How do atoms of
elements exist in the
nature?
How do atoms of elements exist in the nature?
™But noble gases like He,Ne,Ar,Kr,Xe,Rn etc
do not exist in the nature in the combined
state. .They exist as monoatomic gases.
yAtoms of most elements
(except noble gases) are not
able to exist independently.
independently
yA group of two or
more atoms known
as molecules have
independent
existence.
yAtoms
off same element
At
l
t
or of different elements
can join to form
molecules.
yExample:
E
l A
molecule
l
l off oxygen is
i
f
formed
db
by jjoining
i i two
atoms off oxygen (O2)
O
O
O
yExample A molecule
of water is formed by
joining two atoms of
hydrogen with one
atom of oxygen (H2O)
y
H
H
O
What is a molecule?
yThe smallest particle of an element
or a compound
d made
d up off group off
two or more atoms or ions that is
capable of independent existence
and shows all the p
properties
p
of that
substance is called as molecule.
H
C
O
O
H
H
H
N
H
C
H
H
Molecules have free existence
Atoms
cannot exist
freelyy
What is a chemical bond?
yThe attractive force which holds
constituents (atoms,
(atoms ions
ions,
molecules, etc.)together in
diff
different
chemical
h i l species
i iis called
ll d
as chemical bond.
Graphite
Crystal of sodium chloride
y Noble gases or inert gases like Helium
(He), Neon (Ne), Argon (Ar), Krypton
(Kr) Xenon (Xe) and Radon (Rn) are the
(Kr),
most stable elements because atoms of
these
h
elements
l
possess an octet structure
i.e. eight electrons in the outer most orbit
He (2)
Ne (10)
A (18)
Ar
( 8)
Kr (36)
Xe
( )
(54)
Valence
Electronic configuration electrons
1s2
2
1s22s22p6
8
8
1s22s22p63s23p6
1s22s22p63s23p63d104s24p6
5s25p6
8
8
Sodium
(Na)
11
Chlorin
e(Cl)
17
Carbon
(C)
6
Nitroge
n(N)
( )
7
1s22s22p63s1
1
( ,8 ,1))
(2
1s22s22p63s23p5
7
(2 ,8 ,7)
1s22s22p2
4
(2 ,4)
1s22s22p3
( ,5))
(2
6
5
Reason‐1 for chemical bond formation;
Tendency of atoms to attain octet
structure
y The atoms of elements other than noble
gases have no octet structure and hence
try to attain eight electrons
y ( or two electrons)
l
) in
i their
h i valence
l
shell by combining with the same or
other
h atoms.
Octet Rule = atoms tend to gain, lose or share electrons so as to have 8 electrons
9C would liketo
9N
N wouldliketo
9O wouldliketo
Gain 4 electrons
Gain 3 electrons
Gain 2 electrons
yWhen two or more atoms combine to
f
form
a molecule
l
l the
h electrons
l
in
i their
h i
outermost orbits are rearranged
g in
such a way as to achieve octet or
duplet structure of the nearest
noble gas. This is called octet rule.
: Formation of NaCl
Na(2,8,1) transfers it’s excess electron to Cl
atom (2,8,7) and thus Na atom acquires the
configuration of Ne(2,8) and Cl acquires the
configuration of Ar(2,8,8)
_
Na
Cl
281
2,8,1
287
2,8.7
Na+
Cl‐
2,8
2,8,8
, ,
Reason‐2 for chemical bond formation;
Tendency of atoms to attain minimum energy
and maximum stability
(a)When
)
two or more atoms combine to form a
molecule, the potential energy of bonded
y
decreases.(b)Bonded
( )
system
y
has less
system
energy than the unbonded system .( c)Therefore
atoms combine to form molecules to attain
minimum energy and maximum stability
Octet
rule
O
l
(Kossel
(K
l and
d Lewis)
L i )
y Atoms can combine either by transfer of
valence electrons from one atom to another or
by sharing of valence electrons in order to
have an octet electron structure in their
valence shells. This is known as octet rule. This
theoryy of chemical combination is known as
electronic theory of chemical bonding.
Concept of valence electrons
y
The
electrons
present
in
the
outermost shell of an atom are called
valence
electrons
electrons,
because
the
combining capacity of the element (i.e.
valency) depends upon the number of
these electrons.
