Anand Classes offers comprehensive study material on Define Chemical Bond, explaining Why Do Atoms Combine with detailed theories, MCQs, assertion-reason questions, and case Anand Classes offers comprehensive Class 11 Chemistry study material for JEE and NEET preparation on Define Chemical Bond, explaining Why Do Atoms Combine with detailed theories, solved examples, MCQs, assertion-reason questions, and case study-based problems to strengthen chemical bonding concepts for competitive exams. Click the print button to download study material and notes.study-based problems to help students master chemical bonding concepts for exams. Click the print button to download study material and notes.
What is Chemical Bond ?
The attractive force which holds together the constituent particles (atoms, ions, or molecules) in a chemical species is known as a chemical bond.
Why Do Atoms Combine?
A number of attempts were made to explain the formation of chemical bonds between atoms in terms of electrons. In 1916, Kossel and Lewis independently succeeded in giving the first successful explanation about the cause of combination between atoms, based upon the understanding of the electronic configuration of noble gases.
(i) Tendency to Acquire Noble Gas Configuration — Octet Rule
It has been observed that atoms of noble gases have little or no tendency to combine with each other or with atoms of other elements. This means that these atoms must be having stable electronic configurations.
Electronic Configurations of Noble Gases
| Noble Gas | Atomic Number | Electronic Configuration | Valence Shell Configuration |
|---|---|---|---|
| Helium (He) | 2 | $1s^2$ | $1s^2$ |
| Neon (Ne) | 10 | $1s^2,2s^2,2p^6$ | $2s^2 2p^6$ |
| Argon (Ar) | 18 | $1s^2,2s^2,2p^6,3s^2,3p^6$ | $3s^2 3p^6$ |
| Krypton (Kr) | 36 | $1s^2,2s^2,2p^6,3s^2,3p^6,3d^{10},4s^2,4p^6$ | $4s^2 4p^6$ |
| Xenon (Xe) | 54 | $1s^2,2s^2,2p^6,3s^2,3p^6,3d^{10},4s^2,4p^6,4d^{10},5s^2,5p^6$ | $5s^2 5p^6$ |
| Radon (Rn) | 86 | $1s^2,2s^2,2p^6,3s^2,3p^6,3d^{10},4s^2,4p^6,4d^{10},4f^{14},5s^2,5p^6,5d^{10},6s^2,6p^6$ | $6s^2 6p^6$ |
From the table, we find that the atoms of all noble gases (with the exception of helium) have eight electrons in their valence shell.
- Helium has two electrons in its valence shell (first energy shell) because the first shell ($n = 1$) cannot have more than two electrons.
- The electronic configuration of the valence shell for all noble gas atoms except helium can be expressed as:
ns2 np6 (for helium, 1s2)
This represents a stable configuration corresponding to maximum stability.
Thus, noble gases neither gain nor lose electrons, and their combining capacity (valency) is zero.
All atoms other than noble gases have less than eight electrons in their outermost shells. Therefore, they combine with each other or with other atoms to achieve stable noble gas electronic configurations ($ns^2,np^6$ or $1s^2$).
Hence, the tendency of atoms to attain a stable configuration of eight electrons in their valence shells is the cause of chemical combination.
This principle is called the Octet Rule.
Note: For hydrogen and lithium, the stable configuration means two electrons in the valence shell (duplet rule), corresponding to helium.
(ii) Tendency to Acquire Minimum Energy and Stability
According to the modern view, atoms combine to form chemical bonds in order to acquire a state of minimum energy.
- Every system in the universe has a natural tendency to attain a state of minimum energy.
- The state of minimum energy corresponds to maximum stability.
- Atoms combine with each other only if the process leads to a lowering of energy.
If there is no fall in the potential energy of the system, no bonding is possible.
Short Answer Conceptual Types Questions (SAT)
Q1: What is a chemical bond?
A chemical bond is the attractive force that holds together atoms, ions, or molecules in a chemical species. It is formed when atoms combine in order to achieve stable electronic configurations similar to noble gases. The tendency of atoms to attain stability by either losing, gaining, or sharing electrons leads to the formation of chemical bonds.
This concept is a fundamental part of Class 11 Chemistry and is frequently asked in JEE and NEET exams.
Q2: Why do atoms combine?
Atoms combine mainly for two reasons:
- To achieve a stable noble gas configuration (Octet Rule). Atoms other than noble gases have incomplete outer shells, so they combine by gaining, losing, or sharing electrons to attain eight electrons in their valence shell (two in the case of hydrogen and lithium). Example: Sodium ($Na$) loses one electron to achieve the configuration of neon ($1s^2 2s^2 2p^6$).
