Although they are composed of ions, ionic Most ionic ...

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Key Concepts

Although they are composed of ions, ionic compounds are electrically neutral.

Most ionic compounds are crystalline solids at room temperature.

Ionic compounds generally have high melting points.

Ionic compounds can conduct an electric current when melted or dissolved in water.

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CHEMISTRY & YOU

What are some properties that are unique to metals? Wrought iron is a very pure form of iron that contains trace amounts of carbon. It is a tough, malleable, ductile, and corrosion-resistant material that melts at very high temperatures.

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Metallic Bonds and Metallic Properties

Metallic bonds are the forces of attraction between the free-floating valence electrons and the positively charged metal ions. • These bonds hold metals together.

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Properties of Metals Metals are good conductors of electric current because electrons can flow freely in the metal.

Movie-conduction

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Properties of Metals Metals are good conductors of electric current because electrons can flow freely in the metal. • As electrons enter one end of a bar of metal,

an equal number of electrons leave the other end.

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Metals are ductile—that is, they can be drawn into wires.

Properties of Metals

Force

Metal rod

Die

Wire

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When a metal is subjected to pressure, the metal cations easily slide past one another.


Sea of electrons

Metal cation

Force

Metal

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When a metal is subjected to pressure, the metal cations easily slide past one another.

• If an ionic crystal is struck with a hammer, the blow tends to push the positive ions close together.

• The positive ions repel one another, and the crystal shatters.


Sea of electrons

Metal cation

Force Force

Metal cation

Strong repulsions

Nonmetal anion

Metal Ionic crystal

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How are metals and ionic compounds different? How are they similar?

CHEMISTRY & YOU

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How are metals and ionic compounds different? How are they similar?

CHEMISTRY & YOU

Both metals and ionic compounds form crystal structures. However, they have different configurations of electrons. The sea of electrons surrounding cations in a metal allows metals to be ductile and malleable. Ionic crystals will fracture under pressure.

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Alloys

Alloys Why are alloys important?

• Alloys are mixtures of two or more elements, at least one of which is a metal.

– Brass, for example, is an alloy of copper and zinc.

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Alloys

Alloys are important because their properties are often superior to those of their component elements. • Sterling silver (92.5 percent

silver and 7.5 percent copper) is harder and more durable than pure silver, yet it is still soft enough to be made into jewelry and tableware.

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Alloys

The most important alloys today are steels. • The principal elements in most steels, in addition to

iron and carbon, are boron, chromium, manganese, molybdenum, nickel,

tungsten, and vanadium.

• Steels have a wide range of useful properties, such as corrosion resistance, ductility, hardness, and toughness.

Stainless Steel 80.6% Fe 18.0% Cr 0.4% C 1.0% Ni

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Chapter 8 Covalent Bonding 8.1 Molecular Compounds 8.2 The Nature of Covalent Bonding 8.3 Bonding Theories 8.4 Polar Bonds and Molecules

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Although the types of pieces are limited, you can make many different toy models depending on how many pieces you use and how they are arranged.

CHEMISTRY & YOU

How are atoms joined together to make compounds with different structures?

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In nature, only the noble gas elements, such as helium and neon, exist as uncombined atoms.

Molecules and Molecular Compounds

• They are monatomic; that is, they consist of single atoms.

• Helium, which is less dense than air, is often used to inflate balloons.

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But not all elements are monatomic.


• O2 represents two oxygen atoms that are bonded together.

• Scuba divers breath compressed air, a mixture that contains oxygen gas.

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Ionic compounds are generally crystalline solids with high melting points. Other compounds, however, have very different properties.

• Water (H2O) is a liquid at room temperature.

• Carbon dioxide (CO2) and nitrous oxide (N2O) are both gases at room temperature.

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These bonds do not involve the transfer of electrons.


The attractions that hold together the atoms in O2, H2O, CO2, and N2O cannot be explained by ionic bonding.

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Recall that ionic bonds form when the combining atoms give up or accept electrons.

Another way that atoms can combine is by sharing electrons.


Sharing Electrons

– Atoms that are held together by sharing electrons are joined by a covalent bond.

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Oxygen gas consists of oxygen molecules; each oxygen molecule consists of two covalently bonded oxygen atoms.

Sharing Electrons


A molecule is a neutral group of atoms joined together by covalent bonds.

