tube apparatus) at 1.76 x 108 coulombs/gram.
• Charged particle stream can be deflected by both an electric charge and by a magnetic
field
• An electric field can be used to compensate for the magnetic deflection - the resulting
beam thus behaves as if it were neutral
• The required current needed to "neutralize" the magnetic field indicates the charge of the
beam
• The loss of mass of the cathode indicated the "mass" of the stream of electrons
