Chapter Preview Questions 1. Groups of two or more atoms held
together by chemical bonds are a. elements. b. molecules. c.
particles. d. electrons.
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Chapter Preview Questions 1. Groups of two or more atoms held
together by chemical bonds are a. elements. b. molecules. c.
particles. d. electrons.
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Chapter Preview Questions 2. When elements are chemically
combined in a set ratio, they form a. molecules. b. matter. c.
elements. d. compounds.
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Chapter Preview Questions 2. When elements are chemically
combined in a set ratio, they form a. molecules. b. matter. c.
elements. d. compounds.
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Chapter Preview Questions 3. Which of the following is an
example of an element? a. copper b. rust c. concrete d. water
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Chapter Preview Questions 3. Which of the following is an
example of an element? a. copper b. rust c. concrete d. water
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Chapter Preview Questions 4. Two compounds with different
properties are a. C and O. b. H and O. c. CO and CO 2. d. Cl and
Ar.
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Chapter Preview Questions 4. Two compounds with different
properties are a. C and O. b. H and O. c. CO and CO 2. d. Cl and
Ar.
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Whats an atom? Smallest particle of an element
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Atomic Theories There have been many theories of proposed
models of the atom. Some were completely wrong! And some were
partially correct or close to correct. We will look at some of the
most known theories and models.
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Daltons Atomic Theory Dalton thought that atoms were like
smooth, hard balls that could not be broken into smaller pieces. He
was credited with coming up with the earliest model of the atom
Most of this theory is still accepted today
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Thomsons Model Thomson suggested that atoms had negatively
charged electrons embedded in a positive sphere. This is the first
model that included smaller particles of the atom His model is
similar to blueberries in a muffin
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Rutherfords Model Developed the first model that included a
nucleus Similar to a peach with a pit, the pit = the heavy mass of
protons in the nucleus, the fruit = the empty space made up of
electrons
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Rutherfords Gold Foil Experiment Turn to page 126-127 in your
text book Lets read about Rutherfords model/ experiment Who wants
to read first? Everyone else, follow along.
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Rutherfords Gold Foil Experiment Rutherford was surprised that
a few particles were deflected strongly. This led him to propose an
atomic model with a positively charged nucleus.
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Bohrs Model Said electrons could only have a certain amount of
energy, so they must move in certain orbits Similar to planets
orbiting the sun (sun = nucleus), or the rings/layers of an
onion
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Cloud Model Says Bohrs model is wrong, electrons do not orbit
the nucleus like planets do the sun Instead, electrons can be found
anywhere in a cloud-like region around the nucleus, rapidly
orbiting in every direction
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Later Atomic Models Through the first part of the twentieth
century, atomic models continued to change.
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Modern Atomic Model Accepted since the 1930s At the center of
the atom is a massive nucleus, containing positively charged
protons and neutrally charged neutrons (no charge) Surrounding the
nucleus is a cloud-like region of moving negatively charged
electrons # protons in atom = # electrons in atom # neutrons can
change
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Particles in an Atom An atom is composed of positively charged
protons, neutral neutrons, and negatively charged electrons.
Protons and neutrons are about equal in mass. An electron has about
1/2,000 the mass of a proton or neutron.
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Atomic Number Every atom of a given element has the same number
of protons in its nucleus This is known as the elements atomic
number For example, Heliums atomic number is 2, so every atom of
helium has 2 protons in its nucleus. If the element has more than 2
protons, it is NOT helium! Carbons atomic number is 6. How many
protons does an atom of carbon have?
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Atomic Number The atomic number is usually found above the
letter on the periodic table Look at helium below
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Isotopes: Atoms of elements have same # protons, but different
#s of electrons Atoms of all isotopes of carbon contain six protons
and six electrons, but they differ in their number of neutrons.
Carbon-12 is the most common isotope.
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Mass number An isotope is identified by its mass number Mass
number = #protons + #neutrons Example: Carbon-12, the mass number
is 12. We know carbon has 6 protons by looking at its atomic number
on the periodic table. How many neutrons does carbon-12 have? 6 How
do you know? Because if the mass number is 12, 12 - 6protons =
6neutrons
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Problem: The atomic number of nitrogen is 7. How many protons,
neutrons, and electrons make up Nitrogen-15? Protons = 7 Neutrons =
8 Electrons = 7