Core electrons
y All the electrons p
present in the inner
orbitals (core of the atom) excluding
valence electrons are called core electrons
which do not participate in chemical
reaction.
l
l
Valence
electron
Nucleus
Core electron
Sodium atom 2,8,1
18 group
1group
2 group 13 group
14 group
15 group
16group
17group
ELECTRON DOT STRUCTURES
LEWIS SYMBOLS FOR THE ELEMENTS
Symbols of atoms with dots to represent the valence‐
shell electrons
1
2
13
14
15
16
17
18
H•
He:
•
Li• Be•
•
•
•B •
•
C•
•
•
Na• Mg•
•
•
Al•
••
•
N•
•
•
• Si •
•
••
•
O•
••
••
• P•
•
••
•S•
••
••
:F•
••
••
:Cl •
••
••
:Ne :
••
••
:Ar :
••
ELECTRON DOT STRUCTURES
LEWIS SYMBOLS FOR THE ELEMENTS
1
Symbols of atoms with dots to represent the
valence-shell
valence
shell electrons
2
13
14
15
16
17
18
H•
Li•
He:
•
•
•
Be•
•B •
•
••
C•
•
•
Na• Mg•
•
•
•
Al•
• Si •
•
•
N•
••
•
O•
••
••
: F • :Ne :
•
••
••
••
••
•P•
••
•S•
••
••
••
••
•
••
:Cl • :Ar :
element
Atomic Electronic
number configuration
Li
3
Group
Valence
number electron
s
Lewis symbol
1s2 1s1
1
1
Li.
Be
4
1s22s2
2
2
B:
B
5
1s22s22p1
13
3
.B:
C
6
1s22s22p2
14
4
C
element
Atomic
number
Electronic
configuration
Grou
p
num
ber
Valen Lewis
ce
Symbols
elect
rons
N
7
1s22s22p3 15 5
:N.
8-5
=33
O
8
1s22s22p4 16 6
O:
8-6
=2
Valence electrons
F
9
1s22s22p5 17 7
:F.
8-7
=11
Significance of Lewis symbols
y The number of valence electrons
(number of dots) helps to calculate
the valency of the element.
element
y The valency is generally either equal
to the number of dots in Lewis
symbols
y
or 8 minus the number of
dots or valence electrons.
Types of Bonds
yThe type of bond formed
between the atoms depends
upon the way in which the
combining atoms attain octet
structure in their valence shell.
TYPES OF CHEMICAL BONDS
™Ionic or electrovalent bond
™ Covalent bond
™ Coordinate
C
di t covalent
l t bond
b d
™ Metallic bond
™ Hydrogen bond
™ Van
V der
d Waals
W l forces
f
off attraction
tt ti
IONIC BOND OR ELECTROVALENT BOND
yIonic bond is formed by the
transfer of one or more valence
shell electrons from a metal
atom to the
h valence
l
shell
h ll off a
nonmetal
o
eta ato
atom
y The
which
Th metal
t l atom
t
hi h has
h lost
l t electron(s)
l t ( ) is
i
converted into a positive ion (cation)and
the nonmetal atom which gains electron(s)
is converted into a negative ion (anion).
y The cations and anions thus formed attain
stable noble gas electronic
configuration.
y The
h negative and
d positive ions are held
h ld
by electrostatic force of attraction
A metal gives electron(s) to a non-metal. The metal
becomes positive and the non-metal becomes negative.
They now attract each other.
+1
1 +2
H 2
Li
B
Be
Na Mg
K
+3 -4 -3 -2 -1 18
13 14 15 16 17 He
Ca
Rb
3
7
9
C
N
O
F
N
Ne
10 11 12 Al
Si
P
S
Cl
Ar
4
5
6
Ti
V
Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr
Zr
8
B
Ag
Cs Ba
W
Pt Au Hg
Fr
U
Pu
I
Pb
Xe
Rn
Formation of Ions from Metals
zIonic compounds result when metals react
with nonmetals
zMetals lose electrons to match the number
of valence electrons of their nearest noble
gas
g
Group 1 metals ⎯→ ion 1+
Group 2 metals ⎯→ ion 2+
y
Group 13 metals ⎯→ ion 3+
Formation
Ion
F
ti off Sodium
S di
I
Sodium atom
Sodium ion
Na •
– e−
⎯⎯→
Na +
2,8,1
11 p+
11 e‐
0
2,8 ( = Ne)
11 p+
10 e‐
1+
Formation off Magnesium Ion
Magnesium atom
•
Mg •
2,8,2
12 p+
12 e‐
0
Magnesium ion
– 2e− ⎯→
Mg2+
2,8 (=Ne)
12 p+
10 e‐
2+
yp
g
Some Typical
Ions with Positive Charges
(Cations)
Group 1
Group 2
H+
Mg
g2+
Li+
Ca2+
Na+
Sr2+
K+
Ba2+
Group 13
Al3+
Ions from Nonmetal Ions
„In ionic compounds,
compounds nonmetals in 15,
15 16,
16
and 17 gain electrons from metals
„Nonmetal add electrons to achieve the octet
arrangement
„Nonmetal ionic charge:
3‐, 2‐, or 1‐
Fluoride
Fl
id Ion
I
unpaired electron
••
:F•
••
+ e−
octet
•• 1 ‐
: F:
••
2,7
2,8 (= Ne)
9 p+
9 e‐
0
9 p+
10 e‐
1‐
ionic charge
Formation of oxide ion
2 unpaired electrons
••
•O •
••
+ 2e−
octet
•• 2 ‐
:O:
••
2,6
2 ,8 (= Ne)
8 p+
8 e‐
0
8 p+
10 e‐
2‐
ionic charge
Ionic Bonds: One Big Greedy Thief Dog!
