Atoms, Molecules and Ions
The Atomic Theory of Matter
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.
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
Atoms are the smallest particle of an element which retains the chemical properties of that
element
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).
Where's the water?
Hydrogen and oxygen gas can react to form water (H2O). This is a violent reaction which
liberates considerable amounts of heat. A large child's balloon will hold about four liters of gas.
This would represent about 5 grams of oxygen and 0.3 grams of hydrogen if we mixed them in
the appropriate (molar) ratios. Thus, we would expect, at most, to form about 5.3 grams of water
(about 1 tablespoon full). Since some gasses escape before reacting, and since we may not add
the gasses in exactly the correct ratio, the amount of water formed may be far less. The water
that is formed is going to be in the vapor phase.
This demonstrates one of the attractive aspects of hydrogen as a fuel: the only pollution (i.e.
product) is water!
Atoms, Molecules and Ions Contd.
The Discovery of Atomic Structure
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 model...
The behavior of electrically charged particles
Like charges repel each other, unlike charges attract
Behavior of moving charge in magnetic field


A charged particle moving though a magnetic field will feel a force perpendicular to the
plane described by the velocity vector and magnetic field vector
This deflects the moving charged particle according to the "right hand rule" (based on a
positive charge)

A negative charge will be deflected in the opposite direction
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
Thompson determined the charge to mass ratio for the electron, but was not able to determine
the mass of the electron.
However, from his data, if the charge of a single electron could be determined, then the mass of
a single electron could determined.
Robert Millikan (1909) was able to successfully measure the charge on a single electron (the
"Milliken oil drop experiment"). This value was determined to be 1.60 x 10-19 coulombs.
Thus, the mass of a single electron was determined to be:
(1 gram/1.76 x 108 coulombs)*(1.60 x 10-19 coulombs) = 9.10 x 10-28 grams
Note: the currently accepted value for the mass of the electron is 9.10939 x 10-28 grams.
Radioactivity
Wilhelm Roentgen (1895) discovered that when cathode rays struck certain materials (copper for
example) a different type of ray was emitted. This new type of ray, called the "x" ray had the
following properties:



The could pass unimpeded through many objects
They were unaffected by magnetic or electric fields
They produced an image on photographic plates (i.e. they interacted with silver
emulsions like visible light)
Henri Becquerel (1896) was studying materials which would emit light after being exposed to
sunlight (i.e. phosphorescent materials). The discovery by Roentgen made Becquerel wonder if
the phosphorescent materials might also emit x- rays. He discovered that uranium containing
minerals produced x-ray radiation (i.e. high energy photons).
Marie and Pierre Curie set about to isolate the radioactive components in the uranium mineral.
Ernest Rutherford studied alpha rays, beta rays and gamma rays, emitted by certain radioactive
substances. He noticed that each behaved differently in response to an electric field:



The -rays were attracted to the anode
The -rays were attracted to the cathode
The -rays were not affected by the electric field
The  and  "rays" were composed of (charged) particles and the -"ray" was high energy
radiation (photons) similar to x-rays


-particles are high speed electrons (charge = -1) 
-particles are the positively charged core of the helium atom (charge = +2)
The nuclear atom
J.J. Thompson model of the atom (1900)


The atom consists of a sphere of positive charge within which was buried negatively
charged electrons
Also known as the "plum pudding" model of the atom
Rutherford model of the atom (1910)


Most of the mass of the atom, and all its positive charge, reside in a very small dense
centrally located region called the "nucleus"
Most of the total volume of the atom is empty space within which the negatively charged
electrons move around the nucleus
Rutherford (1919) discovers protons - positively charged particles in the nucleus
Chadwick (1932) discovers neutron - neutral charge particles in the nucleus
Adapted from Dr. Michael Blaber’s class