CHEMICAL BONDING AND MOLECULAR STRUCTURE 2 Flashcards
how is variable valency shown by few elements
Variable valencies are shown by those elements which have empty
orbitals in outermost shell.
* Lone pair electrons get excited in the subshell of the same
shell to form the maximum number of unpaired electrons.
Maximum covalency is shown in excited state.
*3, 4, 5, 6 period elements due to the presence of vacant d-orbital
in the presence of highly electro negative elements such as (F, O, Cl)
can show variable valencies.
state promotion rule
Excitation of electrons takes place in the same
orbit.
*The energy required for excitation of electrons is called promotion
energy.
*If an element shows odd or even valency in ground state then it
will show odd or even in excited state also with the difference of 2.
conditions for formation of covalent bond
- Number of valence electrons
- Comparable electronegativity
- Mutual sharing of electrons
*Half filled orbital or unpaired electron orbital accepts one
electron from another atom, to complete its orbital.
*Tendency to complete orbital or to pair the electron is an essential
condition of covalent bond.
*Completion of octet is not the essential condition of covalent
bond.
4.The shared pair of electrons should have opposite spins
*One orbital can accommodate maximum 2 electrons with opposite
spins
who gave vbt
It was presented by Heitler & London to explain how a covalent bond is formed.
It was extended by Pauling & Slater.
what are the postulates of vbt
(a) To form a covalent bond overlapping occurs between half filled valence shell orbitals of the two
atoms.
(b) Orbitals come closer to each other from the direction in which there is maximum overlapping
(c) So covalent bond has directional character.
(d) Strength of covalent bond direcly prop. extent of overlapping.
what does stregth of overlap depend upon
(i) Nature of orbitals –
(a) As the value of n increases, bond strength decreases.
1 - 1 > 1 - 2 > 2 - 2 > 2 - 3 > 3 - 3
1s - 2p > 2s - 2p > 3s - 3p
(b) p, d and f are directional orbitals ® more overlapping
s-orbital ® non directional – less overlapping
The bond is formed by the overlaping of directional orbital is more stronger than the bond
formed by overlapping of non-directional orbital.
p–p > s–p > s–s
(c) If n is same 2p - 2p > 2s - 2p > 2s - 2s
(ii) Nature of overlapping – Co-axial overlapping - extent of overlapping more.
Collateral overlapping - extent of overlapping less
Order of strength of Co - axial overlapping – p - p > s - p > s - s
Two types of bonds are formed on account of overlapping.
(1) Sigma (s) bond
(2) Pi (p) bond
why do hybrid orbitals form strogner bonds
The directional properties in hybrid orbital is more than atomic orbitals. Therefore hybrid orbitals form
stronger sigma bond.
what does bond angle depend upon
(i) Bond agle directly proportional to % of s characte
sp>sp2>sp3
(ii) Bond Angle inversely proportional no of lone pairs
more lone pairs, lesser bond angle
(iii) If side atoms are same, then bond angle direclt proportional to electronegativity of central atom
H2O> H2S
(iv) If central atosm are same, then bond angle inversly proportional to electronegativity of side atom. It is directly proportional to size of side atom.
OCl2> OF2
what does bond length depend upon
(i) Bond length inversely proportional to difference of electronegativity b/w two atoms
(ii) Bond Length direclty proportional to % s character
(iii) bond length inversely prop to bond order
what are 2 types of bond energy
(i) Bond formation energy :- Energy released when any bond is formed is known as bond formation energy
or bond energy.
(ii) Bond dissociation energy :- Energy required to dissociate any bond is known as Bond dissociation
energy.
Calculation of released energy is more difficult than the dissociation energy therefore dissociation energy of bond
is calculated and is assumed as bond energy or bond formation energy.
In diatomic molecules, both values are equal
what are the fators on which bond energy depends
BE directly prop to bond order
BE direclty prop to difference of electrnegativity
BE inversely proportional to size of atom
BE direclty propotional to polarity
BE increases due to resonance
BE inversly proportional to no of lone pairs
BE directly proportinal to s character
BE directly proportional to extent of overlapping directly proportional to directional propoerties
BE inversely proportional to size of orbitals
why are molecules of -O-O- and -F-F have less bond energy
Size of F and O atoms are small so their bond energy should be high (small atomic radius) but it is actually less,
due to lone pair of electrons present on F and O atoms, which repells each other in F—F and —O—O–type of
bonds
give the order of pi bond strength
2pπ – 2pπ > 2pπ– 3dπ > 2pπ – 3pπ > 3pπ – 3pπ
what is a coordinte bond
It is a covalent bond in which the shared e– pair come from one atom is called coordinate bond.
(b) Atom which provide electron pair for sharing is called donor (Lewis base).
(c) Other atom which accepts electron pair is called acceptor (Lewis acid). That is why it is called
donor-acceptor or dative bond.
O3,N2O,N2O5,H2SO4
conditions for formation of coordinate bond
(a) Octet of donor atom should be complete and should have atleast one lone pair of electron.
(b) Acceptor atom should have vacant orbital to accomodate L.P.
