MD. SHAKIL SARKER

  Hydrogen bond  definition of hydrogen bond: the attractive electrostatic force between a hydrogen atom which is already covalently attached with a strongly electronegative atom of a molecule and another electronegative atom of some other molecule is known as hydrogen bond. Properties of hydrogen bond   a hydrogen bond is a bond of hydrogen between two electronegative atoms only. bond energy of a hydrogen bond is in the range of 3 to 10 kilo calorie per mole. the formation of a hydrogen bond does not involved in sharing of electron pairs only oxygen, nitrogen and fluorine are capable of forming hydrogen bond. like a covalent bond hydrogen bond has a preferred bonding direction.  Significance of hydrogen bonding physical state of water: without hydrogen bonding, water would have existed as a gas .structure of compounds: hydrogen bond is is directional and on this account it helps in studying and establishing the structure of many compounds like ice, solids acid etc ....

Properties of Elements in Periods and Groups 1.Valence electrons: In a period, the number of valence electron increase from 1 to 8 from the left to the right. In a group, the number of valence electrons is the same for all the elements. 2.Valency: In a period, the valency of the element increases from 1 to 4 and then decreases from 4 to 0 from the left to the right. In a group, the valency is the same of all elements. 3.Atomic size: In a period, the atomic size of the element decreases from the left to the right. In a group, the atomic size of the element increases from top to bottom. 4.Metallic property: In a period, the metallic property of element decreases from the left to the right. In a group, the metallic property of the elements  increases from the top to the bottom. 5.Non-metallic property: In a period, the non metallic property of the element increases from the left to the right. In a group, the non metallic property of the elements decreases from the top to the bottom....