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Biology
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Copyright Pearson Prentice Hall
2-1 The Nature of Matter
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Copyright Pearson Prentice Hall
2-1 The Nature of Matter
Atoms
Atoms
*The study of chemistry begins with the basic unit
of matter, the atom.
The Greek philosopher Democritus called the
smallest fragment of matter the atom, from the
Greek word atomos.
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2-1 The Nature of Matter
Atoms
Atoms contain subatomic particles that are even
smaller.
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2-1 The Nature of Matter
Atoms
What three subatomic particles make up
atoms?
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2-1 The Nature of Matter
Atoms
The subatomic particles that make up
atoms are
• Protons- (+) positive charge
• Neutrons- (0) no charge neutral
• Electrons- (-) negative charge
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2-1 The Nature of Matter
Atoms
The subatomic
particles in a helium
atom.
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2-1 The Nature of Matter
Atoms
Protons and neutrons are found in the nucleus or
center of the atom. They are held together by strong
forces.
Protons & neutrons have about the same atomic
mass, which is 1.
*The atomic mass is the total number of protons &
neutrons.
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Atoms
* Electrons are in constant motion in the space
surrounding the nucleus.
*They have a negligible atomic mass (0).
Electrons are attracted to the positively charged
nucleus but remain outside the nucleus because
of the energy of their motion.
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2-1 The Nature of Matter
Atoms
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2-1 The Nature of Matter
Atoms
*Because atoms have equal numbers of electrons
and protons, and because these subatomic particles
have equal but opposite charges, atoms are neutral.
*protons (+) = electrons (-)
If an atom has a charge it is called an Ion.
* + ion- protons > electrons
* - ion- protons< electrons
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2-1 The Nature of Matter
Elements and Isotopes
Elements and Isotopes
A chemical element is a pure substance that
consists entirely of one type of atom.
*Elements are represented by a one- or two-letter
symbol.
• Ex: C stands for carbon.
• Ex: Na stands for sodium.
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2-1 The Nature of Matter
Elements and Isotopes
*The number of protons in an atom of an element is
the element's atomic number (symbol Z).
Carbon has 6 protons, so its atomic number is 6.
*Every element has its own atomic number so it
identifies the element.
More than 100 elements are known, but only about
two dozen are commonly found in living organisms.
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2-1 The Nature of Matter
Rules:
1. To find Protons, it is the same as the atomic
number.
2. Protons = Electrons
3. To find Neutrons: atomic mass – atomic number
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*Periodic Table – list of all the elements
Symbols- for the names of the elements
Atomic numbers
Atomic mass
6 atomic number
C symbol
atomic mass
12.01
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2-1 The Nature of Matter
Elements and Isotopes
Isotopes
Def: Atoms of the same element that differ in the
number of neutrons they contain are known as
isotopes.
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2-1 The Nature of Matter
Elements and Isotopes
*Isotopes are identified by their mass numbers.
For example, carbon has three isotopes—carbon-12,
carbon-13, and carbon-14. Each isotope has a
different number of neutrons.
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2-1 The Nature of Matter
Elements and Isotopes
How are all of the isotopes of an element
similar?
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2-1 The Nature of Matter
Elements and Isotopes
Because they have the same number of
electrons, all isotopes of an element have
the same chemical properties.
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2-1 The Nature of Matter
Elements and Isotopes
Isotopes of Carbon
6 electrons
6 protons
6
7
8 neutrons
Copyright Pearson Prentice Hall
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2-1 The Nature of Matter
Elements and Isotopes
Radioactive Isotopes
Def: Some isotopes are radioactive, meaning that
their nuclei are unstable and break down at a
constant rate over time.
Although the radiation these isotopes give off can
be dangerous, they have important scientific and
practical uses.
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2-1 The Nature of Matter
Elements and Isotopes
*Radioactive isotopes can be used:
• to determine the ages of rocks and fossils.
• to treat cancer.
• to kill bacteria that cause food to spoil.
• as labels or “tracers” to follow the movement of
substances within an organism.
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2-1 The Nature of Matter
Chemical Compounds
Chemical Compounds
In nature, most elements are found combined with
other elements in compounds.
