Chapter 4 Structure of Molecules Chemistry Class 9 Notes

Chapter 4 Structure of Molecules Chemistry Class 9 solved exercise, shrot question, long question, and numerical.

Structure of Molecules Chemistry Class 9

What is an electron-sea model of metallic bonding?

In the metal lattice, metal atoms lose their outer electrons and become positively charged ions. The outer electrons of the atom no longer belong to any metal atom and are said to be delocalized, which means that they do not belong to any one atom but move freely between the metal ions like a cloud of negative charge. These mobile electrons form a sea of electrons around the metal atoms, which are held together in a metal lattice.

Why most atoms are chemically bonded to other atoms in nature?

Every system in the universe tends to lower its energy, in order to attain stability. Atoms, in the same way, have a tendency to decrease their energy. The energy of the isolated hydrogen atoms is higher than the two bonded hydrogen atoms. That is, the combination of atoms gives stable molecules through the emission of energy. They can decrease their energy by combining with other atoms and form a chemical bond.

Q.3) Identify and define the four major types of chemcial bonding.

Types of chemical bonding
i) Ionic bond
The chemical bond which is formed due to the complete transfer of electron(s) from one atom to the other atom is called ionic bond.
ii) Covalent bond
The bond which is formed by the mutual sharing of electrons is called covalent bond.
iii) Co-ordinate covalent bond
The covalent bond in which only one atom donates the shared pair of electrons is called co-ordinate covalent bond.
iv) Metallic bond
The chemical bonding that result from the attraction between metal positive ions and the surrounding sea of electrons is called metallic bonding.

Q.4) Arrange the following from strongest to weakest attraction:

a) Covalent bond
b) Dipole-dipole interaction
c) Ionic bond

Ionic bond > Covalent bond > Dipole-dipole interaction

Read more: Structure of Atoms Cha 2 Chemistry 9 class notes

Q.5) Why ionic compounds are good electrolyte in molten and solution form not in solid state.

The molten and solution form of ionic compounds have charged particles free to move. Electricity is conducted by these charged particles. The negatively charged delocalized electrons act as the charge carriers, allowing the molten or solution form of ionic compounds to conduct electricity.
In solid state of ionic compounds,  the ions are tightly packed with each other such that the movement of the ions is restricted and they cannot move freely. So these ions cannot conduct electric current. 

Read more: Class 9 Chemistry Cha 2 Periodic Table and Periodicity of Properties

Q.6) What types of elements/atoms tend to form the following types of bonding?

a) ionic        
b) covalent        
c) metallic

a) ionic
Ionic bonding occurs between metal and non-metal. For example, sodium (metal) forms bond with chlorine (non-metal) to make NaCl.
b) covalent
Covalent bonds occur between non-metals. For example, hydrogen and chlorine share a pair of electrons to form HCl.
c) metallic
Metallic bonds form between metal atoms

Q.7) Give an example of a non-polar molecule with polar bonds. Give reasons.

BF3 is non-polar molecule with polar bonds. The three bonds in BF3 are polar, which are evenly distributed around the central atom B. None of the sides of the molecules has more positive or negative charge than any other side. The bond dipoles cancel and do not create a molecular dipole. So BF3 is polar.

Q.8) Predict the bond type (ionic, polar  covalent, non-polar covalent) in each of the following:

a) CaCl2
b) H2O

a) CaCl2
Ionic bond
b) H2O
Polar covalent bond
c) CO2
Non Polar covalent bond
d) C2H4
Non Polar covalent bond

Q.9) Why ionic compounds are good conductor compared to covalent compounds?

Ionic compounds contain oppositely charged ions. Ionic compounds dissolved in water readily form ions that are free to move. These dissociated ions can carry charge through the solution. Therefore, they conduct electricity.
Covalent compounds do not ionize in solution or in molten form. Therefore, they do not conduct electricity.

Q.10) Give reasons that why dative bond is always polar?

