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.
- 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.
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 .
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 :
- 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 ) .
- The valence shell should be partially filled .
Q#10 : Why HCl has dipole-dipole forces of attraction ?
HCl has diople-dipole forces .
( 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.
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 (≡).
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 .
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 .
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.
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 .
- They are crystalline solids .
- They are non-conductor in solids form but good conductor in molten form or aqueous solution form , due to presence of ions .
- Their melting points and boiling points are very high . For example : Melting point of NaCl is 800°C and boiling point is 1413°C.
- These are soluble in polar solvents like water .
Q#17 :What characteristics properties do the covalent compounds have ?
- They have low melting and boiling points .
- 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 .
- They are usually insoluble in water but are soluble in non-aqueous solvents like benzene , ether , alcohol and acetone .
- 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