End of chapter exercises
Problem 1:
Give one word/term for each of the following descriptions.
1. The distance between two adjacent atoms in a molecule.
2. A type of chemical bond that involves the sharing of electrons between two atoms.
3. A measure of an atom's ability to attract electrons to itself in a chemical bond.
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Answer 1:
1. Bond length
2. Covalent bond
3. Electronegativity
Problem 2:
Which ONE of the following best describes the bond formed between an H+ ion and
the NH3 molecule?
1. Covalent bond
2. Dative covalent (co-ordinate covalent) bond
3. Ionic Bond
4. Hydrogen Bond
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Answer 2:
Dative covalent (co-ordinate covalent) bond
Problem 3:
Explain the meaning of each of the following terms:
1. valence electrons
2. bond energy
3. covalent bond
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Answer 3:
1. The number of electrons in the outermost shell of an atom that are available for use in
bonding either by sharing, donating or accepting.
2. The amount of energy needed for a bond to break.
3. A type of bond that occurs between two atoms with a difference in electronegativity
between 0 and 2,1.
Problem 4:
Which of the following reactions will not take place? Explain your answer.
1. H+H→H2
2. Ne+Ne→Ne2
3. Cl+Cl→Cl2
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Answer 4:
Ne+Ne→Ne2 will not take place as neon does not have electrons available for bonding.
Neon is a noble gas and has a full outer shell of electrons.
Problem 5:
Draw the Lewis diagrams for each of the following:
1. An atom of strontium (Sr). (Hint: Which group is it in? It will have an identical Lewis
diagram to other elements in that group).
2. An atom of iodine.
3. A molecule of hydrogen bromide (HBr).
4. A molecule of nitrogen dioxide (NO2). (Hint: There will be a single unpaired electron).
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Answer 5:
1.
2.
3.
4.
Problem 6:
Given the following Lewis diagram, where X and Y each represent a different element:
1. How many valence electrons does X have?
2. How many valence electrons does Y have?
3. Which elements could X and Y represent?
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Answer 6:
1. 1
2. 5
3. X could be hydrogen and Y could be nitrogen.
Problem 7:
Determine the shape of the following molecules:
1. O2
2. MgI2
3. BCl3
4. CS2
5. CCl4
6. CH3Cl
7. Br2
8. SCl5F
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Answer 7:
1. This is a diatomic molecule and so the molecular shape is linear.
2. The central atom is magnesium (draw the molecules Lewis structure to see this).
There are two electron pairs around magnesium and no lone pairs.
There are two bonding electron pairs and no lone pairs. The molecule has the general
formula AX2. Using this information and Table (Reference) we find that the molecular
shape is linear.
3. The central atom is boron (draw the molecules Lewis structure to see this).
There are three electron pairs around boron and no lone pairs.
There are three bonding electron pairs and no lone pairs. The molecule has the
general formula AX3. Using this information and Table (Reference) we find that the
molecular shape is trigonal planar.
4. The central atom is carbon (draw the molecules Lewis structure to see this).
There are four electron pairs around carbon forming two double bonds. There are no
lone pairs.
The molecule has the general formula AX2. Using this information and
Table (Reference) we find that the molecular shape is linear.
5. The central atom is carbon (draw the molecules Lewis structure to see this).
There are four bonding electron pairs and no lone pairs. The molecule has the general
formula AX4. Using this information and Table (Reference) we find that the molecular
shape is tetrahedral.
6. The central atom is carbon (draw the molecules Lewis structure to see this).
There are four bonding electron pairs and no lone pairs. The molecule has the general
formula AX4. Using this information and Table (Reference) we find that the molecular
shape is tetrahedral.
7. This is a diatomic molecule and so the molecular shape is linear.
8. The central atom is sulfur
There are six electron pairs around beryllium and no lone pairs.
The molecule has the general formula AX6. Using this information and
Table (Reference) we find that the molecular shape is octahedral.
Problem 8:
Complete the following table.
Element pair
Hydrogen and lithium
Hydrogen and boron
Hydrogen and oxygen
Hydrogen and sulfur
Magnesium and
nitrogen
Electronegativity
difference
Type of bond that could
form
Magnesium and
chlorine
Boron and fluorine
Sodium and fluorine
Oxygen and nitrogen
Oxygen and carbon
Table 1
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Answer 8:
Element pair
Electronegativity
difference
Type of bond that could
form
Hydrogen and lithium
1,22
Strong polar covalent bond
Hydrogen and boron
0,16
Weak polar covalent bond
Hydrogen and oxygen
1,24
Strong polar covalent bond
Hydrogen and sulfur
0,38
Weak polar covalent bond
Magnesium and
nitrogen
1,73
Strong polar covalent bond
Magnesium and
chlorine
1,73
Strong polar covalent bond
Boron and fluorine
1,94
Strong polar covalent bond
Sodium and fluorine
3,05
Ionic bond
Oxygen and nitrogen
0,40
Weak polar covalent bond
Oxygen and carbon
0,89
Weak polar covalent bond
Table 2
Problem 9:
Are the following molecules polar or non-polar?
1. O2
2. MgBr2
3. BF3
4. CH2O
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Answer 9:
1. The molecule is linear. There are two bonding pairs forming a double bond and two
lone pairs on each oxygen atom.
There is one bond. The electronegativity difference between oxygen and oxygen is 0.
The bond is non-polar.
The molecule is symmetrical and is non-polar.
2. The molecule is linear. There are two bonding pairs forming two single bonds and three
lone pairs on each bromine atom.
There are two bonds, both of which are between magnesium and bromine. The
electronegativity difference between magnesium and bromine is 1,6. The bonds are
polar.
The molecule is symmetrical and is non-polar.
3. The molecule is trigonal planar. There are three bonding pairs forming three single
bonds and three lone pairs on each fluorine atom.
There are three bonds, all of which are between boron and fluorine. The
electronegativity difference between boron and fluorine is 2,0. The bonds are polar.
The molecule is symmetrical and is non-polar.
4. The molecule is trigonal planar. There are four bonding pairs forming two single bonds
and one double bond. There are two lone pairs on the oxygen atom.
There are three bonds, two of which are between carbon and hydrogen. The
electronegativity difference between carbon and hydrogen is 0,4. The other bond is
between carbon and oxygen. The electronegativity difference between carbon and
oxygen is 1,0. All the bonds are polar.
The molecule is not symmetrical and is polar.
Problem 10:
Given the following graph for hydrogen:
1. The bond length for hydrogen is 74 pm. Indicate this value on the graph. (Remember
that pm is a picometer and means 74 × 10−12 m).
The bond energy for hydrogen is 436 kJ·mol−1. Indicate this value on the graph.
2. What is important about point X?
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Answer 10:
1.
2. At point X the attractive and repulsive forces acting on the two hydrogen atoms are
balanced. The energy is at a minimum.
Problem 11:
Hydrogen chloride has a bond length of 127 pm and a bond energy of 432 kJ·mol −1. Draw a
graph of energy versus distance and indicate these values on your graph. The graph does
not have to be accurate, a rough sketch graph will do.
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Answer 11: