Ch. 8 Chemistry study guide

How many valid electron dot formulas—having the same number of electron pairs for a molecule or ion—can be written when a resonance structure occurs?

0

How many pi bonds are formed when sp[2] hybridization occurs in ethene, C2H4?

1

How many unshared pairs of electrons does the nitrogen atom in ammonia possess?

1

What is the bond angle in a water molecule?

105 degrees

Experimental evidence suggests that the H—C—H bond angles in ethene, C2H4, are ____.

120 degrees

How many electrons are shared in a single covalent bond?

2

Which of the following electron configurations gives the correct arrangement of the four valence electrons of the carbon atom in the molecule methane (CH4)?

2s[1] 2p[3]

How many covalent bonds are in a covalently bonded molecule containing 1 phosphorus atom and 3 chlorine atoms?

3

How many electrons does a nitrogen atom need to gain in order to attain a noble-gas electron configuration?

3

How many unshared pairs of electrons are in a molecule of hydrogen iodide?

3

How many electrons are shared in a double covalent bond?

4

How many electrons does carbon need to gain in order to obtain a noble-gas electron configuration?

4

What is the total number of covalent bonds normally associated with a single carbon atom in a compound?

4

How many valence electrons does an iodine atom have?

7

Explain a pi bond and a sigma bond. Which of these bond types tends to be the weaker? Why?

A pi bond is the bond formed as a result of the side-by-side overlap of two p orbitals. A sigma bond is the bond that results from a combination of two s orbitals, two p orbitals, or a p and an s orbital. Orbital overlap in pi bonding is less extensive than that for sigma bonding. Therefore, pi bonds tend to be weaker than sigma bonds.

What is bond dissociation energy, and how does it affect carbon compounds?

Bond dissociation energy is the energy required to break a single bond. The greater the bond dissociation energy, the more stable the compound. Due in part to the high bond dissociation energy of carbon-carbon bonds, carbon compounds are not very reactive chemically.

Which molecule has a single covalent bond?

Cl2

What are dispersion forces? How is the strength of dispersion forces related to the number of electrons in a molecule? Give an example of molecules that are attracted to each other by dispersion forces.

Dispersion forces are the weakest of all molecular interactions, and are thought to be caused by the motion of electrons. Generally, the strength of dispersion forces increases as the number of electrons in a molecule increases. Diatomic molecules of halogen elements are an example of molecules whose attraction for one another is caused by dispersion forces.

Why is hydrogen bonding only possible with hydrogen?

Hydrogen's nucleus is electron deficient when it bonds with an electronegative atom.

Which of the following covalent bonds is the most polar?

H—F

Which of the following bonds is the least reactive?

H—H

Explain what is meant by orbital hybridization. Give an example of a molecule in which orbital hybridization occurs.

In orbital hybridization, two or more different atomic orbitals mix to form the same total number of equivalent hybrid orbitals. For instance, the s and p orbitals of an atom combine to make hybrid orbitals having the character of both the s orbital and the p orbital. These hybrid orbitals are equivalent. Orbital hybridization occurs in the methane molecule in which one 2s orbital and three 2p orbitals hybridize to form four sp[3] orbitals.

Choose a molecule which displays an exception to the octet rule and illustrate clearly, using Lewis dot structures, why it is considered an exception. Discuss whether resonance structures can account for its stability.

NO2 is an molecule that does not satisfy the octet rule. The Lewis dot structure for nitrogen dioxide is (see attached image). It does not satisfy the octet rule because each atom in the molecule only has access to 7 electrons, instead of the eight required. This molecule can be represented by a resonance structure in which the nitrogen atom and one oxygen atom at a time appear to have a stable octet. The actual bonding is a hybrid, or mixture, of the extremes represented by the resonance forms.

Describe a network solid and give two examples.

Network solids are substances in which all of the atoms are covalently bonded to each other. Melting these substances requires breaking covalent bonds throughout the solid. Two examples are diamond and silicon carbide.

Which of the following atoms acquires the most negative charge in a covalent bond with hydrogen?

O

Which of the following diatomic molecules is joined by a double covalent bond?

O2

In which of the following compounds is the octet expanded to include 12 electrons?

SF6

The molecular formula for the compound hydrogen cyanide is HCN. What information does the molecular formula provide about hydrogen cyanide?

The formula HCN indicates that a molecule of hydrogen cyanide contains one hydrogen atom, one carbon atom, and one nitrogen atom.

What determines the degree of polarity in a bond? Distinguish between nonpolar covalent, polar covalent, and ionic bonds in terms of relative polarity.

The relative electronegativity of the two bonded atoms determines the polarity of a bond. If the difference in electronegativities between the two atoms is less than 0.4, the bond is nonpolar covalent. If the difference in electronegativities between the two atoms is 0.4 to 1.0, the bond is moderately polar covalent. If the difference in electronegativities between the two atoms is 1.0 to 2.0, the bond is highly polar covalent. If the difference in electronegativities between the two atoms is more than 2.0, the bond is ionic.

Once formed, how are coordinate covalent bonds different from other covalent bonds?

