Chemistry Class 9 Chapter 4 Important Question Answers 2023
9th Chemistry
Chapter 4 Structure of Molecules
Q.1
Why do atoms react?
Answer
Atoms react with each other to attain stability. All the atoms except those of noble gases are unstable because their valence shell is incomplete. So, atoms react with each other by loss, gain, or sharing of electrons and attain stability by completing their valence shell.
For example
Fluorine has 7 electrons in its valence shell (L – Shell), but L-shell can accommodate 8 electrons so it means fluorine’s valence shell is incomplete and it is unstable. So fluorine will complete its valence shell by gaining or sharing an electron
Similarly, Li has one electron in its valence shell and it is much easier for Li to give up that one electron than gain 7 electrons to get a complete valence shell. So Li will give up that one electron of valence shell and complete its valence shell.
Q.2
Write a brief note on the general outer electronic configuration of inert gases.
Answer
Chemical inertness of noble gases
Group VIII A members are called noble or inert gases.
Following are the elements of group VIII A
He2, Ne10, Ar18, Kr36, Xe54 and Rn86.
The general outer electronic configuration of noble gases is ns2, np6.
- He has two electrons in its valence shell . i .e it follows the duplet rule.
- The remaining elements in this group have eight electrons in their valence shell i.e following the octet rule.
It means that all noble gases have their valence shell complete.
Q.3
What is the reason behind the stability of elements of group VIII A?
Answer
Group VIII A elements (Noble gases) are usually not involved in reactions this is because their valence shell complete and they are stable. He has two electrons in its valence shell and it follows the duplet rule while the remaining elements of this group have 8 electrons in their valence shell and they follow the octet rule. The chemical reactivity of an element depends upon the number of unpaired electrons while in noble gases there are no unpaired electrons in their valence shell hence they are stable and do not take part in any reaction.
Electronic configuration of a few noble gases are
He = 1s2
Ne = 1s2,2s2,2p6
Ar = 1s2,2s2,2p6, 3s2, 3p6
Q.4
What do you mean by the duplet rule?
Answer
Duplet rule
The duplet rule indicates an atom’s tendency to acquire a two-electron configuration in its valence shell when it bonds. Helium has two electrons in its valence shell and is chemically inert, some elements that are near to He in the periodic table tend to obtain two electron configuration in their outer-most shell
Explanation
Let us consider the following example for an explanation.
Hydrogen has 1 electron in its valence shell and it needs 1 more electron to complete its valence shell so it will obtain that electron by forming a bond with another atom. In this way, its valence shell will have 2 electrons which means its valence shell will be filled and stable and it follows the duplet rule.
Q.5
What is an example of the octet rule
Answer
Octet rule
The octet rule describes an atom’s tendency to acquire eight electrons in its valence shell. Atoms with less than eight electrons are more likely to interact with one another and create more stable compounds.
For Example
Oxygen has 6 electrons in its valence shell and its valence shell can accommodate 8 electrons which means the valence shell of oxygen is incomplete. So Oxygen will take part in a chemical reaction and complete its valence shell by either sharing or gaining electrons.
H2 + O → H2O
In the above example oxygen shares one electron with each hydrogen atom and completes its valence shell by having 8 electrons in it. It shows that Oxygen follows the octet rule.
Q.6
What is a chemical bond?
Answer
Chemical bond
The attractive force that holds the atoms together in a molecule or compound is called a chemical bond.
OR
It is the force that holds ions or molecules together.
Q.7
Enlist the names of types of chemical bondings.
Answer
Following are the three primary types of bonding,
- Ionic Bonding
- Covalent Bonding
- Metallic bonding
Q.8
Discuss the formation of ionic bonds with examples.
Answer
Ionic bond
The bond which is formed by the complete transfer of electrons is called an ionic bond.
Formation of ionic bond
An ionic bond is created between the two atoms when one atom loses an electron to form a cation and the other atom acquires an electron to form an anion.
Explanation
The charges of anions and cations are opposite. Electrostatic forces are what pull them toward one another. Ionic bonds are the forces of attraction that hold together ions with different charges. Compounds made up of ions held together by electrostatic forces are referred to as known as ionic compounds. The cations’ combined positive charge has to match the entire anions’ negative charge. The reason for this is that ionic molecules are generally electrically neutral.
