Structure of Molecule

9th chemistry chapter 3 solved exercise short questions

Q#1 : Why do atoms react ?

Atoms react to attain stability .

Atoms attain stable electronic configuration by following ways :

• By losing electrons
• By gaining electrons
• By mutual sharing of electrons

Q#2 : Why is the bond between an electropositive and electronegative atom ionic in nature ?

Electropositive elements have the tendency to lose electrons i.e., Metals.

Electronegative elements have the tendency to accept electrons i.e., Non-metals.

Ionic Bond :

Type of chemical bond which is formed due to complete transfer of electron from one atom to other atom.

Therefore , electropositive atom give electrons and electronegative atom accept electrons to form ionic bond . i.e., NaCl , KBr

Q#3 : Ionic compounds are solids .Justify .

Ionic compounds are solids.

Reason :

• Due to strong electrostatic force of attraction between cation and anion .
• Ionic compounds are formed as a result of Ionic Bond . Ionic bond is strong bond because it consists of network of positive and negative ions held together by strong electrostatic force of attraction.

For Example : salt at home

Q#4 : More electronegative elements can form bond between themselves . Justify .

More electronegative elements have very small electronegativity difference . Therefore , they cannot lose or gain electrons.

Instead these share their electrons and form covalent bonds.

Examples :

Both Chlorine and Oxygen are more electronegative . They do not want to lose electrons . so , they share electrons .

Q#5 : Metals are good conductors of electricity . Why ?

Metals are good conductors of electricity .

Reason :

Because of the presence of free electrons / mobile electrons . Free electrons carry potential from one place to other place .

Q#6 : Ionic compounds conduct electricity in solution or molten form . Why ?

For conduction of electricity , Ions should be able to move freely .

→ In solid form , the ions are held by strong electrostatic forces . Therefore cannot conduct electricity .

Example : salt (solid) does not allow electricity to pass .

→ In liquid / molten state or in aqueous solution form , ions are able to move and act as carrier of electricity. Hence , conduct electricity.

Example : ( salt + water ) . This solution will conduct electricity.

Q#7 : What type of covalent bond is formed in nitrogen molecule .

Nitrogen forms triple covalent bond . Because each Nitrogen atom can share its three electrons with other Nitrogen atom .

Q#8 : Differentiate between lone-pair and bond-pair of electrons .

Q#9 : Describe at least two necessary conditions for the formation of a covalent bond .

Two conditions for the formation of covalent bond are :

1. The atoms must have comparable electro-negativity values . It means difference of electronegativity between the atoms to be bonded must be small . (less than 1.7 ) .
2. The valence shell should be partially filled .

Q#10 : Why HCl has dipole-dipole forces of attraction ?

HCl has diople-dipole forces .

Reason :

( Diople-Dipole forces are present in polar substances.)

In HCl molecule , Chlorine is more electronegative than Hydrogen.

Therefore , shared electron pair is closer to chlorine creating partial negative (δ–) and partial positive (δ+) charge on Hydrogen . In this way , molecule becomes polar .

→ Partial positive end of one molecule begin to internet with molecule begin to interact with partial negative end of other molecule .

→ Thus , dipole-dipole interactions are developed in H-Cl.

Example :

Q#11 : What is a triple covalent bond . Explain with an example ?

Triple covalent bond :

” The bond in which each atom contributes three electrons in bond formation is called triple covalent bond .”

→ It is represented by triple lines between two atoms (≡).

Examples :

Formation of N2 and C2H4 (Ethylene/ Acetylene ) molecules.

Q#12 : What is difference between polar and non-polar covalent bonds. Explain with one example of each ?

Q#13 : Why does a covalent bond becomes polar ?

Covalent bond becomes polar .

Reason :

When a covalent bond exists between two unlike atoms , the shared electron pair will be close to the atom whose electronegativity is greater . This atom will get partial negative charge (δ–) and other bonded atom will get partial positive charge (δ+) by same amount .

In this way covalent bond becomes polar .

For Example :

In Hydrogen Chloride , polar covalent bond is present .

Q#14 : What is the relationships between electronegativity and polarity ?

• Difference of electronegativity between bonded atoms is directly related to polarity of a molecule .
• Greater the EN difference between bonded atoms , Greater will be the polarity on Molecule .

For Example :

H-F is more polar than H-Cl , because EN difference between hydrogen and Florine is greater in HF as compared to EN difference between Hydrogen and Chlorine in HCl .

Q#15 : Why does ice float on water ?

Ice floats on water .

Reason :

Because the density of ice is less than liquid water .

• Density of ice is 0.917 g/cm³at 0°C.
• Density of water is 1 g/cm³ at 0°C.

Explanation :

Ice floats on the surface of water due to Hydrogen bonding . When water freezes , the molecules arranged themselves regular geometrical form , so that large empty spaces are developed in ice .Due to these empty spaces among water molecules in frozen form density of ice becomes less than liquid water .

Hence , ice floats upon surface of water due to less density .

Q#16 : Give the characteristic properties of ionic compounds .

Characteristics :

1. They are crystalline solids .
2. They are non-conductor in solids form but good conductor in molten form or aqueous solution form , due to presence of ions .
3. Their melting points and boiling points are very high . For example : Melting point of NaCl is 800°C and boiling point is 1413°C.
4. These are soluble in polar solvents like water .

Q#17 :What characteristics properties do the covalent compounds have ?

Characteristics :

1. They have low melting and boiling points .
2. They are usually bad conductors of electricity . The compounds having polar character in their bonding are conductors of electricity when they dissolve in polar solvents .
3. They are usually insoluble in water but are soluble in non-aqueous solvents like benzene , ether , alcohol and acetone .
4. Large molecules with three-dimensional bonding form covalent crystals which are very stable and hard . They have high melting and boiling points . i.e., Diamond and Graphite .

9th Chemistry , Chapter 4, Structure of Molecules, Solved exercise short questions , free notes