Download The Atom

Chapter 11
Introduction to Atoms
Section 1: Objectives
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Describe some of the experiments
that led to the current atomic
theory.
Compare the different models of the
atom.
Explain how the atomic theory has
changed as scientists have
discovered new information about
the atom.
The Atomic Theory
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Around 440 BCE, a Greek
philosopher (Democritus) started
forming this theory.
He thought that if you sliced an
atom enough times, then eventually
you would end up with something
that could not be cut.
The Atomic Theory
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Atom= Greek for “atomos” which
means not able to be divided.
Democritus thought 2 things about
atoms:
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All are small, hard particles
Made from a single material formed
into different shapes and sizes.
The Atomic Theory
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Aristotle disagreed with Democritus.
Aristotle believed that you would
never have with a particle that
could never be cut.
He had such a strong social
influence on people that everyone
believed he was right.
The Atomic Theory
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Democritus was right though.
Matter is made up of particles called
atoms.
Atom: smallest particle into which
an element can be divided and
remain the same substance
Long before actually being able to
see atoms, scientists had ideas
about them.
The Atomic Theory
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By the late 1700s, scientists knew
elements combine to form
compounds.
John Dalton, a British chemist,
experimented with different
substances to figure out why they
combine and create certain
substances.
The Atomic Theory
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Dalton’s Theory stated that:
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All substances are made of atoms
Atoms cannot be created nor destroyed
Atoms of the same element are alike
and atoms of different elements are not
Atoms join with other atoms to make
new substances
The Atomic Theory
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Toward the end of the 1800s,
scientists agreed with most of
Dalton’s Theory.
New information was found that did
not fit his ideas.
The Atomic Theory was then
changed.
The Atomic Theory
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Thomson showed that there was a
mistake in Dalton’s theory.
Thomson discovered that there are
smaller particles inside atoms.
This means that atoms can be
divided into smaller parts.
The Atomic Theory
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Thomson
experimented and
found that negatively
charged particles are
present inside atoms.
Electrons: subatomic
particles with a
negative charge
Thomson
experimented with
a cathode ray tube.
The Atomic Theory
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Cathode = energy
source.
2 plates placed in the
center.
Invisible beam
produced.
Straight beam if no
charge.
If charged, beam
pulled towards
positive plate.
The Atomic Theory
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After learning that atoms contained electrons,
Thomson produced a new model.
Thomson’s new model was called the “PlumPudding Model.”
This was a popular desert during this time period.
Thomson thought electrons were mixed
throughout the atom, like plums are mixed
throughout the pudding.
The Atomic Theory
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In 1909, a former student of
Thomson, (Rutherford) wanted to
test Thomson’s theory.
Rutherford concentrated a beam of
small, positively charged particles at
a thin sheet of gold foil.
Rutherford put a special coating
behind the gold which glowed when
hit by positively charged particles.
The Atomic Theory
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This allowed
Rutherford to see
where the particles
traveled after contact.
Some of the particles
passed straight
through, were
deflected, and
bounced straight
back.
The Atomic Theory
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In 1913, Niel Bohr studied the way
atoms react with light.
He proposed that electrons are
located on different “levels” and
orbit the nucleus.
Nucleus = protons + neutrons
The Atomic Theory
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Electrons do not travel in definite
paths.
The exact path of an electron
cannot be predicted.
There are regions inside the atom
where electrons are likely to be
found. (“electron clouds”)
Calculating Charges: Lithium
Calculating Charges: Nitrogen
Calculating Charges: Oxygen
Section 2: Objectives
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Describe the size of an atom.
Name the parts of an atom.
Describe the relationship between
numbers of protons and neutrons
and atomic number.
State how isotopes differ.
Calculate atomic masses.
Describe the forces within an atom.
The Atom
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Now that we know what an atom is,
let’s discuss what it is made up of.
As tiny as an atom is, it is made up
of smaller parts:
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Protons
Neutrons
Electrons
The Atom
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Protons: subatomic particles that
have a positive charge
Neutron: subatomic particle that
has no charge
Both protons and neutrons located
in the nucleus.
If the nucleus is only made up of
protons and neutrons, what does
this mean about the charge?
The Atom
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Electrons are found around the nucleus
within electron clouds.
Electrons are very small in mass.
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Example: It takes more than 1,800 electrons
to equal the mass of 1 proton.
What happens if the number of protons and
electrons are not equal?
The Atom
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There are more than 110 elements.
Why aren’t all atoms the same?
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Because of the numbers of protons, neutrons,
and electrons.
Start with the basics:
 To build the simplest element (hydrogen),
place one proton in the center of an atom.
 It’s so simple it has no neutron.
 Then add one electron in the electron cloud.
 You have just created a hydrogen atom.
The Atom
The Atom
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If you were to build an atom that
had more than one proton, you
must have neutrons.
Neutrons keep the protons from
repelling. (like charges)
How many electrons would the
atom have to contain to have a
neutral charge?
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You just created helium.
The Atom
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How can you tell which elements
these atoms represent?
The key is the number of protons.
The atomic number is the number
of protons in the nucleus.
The Atom
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Atoms of the same element have
the same atomic number.
Isotopes: atoms that have the
same number of protons but
different numbers of neutrons
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Atoms that are isotopes of each other
have the same number of protons.
They have different numbers of
neutrons which give them different
masses.
The Atom
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In order to sketch an atom:
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Determine mass number and atomic
number.
Subtract atomic number from mass
number.
Determine number of electrons.
Sketch your atom.