By : Made Ayudina Sancitami P
11 (eleven)
~Class : VII a~
1. Early Atomic Theory
The atomic theory, which holds that matter is composed of tiny, indivisible particles in constant
motion, was proposed in the 5th cent. B.C. by the Greek philosophers Leucippus and
Democritus and was adopted by the Roman Lucretius. However, Aristotle did not accept the
theory, and it was ignored for many centuries. Interest in the atomic theory was revived during
the 18th cent. following work on the nature and behavior of gases (see gas laws).
2. Discovery of the Atom's Structure
In 1911, Ernest Rutherford developed the first coherent explanation of the structure of an
atom. Using alpha particles emitted by radioactive atoms, he showed that the atom consists of
a central, positively charged core, the nucleus, and negatively charged particles called electrons
that orbit the nucleus. There was one serious obstacle to acceptance of the nuclear atom,
however. According to classical theory, as the electrons orbit about the nucleus, they are
continuously being accelerated (see acceleration), and all accelerated charges radiate
electromagnetic energy. Thus, they should lose their energy and spiral into the nucleus.
This difficulty was solved by Niels Bohr (1913), who applied the quantum theory developed by
Max Planck and Albert Einstein to the problem of atomic structure. Bohr proposed that
electrons could circle a nucleus without radiating energy only in orbits for which their orbital
angular momentum was an integral multiple of Planck's constant h divided by 2π. The discrete
spectral lines (see spectrum) emitted by each element were produced by electrons dropping
from allowed orbits of higher energy to those of lower energy, the frequency of the photon of
light emitted being proportional to the energy difference between the orbits.
Around the same time, experiments on x-ray spectra (see X ray) by H. G. J. Moseley showed
that each nucleus was characterized by an atomic number, equal to the number of unit positive
charges associated with it. By rearranging the periodic table according to atomic number rather
than atomic weight, a more systematic arrangement was obtained. The development of
quantum mechanics during the 1920s resulted in a satisfactory explanation for all phenomena
related to the role of electrons in atoms and all aspects of their associated spectra. With the
discovery of the neutron in 1932 the modern picture of the atom was complete.
3. Contemporary Studies of the Atom
With many of the problems of individual atomic structure and behavior now solved, attention
has turned to both smaller and larger scales. On a smaller scale the atomic nucleus is being
studied in order to determine the details of its structure and to develop sources of energy from
nuclear fission and fusion (see nuclear energy), for the atom is not at all indivisible, as the
ancient philosophers thought, but can undergo a number of possible changes. On a larger scale
new discoveries about the behavior of large groups of atoms have been made (see solid-state
physics). The question of the basic nature of matter has been carried beyond the atom and now
centers on the nature of and relations between the hundreds of elementary particles that have
been discovered in addition to the proton, neutron, and electron. Some of these particles have
been used to make new types of exotic “atoms” such as positronium (see antiparticle) and
muonium (see muon).
4. From Dalton to the Periodic Table
Modern atomic theory begins with the work of John Dalton, published in 1808. He held that all
the atoms of an element are of exactly the same size and weight (see atomic weight) and are in
these two respects unlike the atoms of any other element. He stated that atoms of the
elements unite chemically in simple numerical ratios to form compounds. The best evidence for
his theory was the experimentally verified law of simple multiple proportions, which gives a
relation between the weights of two elements that combine to form different compounds.
Evidence for Dalton's theory also came from Michael Faraday's law of electrolysis. A major
development was the periodic table, devised simultaneously by Dmitri Mendeleev and J. L.
Meyer, which arranged atoms of different elements in order of increasing atomic weight so that
elements with similar chemical properties fell into groups. By the end of the 19th cent. it was
generally accepted that matter is composed of atoms that combine to form molecules.
There are kind of atomic theory, that are :
a.Dalton Atomic Theory
Democritus first suggested the existence of the atom but it took almost two millennia before
the atom was placed on a solid foothold as a fundamental chemical object by John Dalton
(1766-1844). Although two centuries old, Dalton's atomic theory remains valid in modern
chemical thought.
