a. Dalton's atomic theory
Based on the law of conservation of
mass and the law of constant comparison, John Dalton (1805) theory of atomic states as follows:
1. The atom is the smallest particle
that can not be subdivided.
2. All atoms of a particular chemical
element have the same mass, as well as all other properties.
3. Other chemical element will have
different types of atoms.
4. Atoms combine with each other to
form molecular compounds.
5. Chemical reactions involve only the
rearrangement of the atoms so that no atom has changed as a result of chemical
reactions.
The weakness of Dalton's atomic theory
In further developments discovered the
facts that can not be explained by the theory, among others:
1. Unable to explain the electrical properties
of the material.
2. Unable to explain how atoms bond
together.
3. Dalton's atomic model can not
explain the difference between the atoms of the element with another element.
b. Thompson's atomic theory
Based on the cathode ray experiment,
Joseph John Thompson (1897) states
that the atomic theory of electrons in atoms are evenly distributed in the
solid ball is positively charged.
Excess Thomson's Atomic Theory:
1. Can explain the existence of
particles smaller than subatomic particles called atoms.
2. Can explain the electrical
properties of atoms.
Weakness Thomson's Atomic Theory:
1. Unable to explain the phenomenon of
scattering of alpha particles by a thin film of gold proposed by Rutherford.
2. Unable to explain the existence of
the atomic nucleus
c. Rutherford's atomic theory
Based on the experimental facts alpha
rays, Ernest Rutherford (1911) suggested that the atomic theory of the atom consists of a positively charged
nucleus at the center of mass and the electron moves around the nucleus of an
atom.
After repeated experiment, Rutherford
finally managed to reveal the following facts.
1. Most of the alpha particles to
penetrate the thin film of gold. Meaning, most of the atom is empty space.
2. A little bit of alpha particles
(positively charged) was deflected out by something, it indicates that
something is positively charged alpha particles that can deflect.
3. Fewer still of the alpha particle
(only 1 in 20,000) reflected from the gold film. With this fact, Rutherford was
surprised and commented, "truly extraordinary, as you shoot a piece of
tissue paper with a 40 cm-thick bullet and the bullet hit the back of your
own". This indicates that something is very small (later referred to as
the core), but the mass concentrated there so that alpha particles striking the
center of mass will reflected.
Excess Rutherford Atomic Theory
1. Can explain the phenomena of
scattering of alpha particles by a thin film of gold.
2. To suggest the presence of
positively charged atomic nucleus and the center of mass of the atom.
Weakness Rutherford Atomic Theory
1. Contrary to the classical theory of
electron dynamics, in which an electrically charged particles would radiate
energy when moving.
2. The negatively charged electrons
that orbit around the nucleus would continuously lose energy and eventually
will form a spiral trajectory and fell to the core. In reality this does not
happen, the electrons remain stable in its trajectory.
d. Bohr's atomic theory
Based on the experimental spectrum of
hydrogen atom, Niels Bohr (1913) theory of atomic states as follows:
1. Electrons orbit around the nucleus
of an atom on the track with a certain energy called skin or orbit.
2. During circulated in its trajectory
(trajectory stationary) electron energy does not change.
3. By absorbing energy, the electrons
can move from track to track to a lower energy level is higher energy level.
4. Electron can also switch from the
track with a high energy level to a lower energy level by emitting energy.
The advantages of the Bohr model of
the atom
Atoms consist of several skin /
subshell for a transfer of electrons and atoms form an orbit in which the
atomic nucleus is positive and there is surrounding electrons.
The weakness of the Bohr model of the
atom
a. Unable to explain the Zeeman effect
and the Strack effect.
b. Unable to explain the events in
chemical bonds well, the influence of magnetic fields on atoms, and atoms are
electron spectrum more.