9th Class Chemistry Chapter 4 Structure of Molecules Short Questions Answer
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It is an universal rule that everything in this world tends to become more stable, Atoms achieve stability by attaining electronic configuration of inert gases (He, Ne or Ar etc.) having 2 or 8 electrons in the valence shell is sign of satiability. Attaining two electrons in the valence shell called duplet rule while attaining 8 electron in the valence shell is called octet rule.
Because ionic bond is formed due to transfer of electron from one atom to another atom is called ionic bond. The elements of Group 1 and 2 being metals have the tendency to lose their electrons forming positively charged ions. Whereas no-metals of Group 15 Group 17 have the tendency to gain or accept electrons. They are electronegative elements with high electron affinities.
Ionic compounds are made up of positively and negatively charged ions. Thus, they consist of ions and not molecules. These positively and negatively charged ions are held together in a solid or crystal form with strong electrostatic attractive forces. So, ionic compounds are solid in nature.
Yes, more electronegative elements can form bonds between themselves. Because they have comparable electronegativity and form covalent bond between similar atoms called does diatoms.
Example: Formation of covalent bonds between two atoms of oxygen and chlorine etc.
Metals are good conductors of electricity because metals have free electrons which are mobile in nature. Spaces are resent among the atoms. Mobile electron can move freely in these spaces. Hence, movement or flow of electron is known as electricity that’s why metals are good conductor of electricity.
Ionic compounds conduct electricity in solution or molten form because in these two states ionic compounds have free in them. When these free ions move in solution or molten state they become conductor of electricity.
In nitrogen molecule, three bond pairs are involved in bond formation. This type is called triple covalent bond. Three small lines are used to indicate these three pairs of electrons between those atoms in the molecules of such compounds.
The non-bonded electron pair available on an atom, like the one lone pair is available on nitrogen in ammonia is called a pair e.g, NH₃
The boded electron pair available present within the atoms is called a bond pair e.g. H ÷ Cl.
Two necessary conditions for the formation of covalent bond are:
The atoms must have comparable electronegativity values. It means difference of electronegativity between the atoms to be bonded must be small (less than 1.7).
Maximum 3 to 6 electron are present in their valence shells.
Example: HCI, CI₂, C₆H₆ and C₂H₂
HCI has dipole-dipole forces of attraction. The unequal sharing of slightly positive and other end slightly negatively charged. As shared pair of electron is drawn towards more electronegative atom, it is partially negatively charged, as chlorine in hydrogen chloride. The other end automatically becomes partially positively charged.
When each bonded atom contributes three electrons, three bond pairs are involved in bond formation. This type is called triple covalent bond. Three small lines are used to indicate these three pairs of electrons between those atoms in the molecules of such compounds.
Polar covalent bonds:
If the covalent bond is formed between two different types of atoms then bond pair of electrons will not be attracted equally y the bonded atoms. One of the atoms will attract the bond pair of electrons more strongly than the other one. They form polar covalent bond.
Example: HCI and H₂O
Non-polar covalent bonds:
If a covalent bond is formed between two similar atoms, the shared pair of electrons is attracted by both the atoms equally. Such type of covalent bond is called non-polar covalent bon.
Example: H₂ and CI₂
Ans: When there is a difference of electronegativity between two covalently bonded atoms, there will be unequal attraction for the bond pair of electrons between such atoms. It will result in the formation of polar covalent bond.
Example: HCI, H₂O
Ans: Electronegativity values of the atoms play a very important role in polarity of compounds. These two terms have direct relationship with one another, because of electronegativity difference is high then more polar bond will be and vice versa.
Floating of ice on water is because of hydrogen bonding. The density of ice at 0°C (0.917 g/cm³) is less than that of liquid water at 0°C (1.00 gcm⁻³). In the liquid state water molecules move randomly. However, water freezes, the molecules arrange themselves in an ordered form that gives them open structure. This process expands the molecules that results in ice being less dense as compared to water.
Ionic compound are mostly crystalline solids.
Ionic compounds in solid state have negligible electrical conductance but they are good conductors in solution and n the molten form. It is due to presence of free ions in them.
Ionic compounds have high melting and boiling points. For example NaCl has M.P 800°C and B.P. 1413°C.
Ionic compounds dissolves in polar solvents. Like NaCI dissolves in water.
They have usually low melting and boiling point.
They are usually bad conductors of electricity.
They are usually insoluble in water
Bigger molecules with three dimensional bonding form covalent crystals which are very stable and hard. They have high melting and boiling points.