- To achieve minimum energy and maximum stability. Every system in nature tends to attain a state of lower energy, and bond formation helps atoms achieve this stable state.
These explanations form the basis of chemical bonding in Class 11 Chemistry and are crucial for JEE and NEET preparation.
Q3: What is the Octet Rule?
The Octet Rule states that atoms tend to combine in such a way that each atom achieves eight electrons in its outermost shell, similar to the electronic configuration of noble gases.
- Example: Sodium ($Na$) loses one electron to form $Na^+$, achieving the configuration of neon.
- Example: Chlorine ($Cl$) gains one electron to form $Cl^-$, achieving the configuration of argon.
Exceptions to the Octet Rule:
- Incomplete octet: $H$, $Li$, $Be$, $B$
- Expanded octet: $P$, $S$, $Cl$, $Xe$
- Odd-electron species: $NO$, $NO_2$
Understanding the Octet Rule and its exceptions is very important for Class 11 Chemistry as well as JEE and NEET exam questions.
Q4: Why are noble gases generally chemically inert?
Noble gases are chemically inert because they already have stable electronic configurations.
- General configuration: $ns^2 np^6$
- Exception: Helium ($He$) with $1s^2$
Because of their completely filled valence shells, they neither need to gain, lose, nor share electrons. As a result, their combining capacity (valency) is almost zero.
The concept of noble gas stability is a key part of Class 11 Chemistry and is often tested in JEE and NEET exams.
Q5: How does energy change during bond formation?
When a chemical bond is formed, the potential energy of the system decreases.
- Attractive forces (nucleus–electron) dominate over repulsive forces (nucleus–nucleus, electron–electron).
- This results in a release of energy, making the system more stable.
- The greater the energy released during bond formation, the stronger the bond.
Thus, bond formation is always an energy-releasing (exothermic) process.
Understanding energy changes during bonding is an essential concept in Class 11 Chemistry and is frequently tested in JEE and NEET exams.
Multiple Choice Questions (MCQs) With Answers and Explanation
Which of the following best defines a chemical bond?
(a) A repulsive force between two atoms
(b) An attractive force that holds atoms, ions, or molecules together
(c) A force that increases the energy of a system
(d) A force present only in noble gases
Answer: (b)
Explanation:
A chemical bond is the attractive force that holds together atoms, ions, or molecules to form stable chemical species. It lowers the energy of the system, making atoms more stable. This is a core topic in Class 11 Chemistry, important for JEE and NEET exams.
Why do atoms combine to form molecules?
(a) To increase their energy
(b) To achieve a stable noble gas configuration and minimum energy
(c) To decrease the number of electrons in the valence shell
(d) To make their nucleus more stable
Answer: (b)
Explanation:
Atoms combine mainly to achieve noble gas configuration (Octet Rule) and to attain minimum energy with maximum stability. For example, sodium loses one electron to get the configuration of neon ($1s^2 2s^2 2p^6$). This principle is very important in chemical bonding, as discussed in Class 11 Chemistry, and forms a frequent question in JEE and NEET.
Which of the following is NOT an exception to the Octet Rule?
(a) $NO$ (odd-electron species)
(b) $BF_3$ (incomplete octet)
(c) $SF_6$ (expanded octet)
(d) $NaCl$ (ionic compound)
Answer: (d)
Explanation:
The Octet Rule has three exceptions:
- Incomplete octet ($H$, $Li$, $Be$, $B$)
- Expanded octet ($P$, $S$, $Cl$, $Xe$)
- Odd-electron species ($NO$, $NO_2$)
$NaCl$ follows the Octet Rule because $Na^+$ and $Cl^-$ both attain stable noble gas configurations. Hence, it is not an exception. Such questions are common in Class 11 Chemistry, and frequently tested in JEE and NEET exams.
Why are noble gases generally inert?
(a) They have incomplete valence shells
(b) They easily gain electrons
(c) They have stable electronic configurations with filled valence shells
(d) They readily share electrons
Answer: (c)
Explanation:
Noble gases have a completely filled valence shell configuration ($ns^2 np^6$), except helium with $1s^2$. This makes their combining capacity (valency) nearly zero, hence they are chemically inert. This concept is a frequently asked topic in JEE and NEET Chemistry.
What happens to the energy of a system when a chemical bond is formed?
(a) Energy increases, making the system unstable
(b) Energy remains the same
(c) Energy decreases, and the system becomes more stable
(d) Energy fluctuates continuously
Answer: (c)
Explanation:
During bond formation, attractive forces dominate over repulsive forces, resulting in a decrease in potential energy. Energy is released, and the system becomes more stable. Bond formation is therefore an exothermic process. This concept is fundamental in Class 11 Chemistry and is regularly tested in JEE and NEET exams.
