Monday, March 4, 2013

Download HSEB Notes of Chemistry | Atomic combinations, Molecular structure, Chemical Bonding | Extra Refrence | Class 11 | E-Book

CLASS : 11
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What is a molecule?
All matter consists of atoms, the most basic element of life. When two or more atoms combine or bond they form a molecule. These molecules are neutral and behave as a unit in a chemical reaction. e.g. two hydrogen atoms and one oxygen atom can combine to form one water molecule: H + H + O = H2O Most substances are composed of molecules. However, they can also be made up of atoms or ions. The noble gases (group VIII of the periodic table) are very + stable and therefore prefer not to form bonds. So they are made up of a single atom and are sometimes called monatomic molecules. Most other gases, e.g. O2  and Cl2, are diatomic molecules. Neutral salts like NaCl consist of ions and don't have any formal bonds to make up a molecule. The ions make up a so-called giant molecule.

Van Der Waals forces
There are three types of intermolecular forces which are classified as Van Der Waals forces. These are iondipole, dipole-dipole and London forces. Ion-dipole forces: These occur when an ionic substance dissolves in a polar liquid, e.g. KCl in H2O. The positive and negative poles of the polar liquid attract the negative and positive ions of the ionic substance, respectively. Dipole-dipole forces: These forces occur when polar molecules arrange them selves such that their oppositely charged ends can attract each other, e.g. 
H 2O and NaOH. 

London forces
These occur between nonpolar molecules. The molecules are neutral but when two approach one another, the protons and electrons from diferent particles attract one another slightly. The electron clouds of these molecules then become distorted and result in a weak dipole. This is the weakest of the Van Der Waals forces and so all such substances have low melting points, e.g. CH4 and CCl4

Bonding and energy
The potential energy of two atoms is zero when they are far apart since there is no attraction or repulsion between them. As the atoms approach each other, the negatively charged electron clouds and the positively charged nuclei repel each other. But the electron cloud and the nucleus of diferent atoms can also attract each other. The atoms move closer together until the lowest possible energy state is reached. Bonding energy has a negative value since energy is required to break a bond. 

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