What is an ion or radical?
Radical or an ion is an atom or
group of atoms possessing
electric charge
E g: Na+,Cl
E.g:
Cl‐ , SO422‐
+
y
Names and symbols
of some ions
Names and symbols
of ions
+
POLYATOMIC IONS
Ions with -1
perbromate
bromate
bromite
hypobromite
perchlorate
chlorate
chlorite
hypochlorite
periodate
iodate
iodite
hypoiodite
charge
BrO4-1
BrO3-1
BrO2-1
BrO-1
ClO4-1
ClO3-1
ClO2-1
ClO-1
IO4-1
IO3-1
IO2-1
IO-1
nitrate
nitrite
hydroxide
cyanide
thiocyanate
acetate
permanganate
bicarbonate
NO3-1
NO2-1
OH-1
CN-1
SCN-1
C2H3O2-1
MnO4-1
HCO3-1
Ions with
I
ith a -2
2 Charge
Ch
carbonate
CO3-2
phthalate
C8H4O4-2
sulfate
lf t
SO4-22
sulfite
chromate
dichromate
oxalate
peroxide
SO3-2
CrO4-2
Cr2O7-2
C2O4-2
O2-2
Ions with a -3 Charge
phosphate
PO4-3
phosphite
PO3-3
arsenate
AsO4-3
Ions with +1 charge
ammonium
i
iion NH4+11
Formula of a compound
yThe symbolic representation
of a molecule is called a
formula.
FORMULAE OF SIMPLE IONIC COMPOUNDS
y
y While writing the chemical formulae for
compounds,
p
, we write the constituent
symbols and their valencies as shown.
y .Then we must cross over the valencies of
the combining atoms after removing
common factors.
factors
Formula of hydrogen chloride
Symbols
valency
Formula
H
1
HCl
Cl
1
Formula of hydrogen sulphide
Symbols
H
valency
1
Formula
H 2S
S
2
Formula of magnesium chloride
Symbols
valency
Formula
Mg
2+
Cl
Ignore
g
the charges
g
1‐
Mg Cl
2
Formula of calcium oxide
Symbols
Ca
O
valency
2+
2‐
1
Formula
1
Ca O
Ignore
g
the charges
g
Remove common fact
Formula
sulphate
F
l off ammonium
i
l h
Symbols
valency
NH4 SO4
1+
2‐
Formula (NH4) SO4
2
Ignore
g
the charges
g
Properties of ionic compounds
y Physical
y
state: Ionic compounds
p
consists of three dimensional solid
aggregates of cations and anions which
are arranged in a well defined
geometrical pattern.
pattern Thus ionic
compounds are solids at room
t
temperature
t
y
Electrical conductivity:
y Ionic compounds do not conduct
electricity
l
i i when
h they
h are in
i the
h solid
lid state
because the cations and anions are held
tightl by
tightly
b electrostatic force of attraction.
attraction
y The ionic solids conduct electricity when
th are di
they
dissolved
l d iin water
t or iin th
the molten
lt
state because ions are free to move in water
solution or molten state.
state
Coil of wire
batteryy
+
+
compass
Copper Chloride
_
_
+
anode
_
cathode
+
_
anode
cathode
++ + +
++
++
Copper Chloride CuCl2
yIonic compounds are
quite hard and have
high melting points
yThey
Th are soluble
l bl in
i
polar
l solvents
l
t like
lik water
t
but insoluble in nonpolar
solvents like ether,
ether
alcohol benzene etc
alcohol,