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Other elements found in nature in the form of diatomic molecules include hydrogen, nitrogen, and the halogens.


Sharing Electrons An oxygen molecule is an example of a diatomic molecule—a molecule that contains two atoms.

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A compound composed of molecules is called a molecular compound. – Water is an example of a molecular

compound.


Sharing Electrons Molecules can also be made of atoms of different elements.

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A molecular formula is the chemical formula of a molecular compound.


Representing Molecules

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A molecular formula shows how many atoms of each element a substance contains.



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The molecular formula of water is H2O.

• Notice that the subscript written after an element’s symbol indicates the number of atoms of each element in the molecule.

• If there is only one atom, the subscript 1 is omitted.

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• Butane is commonly used in lighters and household torches.

• The molecular formula for butane is C4H10.

• According to this formula, one molecule of butane contains four carbon atoms and ten hydrogen atoms.


Representing Molecules Butane is also a molecular compound.

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• The subscripts are not necessarily the lowest whole-number ratios.


Representing Molecules A molecular formula reflects the actual number of atoms in each molecule.

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• For example, an oxygen molecule consists of two oxygen atoms bonded together; its molecular formula is O2.


Representing Molecules Note that molecular formulas also describe molecules consisting of atoms of one element.

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In other words, it does not show either the arrangement of the various atoms in space or which atoms are covalently bonded to one another.



A molecular formula does not tell you about a molecule’s structure.

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A variety of diagrams and molecular models can be used to show the arrangement of atoms in a molecule.



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Acetylsalicylic acid, also known as aspirin, has a molecular formula of C9H8O4. What elements make up acetylsalicylic acid? How many atoms of each element are found in one molecule of acetylsalicylic acid?

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Acetylsalicylic acid, also known as aspirin, has a molecular formula of C9H8O4. What elements make up acetylsalicylic acid? How many atoms of each element are found in one molecule of acetylsalicylic acid? One molecule of acetylsalicylic acid is made of 9 carbon atoms, 8 hydrogen atoms, and 4 oxygen atoms.

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Comparing Molecular and Ionic Compounds

What representative units define molecular compounds and ionic compounds?


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The representative unit of a molecular compound is a molecule. For an ionic compound, the smallest representative unit is a formula unit.


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Many molecular compounds are gases or liquids at room temperature.


Molecular compounds tend to have relatively lower melting and boiling points than ionic compounds.

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Many molecular compounds are gases or liquids at room temperature.

In contrast to ionic compounds, which are formed from a metal combined with a nonmetal, most molecular compounds are composed of atoms of two or more nonmetals.


Molecular compounds tend to have relatively lower melting and boiling points than ionic compounds.

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Water, which is a molecular compound, and sodium chloride, which is an ionic compound, are compared here.


Collection of water molecules

Molecule of water

Chemical formula H2O

Array of sodium ions and

chloride ions

Formula unit of sodium chloride

Chemical formula NaCl

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84

ionic-covalent dissolve.swf

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In molecular compounds, bonding occurs when atoms share electrons.

In ionic compounds, bonding occurs

when electrons are transferred between atoms.

BIG IDEA

Bonding and Interactions

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The hydrogen atoms in a hydrogen molecule are held together mainly by the attraction of the shared electrons to the positive nuclei.

Two atoms held together by sharing

one pair of electrons are joined by a single covalent bond.

The Octet Rule in Covalent Bonding

Single Covalent Bonds

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Hydrogen gas consists of diatomic molecules whose atoms share only one pair of electrons, forming a single covalent bond.



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An electron dot structure such as H:H represents the shared pair of electrons of the covalent bond by two dots.

The pair of shared electrons forming the covalent bond

is also often represented as a dash, as in H—H for hydrogen.

A structural formula represents the covalent bonds as

dashes and shows the arrangement of covalently bonded atoms.



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The ammonium ion (NH4+) consists of

atoms joined by covalent bonds. A polyatomic ion, such as NH4

+, is a tightly bound group of atoms that has a positive or negative charge and behaves as a unit.

Coordinate Covalent Bonds

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Most polyatomic cations and anions contain covalent bonds.

Therefore, compounds containing

polyatomic ions include both ionic and covalent bonding.

Coordinate Covalent Bonds

Although they are composed of ions, ionic Most ionic ...

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