Download Atomic Structure

Early Atomic Theory
“Cosmic substance is made up of
an infinite number of elements
or particles
physically invisible,
indestructible, and infinite,
which vary in size and shape,
and are in eternal motion.”
Democritus
Democritus, atomist BC


Matter is made of
finite particles that
collide as they move
through space
Contradicts idea that
matter is continuous
(Plato, etc.)
John Dalton ~ 1800

Dalton’s Atomic Theory –
based on experimentation
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All matter is made of
indivisible atoms
Atoms of the same element
are identical, atoms of
different elements are
different
Atoms combine in wholenumber ratios
Chemical rxns. occur when
atoms rearrange – they
never change identity
JJ Thomson ~ 1900

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Expt. with cathode ray
tube gave evidence of
electrical charges
inside atoms
Magnets and
electrical fields
proved that the ray
was negatively
charged
“Plum-pudding model”
"I can see no escape from the conclusion
that [cathode rays] are charges of negative
electricity carried by particles of matter."
Ernest Rutherford ~1911


Alpha-scattering expt.
(beam of α particles
aimed at thin gold foil
– some deflected)
Most of the mass and
all of the positive
charge in an atom
forms the nucleus –
the rest of the atom is
empty space
“It was as if you fired a 15inch shell at a piece of
tissue paper and it came
back and hit you.”
Neils Bohr ~ 1913

So, why don’t electrons fall
into the nucleus?

… because they possess
energy, and are located in
specific energy levels
around the nucleus… they
remain in a “ground-state”
energy level unless they
absorb energy

Planetary model – now
considered too simplistic…
Quantum Mechanical Model

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Electrons do not travel
in fixed orbits…
… they occupy clouds of
negative charge called
orbitals…
Model is based on the
mathematical probability
of an electron being in a
particular region;
treats electrons as wave
functions
Evolution of Modern Atomic Theory
Inside today’s atom…
What do the Numbers Mean?

Atomic Number
(on a periodic table)
=

The number of protons.

Mass Number
=

The sum of the protons and
neutrons.

Atomic Mass
(on a periodic table)
=

The weighted average mass
of a typical atom.

Ion Charge
=

+ or – value designating a
non-neutral atom; only
electrons can be gained or
lost in chemical rxns.
Using the Numbers
14
mass number
C
6
symbol
atomic number
14
6
This isotope of carbon has 14 (protons + neutrons).
Carbon atoms must have 6 protons.
Therefore, this isotope has (14 - 6 = 8) neutrons.
Since it does not have a charge, there are 6 electrons.
***The isotope symbol could also be written as C-14***
Isotopes

Atoms of the same
element always have
the same number of
protons.

Most elements are
mixtures of atoms
with different numbers
of neutrons called
ISOTOPES

Neon has 10 protons.

Neon consists of 3 isotopes:
Ne-20 has 10 neutrons.
Ne-21 has 11 neutrons.
Ne-22 has 12 neutrons.
Atomic Mass
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A weighted average of
the masses of the
different isotopes
Depends on the
abundance of each
isotope
Atomic mass unit (amu)
is based on C-12 as a
reference isotope:
C-12 = 12.0 amu
Lab:
The Atomic Mass of
Beanium