chemistry

 CHEMICAL BOND IMPORTANT POINTS

 Atoms combine to form molecules.
 Formation of chemical bond involves redistribution of electrons.
 Ionic Bond: "The chemical bond formed by the complete transfer of one or more
electrons from one atom to other".
Eg: NaCl, MgO

Covalent Bond: "The bond formed by the sharing of electrons between two atoms".
Eg: H2, Cl2, F2, H2O
Octet configuration: "The filling up of valence orbital with 8 electrons".
As the atoms approach each other, the attractive forces between electrons of one atom
and the nucleus of other atom increases, results in the decrease of combined potential
energy of combined atoms.
At the critical internuclear distance, the energy of the molecule (or) combined atoms is
less than the sum of the energies of individual atoms and thus a chemical bond is
formed.
Potential- Energy Level Diagram: Zero group elements have stable electronic configuration having 2 (or) 8 electrons.
A covalent bond is formed by the overlap of two pure atomic orbitals (or) two hybridised
orbitals (or) one hybridized and one atomic orbital.
Strong bond is formed by the maximum overlap of orbitals.
 Sigma-bond: End-on-end overlap of orbitals lead to the formation of sigma bond. It is
formed by the overlap of s-s orbitals, p-p orbitals and s-p orbital overlap.
Pi-bond: Partial overlap of orbitals lead to the formation of pi (ð) bond.
 Sigma bond is stronger than pi-bond, sigma bond exists independently.
Eg: H2, HCl, Cl2, CH4
ð-bond has no independent existence. It exists only after the formation of ó bond.
In a double bond, one sigma and one ð bond are present.
Eg: C2H4, O2, CO2, etc.,
 In a triple bond, one sigma and two pi-bonds are present.
Eg: N2, C2H2, HCN etc.,
 In a co-ordinate covalent bond, both the electrons are supplied by one atom and shared
between two atoms.
The primary requirement for the formation of a covalent bond is that one atom shouldhave a lone pair of electrons (with octet configuration) to donate and another atom
should have an empty orbital to receive it.
Eg:Molecules having linear structure are
Eg: CO2, BeCl2, C2H2, HCN etc.,
 Molecules having pyramidal structure are
Eg: NH3, PCl3, PH3, ....
Molecules having double bond are
Eg: O2, CO2, C2H4, ....
 Molecules having triple bond are
Eg: N2, HCN, C2H2...
Water molecule have "V" - shape.
 PCl5 has trigonal bi-pyramidal shape.
   Periodic Classification of ElementsImportant Points:The first classification of elements is due to dobereiner in 1817.
 Dobereiner Triad Theory: "The atomic weight of the middle element is the
arithmetic mean of first and third elements (or) the atomic weights of all the three
elements are approximately the same"
Eg: i) Li, Na, K ii) Fe, Co, Ni iii) Cl, Br, I.
Newland's Concept of Octaves: Every eighth element is having the similar
properties to that of the first element like in octave of music.
 Mendeleef, Lother Meyer used atomic property. Atomic weight for classification
elements.
 Mendeleef's Periodic Law: The properties of the elements are the periodic
functions of their atomic weights".
 Modern Periodic Law: The properties of the elements are the periodic functions of
their atomic number (or) electronic configuration.
 Modern Periodic Table is called long form of periodic table.
Long form of periodic table is divided into seven horizontal rows called "Periods" and
eighteen vertical columns called "Groups"
 The first period has only two elements.
The second and third period have 8 elements each.
 Fourth, Fifth and sixth periods have 18 elements each.
 Lanthanides and actinides are placed at the bottom of the periodic table.
 Seventh period is incomplete.
 Based on electronic configuration, the elements are classified into 4 types. They are
inert gases, representative elements, transition elements representative elements
and inner transition elements.
 Atomic radius: It is the distance between the centre of the nucleus and outermost
orbital.
 It is expressed in units of Angstroms.
 Ionization energy: It is the energy required to remove an electron from the
outermost orbital in the gaseous state.
 IE is expressed in ev or KCal.mol–1 or K.J.mol–1.
 Electronegativity: The ability of the bonded atom to attract the electron density of
the shared electrons.
 Electronegativity is expressed ... pauling scale.
 In a period, Atomic radius decreases and ionisation energy and electronegativity
increases.
In a group, atomic radius increases and IE and EN value decreases.