Assertion Reason Type Questions With Answers and Explanation
Assertion (A): A chemical bond is formed to achieve stable noble gas configuration.
Reason (R): Noble gases are chemically inert due to completely filled valence shells.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true, but R is false
(d) A is false, but R is true
Answer: (a)
Explanation:
Atoms form bonds to achieve stability like noble gases, which are inert due to their completely filled valence shells ($ns^2 np^6$). Hence both A and R are true, and R correctly explains A.
Assertion (A): Atoms combine to lower their potential energy.
Reason (R): Bond formation always increases the energy of the system.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true, but R is false
(d) A is false, but R is true
Answer: (c)
Explanation:
Bond formation decreases potential energy and makes atoms stable. The reason is wrong because bond formation releases energy, not increases it.
Assertion (A): The Octet Rule states that atoms tend to have eight electrons in their valence shell.
Reason (R): Sodium forms $Na^+$ by gaining one electron.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true, but R is false
(d) A is false, but R is true
Answer: (c)
Explanation:
The Octet Rule is correct, but sodium does not gain an electron. Instead, it loses one electron to achieve neon’s configuration ($1s^2 2s^2 2p^6$).
Assertion (A): Noble gases are generally inert.
Reason (R): They have an incomplete octet.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true, but R is false
(d) A is false, but R is true
Answer: (c)
Explanation:
Noble gases are inert, but not because of an incomplete octet. They are inert because they have a complete octet ($ns^2 np^6$), except helium ($1s^2$).
Assertion (A): Bond formation is generally an exothermic process.
Reason (R): During bond formation, repulsive forces dominate over attractive forces.
(a) Both A and R are true, and R is the correct explanation of A
(b) Both A and R are true, but R is not the correct explanation of A
(c) A is true, but R is false
(d) A is false, but R is true
Answer: (c)
Explanation:
Bond formation releases energy (exothermic) because attractive forces dominate over repulsive forces. The given reason is false.
Case Study : Chemical Bonding and Stability
Read the following passage and answer the questions:
Atoms combine with each other to achieve stability by either gaining, losing, or sharing electrons. This tendency is explained by the Octet Rule, which states that atoms tend to achieve eight electrons in their outermost shell. Noble gases are chemically inert because they already have stable electronic configurations ($ns^2 np^6$ for all except helium, which has $1s^2$).
Bond formation results in the release of energy, as attractive forces overcome repulsive forces, thereby lowering the potential energy of the system. The stability of a bond depends on the amount of energy released during its formation.
Q1. Which of the following statements about noble gases is correct?
(a) They have incomplete octets
(b) They are highly reactive due to vacant orbitals
(c) They are chemically inert due to completely filled valence shells
(d) They readily gain electrons to complete the octet
Answer: (c)
Explanation: Noble gases are inert because of their completely filled valence shells ($ns^2 np^6$, except helium with $1s^2$).
Q2. Which of the following correctly represents the Octet Rule?
(a) Atoms combine to have 6 electrons in their outermost shell
(b) Atoms combine to have 8 electrons in their outermost shell
(c) Atoms combine to completely fill the $d$-subshell
(d) Atoms combine to increase their nuclear charge
Answer: (b)
Explanation: The Octet Rule states that atoms combine to achieve 8 electrons in the valence shell, just like noble gases.
Q3. What happens to the energy of the system when a chemical bond is formed?
(a) Energy increases, and the system becomes unstable
(b) Energy decreases, and the system becomes more stable
(c) Energy remains unchanged
(d) Energy fluctuates continuously
Answer: (b)
Explanation: Bond formation releases energy (exothermic), resulting in a decrease in potential energy and greater stability.
Q4. Which of the following species is an exception to the Octet Rule due to an incomplete octet?
(a) $NO_2$
(b) $BF_3$
(c) $SF_6$
(d) $NaCl$
Answer: (b)
Explanation: $BF_3$ (boron trifluoride) is an incomplete octet exception because boron has only 6 valence electrons after bonding.
Q5. Why is bond formation generally an exothermic process?
(a) Repulsive forces dominate during bond formation
(b) Attractive forces dominate, lowering the system’s potential energy
(c) Nucleus-nucleus repulsion releases energy
(d) Electrons absorb energy during bonding
Answer: (b)
Explanation: Bond formation occurs when attractive forces (nucleus–electron) dominate over repulsive forces, resulting in energy release and stability.
