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A covalent bond occurs when the nuclei of atoms share electrons, resulting in a stable balance of attractive and repulsive electrostatic forces. The potential energy of any system of two atoms capable of forming a covalent bond depends on the distance between the atoms. The more negative the potential energy, the greater the attractive force between the atoms.
Bond energy is the amount of energy released when a bond is formed, which is equivalent to the absolute value of the potential energy of two atoms in a covalent bond. Bonds of higher energies are more stable than bonds of lower energies. Bond length is the average distance between the nuclei of atoms in a covalent bond. Bond order is the number of electron pairs shared between the two atoms. For diatomic molecules, bond energy is equivalent to the dissociation energy, which is the amount of energy required to pull apart the atoms in a diatomic molecule.
Figure 1 shows the potential energy curve for a system of two hydrogen atoms. Table 1 presents bond lengths and energies for covalent bonds of varying orders. Table 2 presents the bond length and dissociation energies for a number of diatomic molecules.
Figure 1
Table 1 | |||
bond |
bond order |
bond length (pm) |
bond energy (kJ/mol) |
C–C |
1 |
154 |
347 |
C=C |
2 |
134 |
612 |
C≡C |
3 |
120 |
837 |
N–N |
1 |
145 |
163 |
N=N |
2 |
123 |
418 |
N≡N |
3 |
110 |
946 |
C–O |
1 |
143 |
360 |
C=O |
2 |
120 |
736 |
Table 2 | ||
molecule |
dissociation energy (kJ/mol) |
bond length (pm) |
H2 |
432 |
74 |
N2 |
946 |
110 |
O2 |
495 |
121 |
F2 |
158 |
142 |
Cl2 |
237 |
199 |
Sources:
http://www.science.uwaterloo.ca/~cchieh/cact/c120/bondel.html
http://www.wiredchemist.com/ch...
Based on information in the passage and Figure 1, as the distance between two hydrogen atoms decreases, the attractive force between the atoms:
Title
A covalent bond occurs
Your Result
Correct
Difficulty
Very Hard
Your Pace
0:02
Others' Pace
1:05
Video Explanation
Text Explanation
The correct answer is (C).
Starting from the right side of the figure (region 1 in the the figure), note that at large distances the potential energy is zero. Moving leftward (decreasing distance), the potential energy becomes more negative (region 2), eventually reaching a minimum (region 3), and then increasing back to zero at very short distances (region 4). Based on this information alone, you might be tempted to choose answer choice (D). However, the introduction to the passage states, “the more negative the potential energy, the greater the attractive force between the atoms.” Therefore, as potential energy decreases, the attractive force increases.
So at large distances, the potential energy is close to zero, and there is essentially no attractive force between the hydrogen atoms. At smaller distances (moving leftward on the curve to position 2) the attractive force increases. The maximum attractive force occurs at the minimum in the potential energy curve (position 3). Finally, at very small distances, the potential energy becomes less negative. Therefore the attractive force decreases again. (At even smaller distances, the electrostatic force between the hydrogen nuclei is repulsive, causing the energy to rise.) Therefore, answer choice (C) is the correct answer: As the distance between two atoms decreases, the attractive force increases, then decreases.
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