There is no difference.

How do atoms achieve noble-gas electron configurations in single covalent bonds?

Two atoms share two electrons.

Explain what is meant by VSEPR theory. Give an example of how VSEPR theory can be applied to predict the shape of a molecule.

VSEPR (valence-shell electron-pair repulsion) theory states that because electron pairs repel, molecules adjust their shapes so that the valence-electron pairs, both bonding and non-bonding, are as far apart as possible. Methane, CH4, for example, has four bonding electron pairs and no unshared pairs. The bonding pairs are farthest apart when the angle between the central carbon and each of its attached hydrogens is 109.5 degrees. This is the angle that is observed experimentally.

shapes adjust so valence-electron pairs are as far apart as possible

VSEPR theory

What are the weakest attractions between molecules?

Van der Waals forces

Indicate how bonding is explained in terms of molecular orbitals.

When two atoms combine, the overlap of their atomic orbitals produces molecular orbitals. An atomic orbital belongs to a particular atom, whereas a molecular orbital belongs to a molecule as a whole. Much like an atomic orbital, two electrons are required to fill a molecular orbital. A bonding orbital is a molecular orbital occupied by the two electrons of a covalent bond.

Can some atoms exceed the limits of the octet rule in bonding? If so, give an example.

Yes, sulfur and phosphorus can expand the octet. They can have 12 or 10 valence electrons, respectively, when combined with small halogens. In PCl5, phosphorus has 10 valence electrons.

A resonance structure, like the one in the image on the study guide, represents

a hybrid of the extremes represented by the resonance forms.

What is the representative unit in a molecular compound?

a molecule

What causes dipole interactions?

attraction between polar molecules

Where are the electrons most probably located in a molecular bonding orbital?

between the two atomic nuclei

energy needed to break a single bond between two covalently bonded atoms

bond dissociation energy

What causes hydrogen bonding?

bonding of a covalently bonded hydrogen atom with an unshared electron pair

molecular orbital that can be occupied by two electrons of a covalent bond

bonding orbital

When H+ forms a bond with HO to form the hydronium ion H3O+, this bond is called a coordinate covalent bond because

both bonding electrons come from the oxygen atom

What is required in order to melt a network solid?

breaking covalent bonds

a covalent bond in which the shared electron pair comes from only one of the atoms

coordinate covalent bond

attraction between polar molecules

dipole interaction

Which of the forces of molecular attraction is the weakest?

dispersion

a covalent bond in which two pairs of electrons are shared

double covalent bond

Which elements can form diatomic molecules joined by a single covalent bond?

hydrogen and the halogens only

a type of bond that is very important in determining the properties of water and of important biological molecules such as proteins and DNA

hydrogen bond

When placed between oppositely charged metal plates, the region of a water molecule attracted to the negative plate is the ____.

hydrogen region of the molecule

What is the shape of a molecule with a triple bond?

linear

What is thought to cause the dispersion forces?

motion of electrons

Which noble gas has the same electron configuration as the oxygen in a water molecule?

neon

Which type of solid has the highest melting point?

network solid

crystal in which all the atoms are covalently bonded to each other

network solid

Which of the following elements can form diatomic molecules held together by triple covalent bonds?

nitrogen

Which of the following theories provides information concerning both molecular shape and molecular bonding?

orbital hybridization theory

A pair of molecular orbitals is formed by the

overlap of two atomic orbitals from different atoms.

According to VSEPR theory, molecules adjust their shapes to keep which of the following as far apart as possible?

pairs of valence electrons

A bond that is not symmetrical along the axis between two atomic nuclei is a(n) ____.

pi bond

The side-by-side overlap of p orbitals produces what kind of bond?

pi bond

Which of the following bond types is normally the weakest?

pi bond formed by the overlap of two p orbitals

a covalent bond between two atoms of significantly different electronegativities

polar bond

A bond formed between a silicon atom and an oxygen atom is likely to be ____.

polar covalent

Molecular orbital theory is based upon which of the following models of the atom?

quantum mechanical model

What causes water molecules to have a bent shape, according to VSEPR theory?

repulsive forces between unshared pairs of electrons

Sigma bonds are formed as a result of the overlapping of which type(s) of atomic orbital(s)?

s and p

symmetrical bond along the axis between the two nuclei

sigma bond

a covalent bond in which only one pair of electrons is shared

single covalent bond

What type of hybridization occurs in the orbitals of a carbon atom participating in a triple bond with another carbon atom?

sp

What type of hybrid orbital exists in the methane molecule?

sp[3]

a depiction of the arrangement of atoms in molecules and polyatomic ions

structural formula

The shape of the methane molecule is ____.

tetrahedral

109.5 degrees

tetrahedral angle

What is shown by the structural formula of a molecule or polyatomic ion?

the arrangement of bonded atoms

The chemical formula of an ionic compound shows

the lowest whole-number ratio between ions in the ionic compound.

What information does a molecular formula provide?

the number and kind of atoms present in a molecule

Why do atoms share electrons in covalent bonds?

to attain a noble-gas electron configuration