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Atomic Theory
Chapter 17
• Atom: the smallest
particle which an
element can be
divided into and still
be the same
• The theory has been
around for over 2000
years. But no one
saw an atom until
1981
Theories
• Democritus proposed
the idea of the atom
around 440 BC,
called the smallest
particle atomos
meaning indivisible
• Aristotle thought he
was wrong so
Democritus got
• 1700’s scientists
knew elements
combine in specific
proportions based on
mass
Dalton
• 1700’s scientists knew
elements combine in
specific proportions
based on mass
• Not all of this was exactly
correct
• John Dalton published his
atomic theory in 1803
• 3 parts
• All substances are made
of atoms. Atoms cannot
be created, divided or
destroyed
• Atoms of the same
element are exactly alike
• Atoms join with other
atoms to form new
substances
Plum Pudding
• J.J. Thomson
discovered the
electron in 1897,
revealing that atoms
were made of smaller
particles
• Plum pudding model:
the atom is mostly
positively charged
with electrons
scattered throughout.
Nucleus
• Ernest Rutherford:
1909 discovered the
nucleus and the
proton
• Most of the atoms
mass is in the
nucleus.
• Protons and neutrons
are found in the
nucleus
• Rutherford calculated
that the nucleus must
be 100,000 times
smaller then the
diameter of the atom
• The atom is mostly
empty space
• Neutron discovered in
1932 by Chadwick
• Electron- charge = -1
• Mass = 9.10938188
× 10-31 kg
• Proton = charge =+1
• Mass = 1.67262158 ×
10-27 kg
• Neutron = charge = 0
• Mass = 1.67492716 ×
10-27 kg
•
The electron can’t go half way
– Can you stand between the
rungs of a ladder?
• Electrons exist in energy
levels
• Often called orbits but
they aren’t really
• Higher the orbit – the
higher the energy
• As they change levels
they absorb or give off
energy in exact amounts
• These are called photons
• The number of electrons
is equal to the number of
protons in a neutral atom
Electron cloud
• Electron exist in a
cloud surrounding the
nucleus
• we can only predict
about where
something is- the
exact path can’t be
predicted if we try
crazy things happen
Elements
• The atomic number is
the number of protons
in the atoms nucleus
• The number of
protons determine the
element and the
properties
Atomic mass
• Atomic mass- the
sum of the protons
and neutrons in an
atom
• Average atomic
mass- weighted
average of all of
the naturally
occuring isotopes
AMU
• Amu- atomic mass
unit
• amu’s are used to
make adding the
masses of
elements easier
• Proton = 1 amu
• Neutron = 1 amu
• Electron = 0 amu
Isotopes
• An isotopes are
atoms with the same
number of protons but
different numbers of
neutrons.
• Most elements have
several naturally
occuring isotopes
Quarks
• Discovered in 1968
• 6 types
• Top, bottom, up,
down, strange, and
charm
• Top quark discoved in
1995
• Dmitri Medeleev-Father of the modern
periodic table.
– Mendeleev’s scheme was based on chemical
properties of the elements.
– It was noticed that the chemical properties of
elements increased in a periodic manner.
– The periodicity of the elements was demonstrated
by Medeleev when he used the table to predict to
occurrence and chemical properties of elements
which had not yet been discovered.
• Mendeleev left blank
spaces in his table
when the properties of
the elements above
and below did not
seem to match. The
existence of unknown
elements was
predicted by
Mendeleev on the
basis of the blank
spaces. When the
unknown elements
were discovered, it was
found that Mendeleev
had closely predicted
the properties of the
• Introduction
– The periodic table is made up of rows of elements
and columns.
– An element is identified by its chemical symbol.
– The number above the symbol is the atomic
number
– The number below the symbol is the rounded
atomic weight of the element.
– A row is called a period
– A column is called a group
• Periodic Patterns
– The chemical behavior of elements is determined
by its electron configuration
– Energy levels are quantized so roughly correspond
to layers of electrons around the nucleus.
– A shell is all the electrons with the same value of n.
• n is a row in the periodic table.
– Each period begins with a new outer electron shell
– Each period ends with a completely filled outer shell
that has the maximum number of electrons for that
shell.
– The number identifying the A families identifies the
number of electrons in the outer shell, except
helium
– The outer shell electrons are responsible for
chemical reactions.
– Group A elements are called representative
elements
– Group B elements are called transition elements.
Lewis Dot Structure- Element
symbol with outer electrons
shown as dots surrounding the
symbol
Metal: Elements that are usually solids at room temperature.
Most elements are metals.
Non-Metal: Elements in the upper right corner of the periodic
Table. Their chemical and physical properties are different
from metals.
Metalloid: Elements that lie on a diagonal line between the
Metals and non-metals. Their chemical and physical
properties are intermediate between the two.
– When an atom or molecule gain or loses an
electron it becomes an ion.
• A cation has lost an electron and therefore has a
positive charge
• An anion has gained an electron and therefore
has a negative charge.
• Octet Rule - Atoms prefer to gain or lose
electrons to fill their outer shells, usually with 8
electrons
– Elements with 1, 2, or 3 electrons in their outer shell
tend to lose electrons to fill their outer shell and
become cations.
• These are the metals which always tend to lose
electrons.
– Elements with 5 to 7 electrons in their outer shell
tend to gain electrons to fill their outer shell and
become anions.
• These are the nonmetals which always tend to gain
electrons.
– Semiconductors (metalloids) occur at the dividing
line between metals and nonmetals. Indicated by a
staircase line
EXAMPLE
What would the charge be on a sodium ion?
Since sodium in in Group IA it is a metal and so would
LOSE an electron
You can tell how many would be lost by the group number
Group 1A elements lose 1 electron
So the charge would be +1
Remember an electron is negatively charged. When you lose
them atom becomes positively charged…
when you gain them it becomes negatively charged
EXAMPLE
How would you right the symbol for the sodium CATION?
Na
+1
How many outer electrons does sodium have before it
loses one?
It has 1…remember the group number!