Dalton's Atomic Theory
1) All matter is made of atoms. Atoms are indivisible and
indestructible.
2) All atoms of a given element are identical in mass and
properties
3) Compounds are formed by a combination of two or more
different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms.
Modern atomic theory is, of course, a little more involved than Dalton's theory but the essence
of Dalton's theory remains valid. Today we know that atoms can be destroyed via nuclear
reactions but not by chemical reactions. Also, there are different kinds of atoms (differing by
their masses) within an element that are known as "isotopes", but isotopes of an element have
the same chemical properties.
Many heretofore unexplained chemical phenomena were quickly explained by Dalton with his
theory. Dalton's theory quickly became the theoretical foundation in chemistry.
b.Thomson Atomic Theory
Thomson presented three hypotheses about cathode rays based on his 1897 experiments :
1. Cathode rays are charged particles (which he called "corpuscles").
2. These corpuscles are constituents of the atom.
3. These corpuscles are the only constituents of the atom.
Thomson's speculations met with some skepticism. The second and third hypotheses were
especially controversial (the third hypothesis indeed turned
out to be false). Years later he recalled, "At first there were
very few who believed in the existence of these bodies
smaller than atoms. I was even told long afterwards by a
distinguished physicist who had been present at my lecture
at the Royal Institution that he thought I had been 'pulling
their legs.'"
The word "electron," coined by G. Johnstone Stoney in
1891, had been used to denote the unit of charge found in
experiments that passed electric current through chemicals.
In this sense the term was used by Joseph Larmor, J. J.
Thomson's Cambridge classmate. Larmor devised a theory
of the electron that described it as a structure in the ether (the invisible elastic fluid that was
proposed as a substrate for light and other electrical phenomena). But Larmor's theory did not
describe the electron as a part of the atom. When the Irish physicist George Francis FitzGerald
suggested in 1897 that Thomson's corpuscles were really "free electrons," he was actually
disagreeing with Thomson's hypotheses. FitzGerald had in mind the kind of "electron"
described by Larmor's theory.
Gradually scientists accepted Thomson's first and second hypotheses, although with some
subtle changes in their meaning. Experiments by Thomson, Lenard, and others through the
crucial year of 1897 were not enough to settle the uncertainties. Real understanding required
many more experiments over later years.
Theories about the atom proliferated in the wake of Thomson's 1897 work. If Thomson had
found the single building block of all atoms, how could atoms be built up out of these
corpuscles? Thomson proposed a model, sometimes called the "plum pudding" or "raisin cake"
model, in which thousands of tiny, negatively charged corpuscles swarm
inside a sort of cloud of massless positive charge. This theory was struck
down by Thomson's own former student, Ernest Rutherford. Using a
different kind of particle beam, Rutherford found evidence that the
atom has a small core, a nucleus. Rutherford suggested that the atom
might resemble a tiny solar system, with a massive, positively charged
center circled by only a few electrons. Later this nucleus was found to be
built of new kinds of particles (protons and neutrons), much heavier
than electrons.
c.Rutherford Atomic Theory
Rutherford's Atomic Theory was a revolutionary theory regarding the nature of atomic structure
that varied significantly from past theories on the same subject matter. In fact, although the
Rutherford Atomic Theory was first posited in 1911, many facets of it are still accepted by the
majority of the scientific community.
to do with the fact that the Plum Pudding Model theorized that an atom was made up electrons
(the plums) surrounded by a positively charged mass (the pudding). Rutherford later proved
that this wasn't the case and theorized that atoms were comprised of a very small nucleus
surrounded by electrons. The basic tenets of that statement are still held to be true in the
modern era.