Def: A chemical compound is a substance formed
by the chemical combination of two or more
elements in definite proportions.
*The physical and chemical properties of a
compound are different from the elements from
which it is formed.
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Chemical Compounds
*Scientists show the composition of compounds by a
kind of shorthand known as a chemical formula.
Ex: Water, H2O, contains two atoms of hydrogen for
each atom of oxygen.
Ex:The formula for table salt, NaCl, indicates that
sodium and chlorine combine in a 1 : 1 ratio.
*When no subscript is present, 1 is understood.
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2-1 The Nature of Matter
Chemical Bonds
What are the two main types of chemical
bonds?
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2-1 The Nature of Matter
Chemical Bonds
Chemical Bonds
Def: The atoms in compounds are held together by
chemical bonds.
*Bond formation involves the electrons that
surround each atomic nucleus. These are called
Valence electrons.
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2-1 The Nature of Matter
Chemical Bonds
The main types of chemical bonds are:
• ionic bonds
• covalent bonds
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2-1 The Nature of Matter
Chemical Bonds
Ionic Bonds
Def: An ionic bond is formed when one or more
electrons are transferred from one atom to
another.
*An atom that loses electrons has a positive
charge.
*An atom that gains electrons has a negative
charge.
These positively and negatively charged atoms are
known as ions.
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2-1 The Nature of Matter
Chemical Bonds
Chlorine atom (Cl)
Sodium atom (Na)
Chloride ion (Cl-)
Sodium ion (Na+)
Protons
Electrons
+11
- 11
10
Protons
Electrons
+17
18
- 17
Charge
+10
Charge
-10
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2-1 The Nature of Matter
Chemical Bonds
Covalent Bonds
*Sometimes electrons are shared by atoms
instead of being transferred.
Sharing electrons means that the moving electrons
actually travel in the orbitals of both atoms.
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Chemical Bonds
A covalent bond forms when electrons are shared
between atoms.
• 1 When the atoms share two electrons, the bond
is called a single covalent bond. Ex: C-C
• 2 When atoms share four electrons it is called a
double bond. Ex: C
C
• 3 When atoms share six electrons it is called a
triple bond. Ex:
C
C
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Chemical Bonds
Def: The structure that results when atoms are joined
together by covalent bonds is called a molecule.
*A molecule is the smallest unit of most compounds.
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Chemical Bonds
In a water molecule, each
hydrogen atom forms a
single covalent bond with
the oxygen atom.
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2-1 The Nature of Matter
Chemical Bonds
Van der Waals Forces
*When molecules are close together, a slight
attraction can develop between the oppositely
charged regions of nearby molecules.
Chemists call such intermolecular forces of
attraction van der Waals forces, after the scientist
who discovered them.
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2-1 The Nature of Matter
Chemical Bonds
Although van der Waals forces are not as strong
as ionic bonds or covalent bonds, they can hold
molecules together, especially when the molecules
are large.
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2-1 The Nature of Matter
Chemical Bonds
*For example, van der
Waals forces form between
the molecules on the surface
of a gecko’s foot and the
molecules on the surface of
the wall.
The combined strength of all
the van der Waals forces
allows the gecko to grip the
wall.
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2-1
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Continue to:
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2-1
The particles that move around the nucleus of
an atom are called
a. neutrons.
b. protons.
c. electrons.
d. isotopes.
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2-1
The atomic number of a carbon atom is 6. How
many neutrons does the isotope carbon-14
have?
a. 6
b. 8
c. 12
d. 14
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2-1
Which of the following statements about the
three isotopes of carbon is true?
a. They are all radioactive.
b. They have different numbers of electrons.
c. They have the same chemical properties but
differ in atomic mass.
d. They have the same number of protons and
neutrons.
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2-1
A chemical compound consists of
a. electrons mixed with neutrons.
b. two or more elements combined in a definite
proportion.
c. two or more elements combined in any
proportion.
d. at least three elements combined by ionic or
covalent bonds.
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2-1
Van der Waals forces are the result of
a. unequal sharing of electrons.
b. ionic bonds.
c. the bonding of different isotopes.
d. the chemical combination of sodium and
chlorine.
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END OF SECTION
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