9.3.1 Why do Atoms Form Chemical Bonds
This is the first lecture from Chapter 3: “Chemical Bonding” in the new Class 9 Chemistry book (Punjab Board – PCTB). It explains why atoms form chemical bonds and briefly describes how these bonds are formed. The lecture also includes a multiple-choice quiz, short question and long question notes.
MCQs Based Quiz
Short Questions
Q1. How can an element lower its energy?
An atom can lower its energy by:
- Gaining electrons
- Losing electrons
- Sharing electrons
Q2. Why are noble gases the least reactive elements?
Noble gases do not react with other elements because they are stable due to their complete duplet or octet shells. This complete shell gives them stability.
Q3. What do you mean by the duplet rule?
The Duplet Rule states that atoms like hydrogen (H) and lithium (Li) can achieve stability by completing their first shell with two electrons.
Q4. What is meant by the octet rule?
The Octet Rule means that atoms achieve stability by completing their outermost shell with eight electrons.
Q5. Why do sodium atoms tend to lose one electron?
Sodium atoms tend to lose one electron to complete their outermost shell (2nd shell) with 8 electrons and achieve stability.
Q6. Is hydrogen atom (H) stable compared to hydride ion (H–)?
No, the neutral hydrogen atom (H) is not as stable as the hydride ion (H–) because only the hydride ion has a complete duplet, whereas hydrogen atom (H) does not.
$\ce{
\underset{\text{(Unstable)}}{H} + {e^-} -> \underset{\text{(Stable)}}{H^-}
}$
Q7. Why doesn’t a sodium atom (Na) gain seven electrons to achieve stability?
Sodium atom does not gain 7 electrons because it is energetically unfavourable. Instead, it achieves stability by losing just one electron.
$\ce{
\underset{\text{(Unstable)}}{Na}
->
\underset{\text{(Stable)}}{Na^+} + e^-
}$
Q8. Which elements do alkali and alkaline earth metals tend to bond with?
Alkali and alkaline earth metals tend to bond with electropositive elements like chlorine and oxygen.
Q9. How would you define a chemical bond?
A chemical bond is a force of attraction that holds two atoms in the form of a molecule. a compound or metal.
Q10. What is the basic process of the formation of a chemical bond?
The formation of a chemical bond involves the following steps:
- Two atoms come closer.
- Attractive forces dominate between them.
- The energy of the system is lowered
- A chemical bond is formed between the atoms
Q11. What do you mean by electronic configuration?
The arrangement of electrons around the nucleus of an atom in shells and sub-shells is called electronic configuration.
Q12. Name three types of chemical bonds.
- Ionic Bond / Electrovalent Bond
- Covalent Bond
- Coordinate Covalent Bond
Descriptive Question
Q1. Explain the reason why atoms form chemical bonds. Also explain the relation between energy of the atoms and their stability.
Duplet and Octet Rule:
Chemists found that noble gases neither react with themselves nor with other elements. Based on this, they concluded that noble gases are the most stable elements. It was suggested that their stability is due to their completely filled outermost shells.
- Helium (He) has two electrons in its outermost shell (duplet).
- Ne, Ar, Kr and Xe have eight electrons in their outermost shell (octet).
By studying the stability of these noble gases, chemists proposed the duplet and octet rules:
- Atoms like hydrogen (H) and Lithium (Li) can become stable by completing their first shell with two electrons. This is called the duplet rule.
- Other elements can become stable by completing their outermost shell with eight electrons. This is called the octet rule.
Energy and Stability:
Atoms form chemical bonds because this lowers their energy and makes them stable. Completion of a duplet or octet is one way to achieve this.
- It is easier for a sodium atom to lower its energy by losing one electron rather than gaining seven electrons to complete its octet.
$\ce{
\underset{\text{(Unstable)}}{Na}
->
\underset{\text{(Stable)}}{Na^+} + e^-
}$
- A hydrogen atom can decrease its energy either by losing its only electron or by gaining one electron to complete its duplet.
$\ce{
\underset{\text{(Unstable)}}{H} -> \underset{\text{(Stable)}}{H^+} + e^-
}$
$\ce{
\underset{\text{(Unstable)}}{H} + {e^-} -> \underset{\text{(Stable)}}{H^-}
}$
Bonding Between Metals and Non-Metals:
All elements react based on this principle. For example, Alkali and alkaline earth metals (electropositive) form bonds with non-metals (electronegative) such as oxygen (O) and chlorine (Cl).
Exceptions to Duplet and Octet Rule:
Even though duplet and octet rule are useful for understanding the basics of chemical bonding, but it is not 100% accurate and have some exceptions.
Q2. Write an introductory note on chemical bonds.
A chemical bond is a force of attraction that holds two atoms in the form of a molecule. a compound or metal.
Formation of a Chemical Bond:
The formation of a chemical bond involves the following steps:
- Two atoms come closer.
- Attractive forces dominate between them.
- The energy of the system is lowered
- A chemical bond is formed between the atoms
Type of chemical bonds
- Ionic Bond / Electrovalent Bond
- Covalent Bond
- Coordinate Covalent Bond
- Metallic Bond