A coordinate covalent bond or dative covalent bond is always polar because it is always formed between unlike atoms. The atom which shares the pair of electron is donor and usually carries lone pair of electron thus acting electronegative specie. While the atom which accepts electron is electron deficient and acts as acceptor.  This electronegativity difference creates polarity in the bond.

Long Questions Chemistry 9 Notes chapter 4

Q.1) Describe the octet rule in terms of noble gas configurations and stability.

Octet rule
The octet rule states that atoms gain or lose electrons to attain a maximum of eight electrons in their valence shell giving them the same electronic configuration as a noble gas. They do this by bonding with other atoms.
These atoms can be of the same element, as when oxygen bonds with itself to form O2:

or of different elements, such as water (H2O).

The exceptions to the octet rule are hydrogen and helium, which have two electrons in their outermost shells.
All atoms, except for the noble gases, are unstable because their outermost shell is not full. They achieve stability by attaining electronic configuration of inert gases. Atoms react with other to gain or lose or to share electrons to fill their outermost shell.

Q.2 a) What is the main distinction between ionic and covalent bonding? Explain your answer with suitable examples.


Ionic bonding

An ionic bond is formed between a metal and a non-metal.
It is formed by the transfer and acceptance of electrons between atoms.
Ionic bonding is relatively stronger.
They have high melting and boiling points.
Methane (CH4), hydrochloric acid (HCl)

Covalent bonding

A covalent bond is formed between two non-metals.
It is formed by the sharing of electrons between atoms.
Covalent bonding is relatively weaker.
They have a quite low melting and boiling points.
Sodium chloride (NaCl), sulphric acid (H2SO4)

Q.2 b) How is electronegativity used in determining the ionic or covalent character of the bonding between two elements?

Sharing of electrons in the formation of covalent bonds rather than complete transfer from one atom to the other as in the formation of ionic bonds depends upon the differences in electronegativity of the bonding atoms. If this difference is more than 1.7, electron transfer will occur and an ionic bond will form. If the electronegativity difference is less than 1.7, electrons will be shared and a covalent bond will form.

Q.4) Explain why most metals are malleable and ductile but ionic crystals are not.

Metals are malleable and ductile because the metal ions can easily slide past one another. Because of this metals can be beaten into sheets and foils and can be drawn into wires without breaking the metallic bonds.
In ionic crystals, electrons are tightly held by the ions, and are not free to slide over each other. Ionic compound must break bond to slide past one another, which causes ionic material to split and crack. Therefore, they are hard and are not malleable or ductile.

Q.5 a) What is the meaning of the term polar, as applied to chemical bonding?

When two atoms of different elements share electrons to form a covalent bond, the shared electrons are not attracted evenly by the two nuclei of bonding atoms. If the bonded atoms have more electronegativity difference, the shared pair of electrons is attracted strongly towards the more electronegative atom than the less electronegative one. Thus, there will be unequal attraction for the bonded pair. As a result opposite poles develop creating polarity in the bond and is called polar covalent bond.

In the hydrogen fluoride (HF) molecule, fluorine is more electronegative and attracts the shared pair of electrons towards itself strongly. Therefore, F atom acquires a partial negative charge, and H atom a partial positive charge. It creates polarity in the bond.

Q.5 b) Distinguish between polar-covalent and non polar covalent bonds.


Polar covalent bondNon-polar covalent bond
i). The covalent bond which is formed by the mutual sharing of electrons between atoms, having different electronegativities is called polar covalent bond.The covalent bond which is formed by the mutual sharing of electrons between atoms, having similar electronegativities is called non-polar covalent bond.
ii). The shared pair of electrons is attracted more towards the more electronegative atom.The shared pair of electrons is attracted towards both atoms equally.
iii). Opposite poles are created i.e. partial positive and partial negative.No opposite poles are created.
iv). For example, in HCl, the electronegativity of Cl is greater than that of hydrogen atom. Therefore, the shared pair of electrons is attracted more towards Cl than the hydrogen atom.For example, in H2, both the atoms attract the shared pair of electrons equally.

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