Example of formation ionic bond
- Formation of sodium chloride (NaCl)
The electronic configuration of sodium is 2,8,1 and that of chlorine is 2,8,7. The electronegativity difference between Na and Cl is 2.1.
When Na and Cl react with each other sodium gives away an electron to chlorine.
This process is explained in the following reactions:
Na 🡪 Na+ + e–
Cl + e– 🡪 Cl–
In this way, both the ions attain noble gas electronic configuration. The ions, Na+ and Cl– are held together by the electrostatic force of attraction.
Energy is emitted during the formation of NaCl.
Examples of ionic bond
MgO,NaCl,CaCl,CaF2 etc
Properties of ionic bond
1. Atoms that obtain an electron become negatively charged ions known as an anion, while atoms that lose an electron become positively charged ions known as cations.
2. The electrostatic force of attraction holds these two oppositely charged ions together, forming an ionic bond.
3. Ionic compounds are those in which ions are held together by ionic bonds.
4. A low ionization energy element and an element with a high affinity for electrons form an ionic bond.
5. Group I A and II A elements form ionic bonds with group VI A and VII A elements.
Q.9
Discuss the factors favoring the formation of ionic bond.
Answer
Factors favoring the formation of ionic bond
- Low ionization energy
Ionization energy, the more likely the creation of a cation from an atom.
e.g. Alkali metals have low ionization energies so they can easily lose electros/s to form a cation, typically forming ionic compounds.
- High electron affinity
The formation of anions is favored when the electron affinity is high. Due to their high electron affinities, halogens typically form ionic compounds.
Q.10
What is covalent bond and its types? Give an example of each kind.
Answer
The bond which is formed by the mutual sharing of electrons is called a covalent bond.
Example
Consider the formation of covalent bond in the hydrogen molecule.
A hydrogen atom has one electron in its valence shell. So, two hydrogen atoms share their electrons and form a diatomic molecule.
H. + H . → H:H
In this way, each hydrogen atom achieves the electronic configuration of the noble gas, Helium which has two valence electrons.
A shared pair of electrons is also represented by a dash (-) in a molecule.
Types of covalent bond
Single covalent bond
Single covalent bonds are covalent bonds formed when one electron pair is shared. Molecules of H2 and F2 have a single covalent bond.
Explanation
Let us consider the following example for explanation,
Take into account the joining of two fluorine atoms to form a bond. Since fluorine is a member of Group VII A, its valence shell contains seven electrons. To achieve the electron configuration of a noble gas, it needs one additional electron. By sharing an electron pair, two F-atoms achieve the Ne electron configuration. To satisfy the octet rule, each F-atom shares one of its electrons.
F + F → F2
Lone pair
Lone pairs are valence electron pairs that are not shared by two atoms.
Double covalent bond
Double covalent bonds are formed by the sharing of two electron pairs.
Explanation
Let us consider the following example for explanation,
Think about how the O2 molecule is formed. Since oxygen belongs to Group VI A, its valence shell contains six electrons. To satisfy the octet rule, it needs two electrons. Therefore, each O-atom contributes two electrons to the sharing, and as a whole two electron pairs are shared.
Other examples are: O2, C2H4, CO2
Triple covalent bond
Triple covalent bonds are bonds that involve the sharing of three electron pairs.
Explanation
Let us consider the following example for explanation,
Consider the formation of the N2 molecule. Since Nitrogen belongs to Group V A, its valence shell contains five electrons. To satisfy the octet rule, it needs three electrons. Therefore, each N-atom contributes three electrons to the sharing and as a whole three electron pairs are shared.
Other examples are: C2H2, C2N2
Q.11
Mention the differences between ionic and covalent bond.
Answer
| Electrovalent bond(ionic bond) |
Covalent bond |
|
It is formed by the complete transfer of electrons from one atom to another. |
It is formed as a result of sharing of electrons between two atoms. |
|
It is indicated by positive and negative charges. |
It is represented by a small straight line. |
|
These are non-directional. |
These are directional |
|
An ionic bond is formed between atoms having electronegativity difference greater than 1.7 |
A covalent bond is formed between atoms having an electronegativity difference less than 1.7 |
|
EXAMPLES |
EXAMPLES |
|
NaCl , CaCl2, K2O |
H2, Cl2 , O2 |
Q.12
What is intermolecular forces and its types?