Effects
The effects of Rutherford's Atomic Theory are truly awe inspiring and the shockwaves are still
being felt today. As mentioned, Rutherford's work paved the way for the Bohr Model of the
atom, which is still widely accepted. Much of modern science and medicine has atomic theory
at its very root. Without Rutherford's previous work, then, and without the later polishing work
done by Niels Bohr, the face of contemporary science would be unimaginably different.
d.Bohr Atomic Theory
Bohr Theory was the foundation which started many people to look at the behavior of
electrons in new ways. The theory has some clearly defined problems, ideas that we now
believe were incorrect. Those problems have been modified over the years so that the final
description of the behavior of the electron is now more accurate. Ultimately, the Bohr
Theory led scientists, specifically Erwin Schrodinger, to the Modern Theory of Atomic
Structure. The primary limitation to Bohr Theory is that it was limited to a description of a
one electron system, namely Hydrogen. The description of multiple electron systems is
much more complex, and was only adequately handled by the Modern Theory of Atomic
Structure.
Basically, the Bohr Theory consists of six parts. Each part contributes a component to the,
overall, understanding of electron behavior. Listed, here, are those six ideas.
1. The electron travels in a circular path around the nucleus. This path is called an
orbit.
2. At normal living conditions, room temperature, the electron resides in the orbit
which is closest to the nucleus. This is the position of lowest energy content for the
electron, and is referred to as the Ground State. (This statement implies that there
will be more that one orbit available to an electron.)
3. As long as the electron remains in a specific orbit, no energy is gained or lost by the
system.
4. If energy is added to an electron, the electron will move to a new orbit. This orbit
will be farther from the nucleus, and is a position of higher energy content. This
new position is known as an excited state.
5. When an electron moves from one orbit to another orbit, it does so without ever
passing through the space between the orbits. In other words, the electron is only
allowed to exist at very specific distances from the nucleus, or positions of very
specific energy content. (This idea is much like climbing a ladder. The foot is only
allowed to be placed in very specific locations.) This idea is known as a quantum
jump, a transition in which the electron gains or loses a very specific amount of
energy.
6. When an electron is in an excited state, it will always drop down to a lower energy
state, ultimately returning to ground state. Each electron transition to a lower
energy state will be accompanied by the simultaneous release of energy. This
energy is released as electromagnetic radiation. The energy of the released
radiation will correspond to the difference in energy content between the two
levels.
e.Modern Atomic Theory
Atom quantum mechanics model developed by Erwin Schrodinger (1926).Before Erwin
Schrodinger, German expert of Werner Heisenberg develop recognized quantum mechanics
theory with uncertainty principle that is " Not possible to earn to be
determined to domicile and momentum an object by seksama at the time of
at the same time, which can determined by ability find electron at certain
distance of atomic nucleus".
Room area around core of with ability to get electron referred by orbital.
Form and storey level of energy orbital formulated by Erwin Schrodinger.
Erwin Schrodinger solve an equation to get wave function to depict possibility boundary finding
of electron in three dimension.
Atom model with this electron trajectory orbital is referred as modern atom model or quantum
mechanics atom model going into effect till now, like seen at picture following.
Electron cloud around core of place to point ability of electron. Orbital depict storey level of
energy electron. Orbital-Orbital with storey level of energy is same or much the same to will
form husk sub. Some husk sub join to form skin .That way husk consist of some husk sub and of
sub skin consist of some orbital. Although its same husk position but its position of him not yet
is same of course.
INDIVIDUALITY MODEL ATOM MECHANICS WAVING.
1. Electron movement measure up to wave, so that its trajectory do not stationer like Bohr model, but
following the solving of wave function square of[is so-called orbital ( form three dimension of from ability
biggest finding of electron with certain situation in an atom.
2. Form and size measure of orbital base on price from third its quantum number. Electron occupying
orbital expressed in quantum number.
3. Electron position as far as 0,529 Armstrong from core of H according to Bohr rather than cinch, but
ability represent biggest opportunity finding of electron\
Attempt of Chadwick
Weakness Of Modern Model Atom
Wave equation of Schrodinger can only be applied by eksak for particle in atom and box with
single electron