2-1 The Nature of Matter

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 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

Placed side by side, 100 million atoms would make a

row only about 1 centimeter long.
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
• neutrons
• electrons

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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 have about the same mass.

Protons are positively charged particles (+).
Neutrons carry no charge.
Strong forces bind protons and neutrons together to
form the nucleus, which is at the center of the atom.

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 2-1 The Nature of Matter Atoms

The electron is a negatively charged particle (−) with

1/1840 the mass of a proton.
Electrons are in constant motion in the space
surrounding the nucleus.

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 2-1 The Nature of Matter Atoms

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

Because atoms have equal numbers of electrons

and protons, and because these subatomic particles
have equal but opposite charges, atoms are neutral.

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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.
• C stands for carbon.
• 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.

Carbon has 6 protons, so its atomic number is 6.

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 Elements and Isotopes

Isotopes

Atoms of the same element that differ in

the number of neutrons they contain are
known as isotopes.
The sum of the protons and neutrons in
the nucleus of an atom is called its mass
number.

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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 Slide
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 2-1 The Nature of Matter Elements and Isotopes

Radioactive Isotopes

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.
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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 2-1 The Nature of Matter Chemical Compounds

Scientists show the composition of compounds by a

kind of shorthand known as a chemical formula.

Water, H2O, contains two atoms of hydrogen for each

atom of oxygen.

The formula for table salt, NaCl, indicates that

sodium and chlorine combine in a 1 : 1 ratio.

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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

The atoms in compounds are held

together by chemical bonds.
Bond formation involves the electrons
that surround each atomic nucleus.
The electrons that are available to form
bonds 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
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

Sodium atom (Na)

Sodium ion (Cl-)
Sodium ion (Na+) Sodium atom (Cl)

Protons +11 Protons +17

Electrons - 11
10 Electrons 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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 2-1 The Nature of Matter Chemical Bonds

A covalent bond forms when electrons are shared

between atoms.

• When the atoms share two electrons, the bond is

called a single covalent bond.
• When atoms share four electrons it is called a
double bond.
• When atoms share six electrons it is called a
triple bond.

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 2-1 The Nature of Matter Chemical Bonds

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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 2-1 The Nature of Matter 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

Continue to: Click to Launch:

- or -

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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