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Chapter 5 Atomic Structure
• The atom is the smallest particle of an
element that retains the properties of that
element.
• A typical atom has a diameter of 10-8 cm
and a mass of 10-23 grams.
• The word atom comes from the Greek word
atomos meaning indivisible.
• The idea of the atom was 1st
developed by Democritus about
400 B.C.
• In the 1700’s, John Dalton, an
English chemist, first stated his
atomic theory which was based
on the ideas set by Democritus
almost 2000 yrs. ago.
The atom is made up of the following subatomic
particles:
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1. Electrons (e-) :
Are negatively charged particles
Are located in the outer region of the atom called
energy levels (electron clouds)
Its mass is about 1/1840 amu or 9.11 x 10-28grams.
The number of electrons in a neutral atom is
determined by the element’s atomic number.
2. Protons (p+)
• Are positively charged particles
• Are located in the dense nucleus of the atom
• It has a mass of 1 amu or 1.67 x 10-24grams.
• The number of protons in the atom is determined by
the element’s atomic number.
3.
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Neutrons (nº)
Are neutral particles having no charge.
Are located in the nucleus of an atom.
It has a mass of 1 amu or 1.67 x 10-24grams.
The # of neutrons are calculated by subtracting the
atomic number from the mass number.
Remember: a.m.u. stands for atomic mass unit.
The nucleus is the dense center of the atom. It contains
protons and neutrons. Since the protons are (+) and the
neutrons have no charge, the nucleus is therefore positive
(+).
Atomic number: (see periodic table)
• designates the # of protons in the nucleus
of an atom
• designates the number of electrons in the
energy levels of a neutral atom
Atomic #
Atomic mass:
• corresponds to the weighted average of the
masses of the isotopes for a certain element
ATOMIC
MASS
Mass number:
• corresponds to the total number of protons
and neutrons in the nucleus of an atom
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it is the rounded-off atomic mass
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round-off to
whole # (70)
this would be the
mass #
The number of energy levels that an atom
contains is given by the period # on the sides
of the periodic table.
Valence electrons are:
• the outermost electrons in an atom
• responsible for chemical bonding and thus
chemical reactions
• given by the group number in the periodic
table
• The chemical properties of atoms, ions, and
molecules are related to the arrangement of
electrons.
• The energy level of an electron is the region
around the nucleus where the electron is
likely to be moving.
• Electrons in an atom cannot exit between
energy levels.
• A quantum of energy is the amount of
energy required to move an electron from
its present energy level to the next higher
one.
Four forces govern the behavior of the subatomic
particles in an atom:
1. 1. Electrons are kept in orbit around the nucleus by the
electromagnetic force by acting as a force of attraction
between the protons(+) in the nucleus and the electrons(-)
in the energy levels. The electromagnetic force acts as a
force of repulsion between the positively charged protons
inside the nucleus.
2. 2. The strong force opposes the electromagnetic force
of repulsion between the protons. The strong force “glues”
protons together to form the nucleus. Without the strong
force, there would be no atoms. The strong force works
only when the protons are very close, hence, it has a
limited range. This force is the greatest of the four forces.
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3. The weak force is the key to the power
of the sun. This force is responsible for a
process called radioactive decay. During
this process, a neutron in the nucleus
changes into a proton and an electron.
4. The final force is gravity. This is the
weakest force known in nature. Its effect
on the atoms is very small compared to the
other three forces. Its role in the atom is
not clearly understood.
History of the Development of the Atomic Theory
• 400 B.C. Democritus 1st proposed the idea of the
atom
• 1803
John Dalton proposed the 1st elaborate
“atomic theory”
• 1897
J.J. Thomson discovered the electron
using cathode ray tubes
• 1909
Millikan used the oil drop experiment
that determined the charge and mass of
the electron
• 1911
Rutherford discovered the nucleus
using the gold foil experiment
• 1930
Shrodinger introduced “wave
mechanics” as a mathematical
model of the atom
• 1932
Chadwick discovered the neutron
using alpha particles
• 1981
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Bennig & Rohrer completed the
construction of the STM
Energy from the Nucleus
Nuclear energy, often mistakenly called
atomic energy, is the most powerful kind of
energy known. It produces the tremendous
heat and light of the sun and the shattering
blast of thermo-nuclear bombs. Nuclear
energy results from changes in the nucleus
of atoms. The first atomic bomb, or A-bomb,
exploded on July 16, 1945 in Alamogordo,
New Mexico. It produced and explosion
equal to that of 19,000 short tons (17,000
metric tons) of TNT.
Transmutation
The conversion of an atom of one element to an
atom of another element is called transmutation.
Radioactive decay is one way in which
transmutation occurs. A transmutation can also
occur when high-energy particles bombard the
nucleus of an atom. The high-energy particles
may be protons, neutrons, or alpha particles. The
elements with atomic numbers above 92 are
called transuranium elements and all undergo
transmutation. None of them occurs in nature, and
all of them are radioactive.
Define the following in your notes:
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Isotope
Ion
Cation
Anion
Cathode ray
STM (see p. 107)
Metal
Nonmetal
Metalloid
Fission
Fusion
Molar mass
Mole
Gram atomic mass
Ground state
group
period
electron configuration
electron dot structure
electronegativity
monatomic ion
polyatomic ion
paramagnetic