Answer
Inter-molecular forces
The attractive forces among the molecules are called inter-molecular forces.
Types of intermolecular forces
There are four types of intermolecular forces and they are collectively called van der Waal’s forces.
1. Dipole – dipole interaction 2. Ion-dipole forces
3. London dispersion forces 4. Hydrogen forces
Q.13
What is non-polar covalent bond with example?
Answer
Non polar covalent bond
When two atoms share electrons evenly, a type of chemical bond known as a non-polar covalent bond is formed. The number of electrons shared by the involved atoms in the compound will therefore be equal. Because of the relatively minor difference in electronegativity, the covalent bond is also referred to as nonpolar.
Examples
H2, F2, Cl2
Q.14
Why is it called a polar covalent bond?
Answer
Polar covalent bond
A polar covalent bond is one in which the electron pair is unequally distributed between the two connected atoms. Polar covalent bonds are caused by variations in electronegativity.
Explanation
When two different atoms share an electron pair, the two atoms’ force on the shared electron pair is different.
Greater force is applied to the pull of the shared electron pair by the more electronegative atom. Therefore, an atom with greater electronegativity partially attracts electron density to it. It now has a partial negative charge, whereas another atom now has a partial positive charge. The term “polar covalent bond” refers to such a covalent bond.
Q.15
How does dipole-dipole interaction form?
Answer
Dipole – dipole interaction
Dipole-dipole interactions are attractive forces between polar molecules. Due to differences in the electronegativities of the atoms involved in a covalent bond, polar molecules contain permanent dipoles.
OR
A polar molecule’s slightly negative end is weakly drawn to another molecule’s slightly positive end. Dipole-dipole interactions are the name for these attracting forces.
Explanation
Let us consider the following example for an explanation
In HCl molecules, dipole-dipole interactions take place. Chlorine obtains a partial negative charge because it is relatively more electronegative than hydrogen (whereas hydrogen acquires a partial positive charge). The dipole-dipole interaction between the HCl molecules then occurs as a result of these appearing charges. These linkages are weak and temporary.
Q.16
Define hydrogen bond with example?
Answer
Hydrogen bonding
The interaction of electron-deficient hydrogen and a lone pair on a nearby highly electronegative atom such as N, O, or F is known as a hydrogen bond.
Explanation
In molecules where hydrogen is covalently bonded to the element that is extremely electronegative, such as oxygen, nitrogen, or fluorine, and is also weakly bonded to a lone pair of electrons of another electronegative atom. This additional atom might be found in the same molecule or one that is close by. The term “hydrogen bonding” refers to this intermolecular Force. Hydrogen is extremely electron-deficient when combined with oxygen, nitrogen, or fluorine. As a result, the interaction between such a severely electron-deficient hydrogen and a lone pair on a nearby electronegative atom compensates for the deficiency. This phenomenon is called hydrogen bonding.
Examples of compounds showing hydrogen – bonding
- H2O ( Water molecule )
- HF ( Hydro-fluoric acid)
Q.17
Discuss the nature of bonding and properties of NaCl and CsCl crystals.
Answer
Nature of bonding and properties of NaCl and CsCl crystals
Compounds that consist of ions held by electrostatic forces are called ionic compounds. At room temperature majority of the ionic compounds are crystalline solids.
Notably, both NaCl and CsCl produce colorless cubic crystals. Six Cl ions circle each Na+ ion, while six + Na ions surround each Cl ion in NaCl. Compared to NaCl, CsCl has a distinct internal structure. In CsCl, each + Cs ion has eight Cl ions around it, and each Cl ion has eight + Cs ions surrounding it. Each ion is hence strongly attracted to each of its neighbors in crystals. A very stable structure is produced by strong attracting forces. This is the reason why ionic compounds have a high melting point. NaCl, for example, has a melting point of 801˚C.
