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Atomic Structure
SMA MAARIF NU PANDAAN
RINTISAN SEKOLAH KATEGORI MANDIRI
2010
Moh. Suwandi, S.Pt.,M.Pd
Atomic Structure
3
1
Development of Atomic Models
2
Subatomic Particles
3
Atomic Number and Mass Number of Atoms
4
Exercise
Development of Atomic Models
One of concept of atom was proposed by the ancient
Greek philosopher, Democritus. Democritus and his followers
believed that every object or matter found in this world is
composed by the minute, invisible, indivisible particles called
atom. Atom is the simplest and smallest division of an object or
matter. The word atom is derived from Greek atomos meaning
“unable to be cut”
According to Democritos, atoms are made up of precisely the same
matter, but atoms of different elements differ in shape, size, weight,
arrangement, and position. The size, shape, and arrangement of atoms of a
substance determinethe substance properties.
The followings are descriptions of atomic models development by
scientists after Democritus and the modern atomic model believed at the
present day.
1. Dalton’s Atomic Model
In the early 19-th century (1808), a British
chemist and physicist “John Dalton” (1766-1844) studied
thoroughly how different elements, such as hydrogen
and oxygen, could combine to form another substance
ex. water. Dalton made two assertions about atoms,
which is :
a. Atoms of each element are all identical to one another, but different from
the atoms of all other elements
b. Atom of different elements can combine to form more complex
substances
2. Thomson’s Atomic Model
Since the discovery of electron as an
elementary particle which is negatively charged, the
validity of Dalton’s atomic teory begans to be
questionable. In 1899, a british physicist, Sir Joseph
Thomson, proposed :
a. An atomic model callaed raisin plum pudding model.
b. Thomson described atom as a positively charged sphere containing
several negatively charged particles called electrons. Electrons are
scattered in the sphere like raisins in a plum pudding
+
+
-
-
+
+
3. Rutherford’s Atomic Model
In 1911, Ernest Rutherford. Geiger and
Marsden carried out an experiment by shooting
the particles (α) on a thin plate of gold to prove
the Thomson’s atomic theory or model as
follows
a. An atom consists of an atomic nucleus which is positively charged. The
nucleus of atom contains almost entire mass of the atom and is
surrounded by electrons which are negatively charged as in the solar
system model
b. Entirely, an atom is neutral because the number of its positive charges
(protons) is equal to that of its negative charges.
c. During revolving the nucleus, the centripetal force on electrons in an
atom is produced by the forces of attraction between the electrons and
nucleus ( coulomb force )
4. Bohr’s Atomic Model
The weaknesses of Rutherford’s atomic
n=3
theory in explaining the line spectrum of
n=2
hydrogen atom was succesfully corerected by a
n=1
Danish physicist, Neils Bohr, in 1913. Based on
the Rutherford’s atomic theory and Planck’s
quantum theory, Bohr proposed postulates
dealing with an atomic model as follows :
a. Electrons of an atom revolve around the nucleus at a certain path called
stationary path. At This path, the electrons do not absorb or release
energy and they have an angular momentum whose magnitude is the
h
multiplication of
2Π
b. Electrons will release energy (in the form photon) if they are moving
from a higher to a lower energy level (from outer to the inner path) and
the electrons will absorb energy when they are moving from a lower to a
higher energi level (from the inner to the outer path)
c. Bohr’s atomic theory brings about the “Quantum number” concept. The
principal quantum number in Bohr’s atomic theory represents the
positive integer (n = 1, 2, 3 ….) denoting the electron’s trajectory (orbit)
in the atom as well as denoting the energy level of electron.
Subatomic Particles
In the modern atomic theory, atom is composed by subatomic
subatomic particles : electrons, protons and nuetrons. This statement has
something to do with the discoveries of particles of electrons, protons and
neutrons by the scientists, as we are going to study as follws.
1. The Discovery of Electron
In 1897, Thomson discovered the value of specific charge of electron
(e/m), that is the ratio of charge to mass of electron. To determine the value
of e/m, Thomson used the magnetic and electric fields which are set in the
path of electron.
In 1906, Robert A. Millikan succeeded in determining the value of
electron charge from oil drop experiment. In this case the value of
e = 1,6 x 10-19 C is the value of an elementary particle, that is electron.
Accordingly, from the value of spesific change (e/m), tha mass of an
electron can be determined, that is 9,11 x 10-31 kg.
2. The Discovery of Proton
Proton is a positively charged particle which was discovered by Eugen
Goldstein through the continuation ray experiment.
According to Goldstein, when cathode rays move toward the anode in
Crookes tube, their particles will collide with the hydrogen gas in the tube.
The collision will cause electrons of hydrogen atom to be released, making
it hydrogen atom be positively charged in the form of hydrogen ion (H+).
This ion moves toward the chatode through the hole and collidies at the
glass wall behind the cathode. The hydrogen ion is then called proton and it
has mass of 1,67 x 10-27 kg and charge of +1,6 x 10-19 C.
3. The Discovery of Neutron
Ruhtherford was succesful in measuring (calculating) the mass of
atomic nucleus which is approximately twice as much as mass of proton in
the nucleus. Therefore, according to Rutherfor, there must be another
particle in the nucleus and that particle is uncharged. It was not until 1932
that J. Chadwick discovered neutron from the shooting of beryllium using
alpha particles and this neutron is, then, known as the paricle which has
mass of approximately 1,6 x 10-27 kg
PROTON
NEUTRON
ELECTRON
Atomic Number and Mass Number of Atoms
Every element has a uniqe number of protons in its atoms. This
number is called the atomic number ( symbolized as Z ). Because normally
atoms are electrically neutral the atomic number also specifies how many
electrons an atom has. In this case, the number of electrons determines
several chemical and physical properties of the atom.
The total number of protons and neutrons in the nucleus of an atom is
the mass number of the atom ( symbolized as A ). The mass number of an
atom is an approximation of the mass of the atom. This is because the
alectron mass in the atom can be neglected to the mass of protons and
neutrons, so the mass of atom is determined by the mass of protons and
neutrons in the nucleus.
In its relation to the atomic number and mass number, the atom of an
element is usually expressed by the following symbol.
A
X
Z
X = the name of atom
A = mass number of atom
Z = atomic number
Notes :
Z = number of protons = number of electrons
A = number of protons + number of neutrons
A – Z = number of neutrons
1. Isotope
Isotope is one of two or more species of an element atom having
the same atomic number, but differ in mass number. The examples of
isotopes are carbon isotopes, namely
12
6
C and 136C
2. Isobar
Isobar is an atom which comes from different element but has the
same mass number. The examples of the isobar are
14
6
C and
14
7
N
3. Isotone
Isotone is an atom which comes from different element but has
same the number of neutrons. The examples of the isotone are
32
16
31
S and 15
P
Exercise
1. Why is the Thomson’s atomic model called by plum pudding model
2. Explain the weaknesses of Bohr’s atomic theory
3. Determine the number of protons, electrons and neutrons for
16
19
39
207
238
O, 9 F, 19 K, 82 Pb, and 92 U
8
4. Classify the following atoms into isotope, isobar or isotone
16
15
O, 7
8
N,
14
7
N,
18
14
16
C, 8 O, 12
6
C
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