JEE Main & Advanced Physics Semiconducting Devices Bonding Forces in Crystals

The properties of a solid are mainly determined by the type of bonding that exists between the atoms. According to bonding in crystals they are classified into following types.

(1) Ionic crystal : This type of bonding is formed due to transfer of electrons between atoms and consequent attraction between them.

(i) In NaCl crystal, the electron of Na atom is transferred to chlorine atom. In this way Na atom changes in to Na+ ion and Cl atom changes into \[C{{l}^{-}}\] ion.

(ii) Cause of binding is electrostatic force between positive and negative ion.

(iii) These crystal are usually hard, brittle and possesses high melting and boiling point.

(iv) These are bad conductor of electricity.

(v) Common example are NaCl, CsCl, LiF etc. 

(2) Covalent crystal : Covalent bonding is formed by sharing of electrons of opposite spins between two atoms

(i) The conductivity of these solids rise with rise in temperature.

(ii) These crystal posses high melting point.

(iii) Bonding between \[{{H}_{2}},\,C{{l}_{2}}\] molecules Ge, Si, Quartz, diamond etc. are common example of covalent bonding

(3) Metallic bonds : This type of bonding is formed due to attraction of valence (free) electrons with the positive ion cores

(i) Their conductivity decreases with rise of temperature.

(ii) When visible light falls on a metallic crystal, the electrons of atom absorb visible light, so they are opaque to visible light. However some orbital electrons absorb energy and reach in excited state. They then return to their normal states, remitting light of same frequency.

Common examples are Na, Li, K, Cs, Au, Hg  etc.

(4) Vander waal's crystal : These crystal consists of neutral atoms or molecules bonded together in solid phase by weak, short range attractive forces called vander Waal's forces.

(i) This bonding is weakest and occurs in solid \[C{{O}_{2}},\] methane, paraffin, ice, etc.

(ii) They are normally insulator, they are soft, easily compressible and posses low melting point.

(5) Hydrogen bonding : Hydrogen bonding is due to permanent dipole interaction.

(i) This bond is stronger than vander Waal's bond but much weaker than ionic and covalent bond.

(ii) They possesses low melting point.

(iii) Common examples are \[{{H}_{2}}O,\] HF etc.