Properties of melted ionic compounds
When NaCl melts, the Na+ and Cl ions are free to move throughout the molten salt. Melted ionic compounds conduct electricity. The + Na ions travel toward the negative electrode when a voltage is applied. Cl ions move in the direction of the positive electrode at the same time. This ion mobility within a cell is what causes the electricity to travel between the electrodes on the external wire.
Properties of aqueous solutions of ionic compounds
Aqueous solutions of ionic compounds also conduct electricity. This is because the ions can freely move around in the aqueous solution when an ionic molecule dissolves in water.
Examples
Example 4.1: Determine the number of valence electrons in an atom using the periodic table.
Determine the number of valence electrons in the given atoms using the periodic table.
(a) Carbon (b) Magnesium (c) Phosphorus
Problem-Solving Strategy:
Remember that the group number of main group elements indicates the number of valence electrons in an atom. Check the group of the elements in the periodic table and Determine the number of valence electrons.
Solution:
(a) Carbon is a part of Group IVA, it has four electrons in the valence shell.
(b) Magnesium is a part of Group IIA, so it has two electrons in the valence shell.
(c) Phosphorus is present in Group VA, so it has five electrons in the valence shell
Example 4.2: Describing the formation of cations.
Describe the formation of Na+ and Mg+2 cations.
Problem-Solving Strategy:
1. Sodium is a part of Group IA on the periodic table. It contains only one electron in the outermost shell. Sodium loses its valence shell electron and is left with an octet. Represent this by writing the complete electronic configuration.
2. Magnesium is a part of Group IIA in the periodic table. It contains two valence electrons.
Magnesium atom loses these electrons to attain noble gas configuration. Represent this by writing the complete electronic configuration.
This number also corresponds to the Group number in the periodic table.
Solution:
- Formation of Na+ ion
Na 1s2, 2s2, 2p6, 3s1 e– Na+ 1s2,2s2,2p6
- Formation of Mg+2 ion
Mg 1s2,2s2,2p6,3s2 e– Mg+ 1s2,2s2,2p6
Example 4.3: Describing the formation of anions
Explain the making of anions for the following non-metal atoms:
(a) O (atomic no.8) (b) F (atomic no. 9)
Problem-Solving Strategy:
1. Write electronic configuration or dot structure.
2. Determine the number of electrons needed to acquire eight electron configuration.
3. Represent the addition of electrons.
Solution:
(a) Formation of anion by an oxygen atom.
Oxygen belongs to Group VIA on the periodic table. So it has six electrons in its
valence shell. It needs two electrons to obtain an inert gas configuration.
O 1s2,2s2,2p4 + 2e– O-2 1s2,2s2,2p6
(b) Making of anion by a fluorine atom
Fluorine is a part of Group VIIA on the periodic table. So it contains seven electrons in the valence shell. A fluorine atom, therefore, needs only one electron to satisfy the octet rule.
F 1s2,2s2,2p5 + e– F– 1s2,2s2,2p6
Example 4.4: Recognizing a compound as having ionic bonds.
Identify the given compounds as having ionic bonds.
(a) MgO (b) NaF
Problem-Solving Strategy:
1. The metal atom releases electrons to form cations and the non-metal atom receives electrons to form anions.
2. The number of electrons lost by metallic atoms of group IA, IIA, and IIIA levels the group number. The amount of electrons gained by the non-metallic atoms is equal to 8 minus the group number.
3. Find the simplest ratio of cations to anions, to identify the compound.
Solution:
(a) MgO
Mg is metal and O is non-metal. Mg atom consists of two electrons in the outermost shell. It loses two electrons and the Mg+2 ion is formed. Since the O atom contains 6 electrons in the valence shell, so it gains two more electrons to form an O-2 ion. In the following way, both atoms attain the nearest noble gas configuration. For every Mg+2 ion, you need one O-2 ion. The chemical formula of the resulting compound is MgO. Therefore MgO is an ionic compound.
(b) NaF
Na is metal and F is non-metal. Na atom has one electron in valence or shell. So it loses
one electron to form a Na+ ion. Since the F atom contains seven electrons in the valence shell, it gains one electron to form an F– ion. The sodium atom by losing one electron and the F atom by gaining one electron acquire the nearest noble gas electronic configuration. You need one F– ion for each Na+ ion. Therefore, NaF is an ionic compound.
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