chemical bonding and molecular structure

Molecule :

A group of atoms is found to exist together as one species having characteristic properties. Such a group of atoms is called a molecule.

Chemical Bond :

The attractive force which holds various constituents (atoms, ions, etc.) together in different chemical species is called a chemical bond.

KÖSSEL-LEWIS APPROACH TO CHEMICAL BONDING :

Lewis given the concept of stable valence  shell by considering nucleus and inner electron as a kernel  and outermost shell around it.so that all eight electron occupy

eight corners of cubic lattics and outermost shell tend to get inert gas configuration by gaining or losing electrons.Lewis also postulated that atoms achieve the

stable octet when they are linked by chemical bonds

Lewsis symbols :

Valence electrons : In the formation of a molecule, only the outer shell electrons take part in chemical combination and they are known as valence electrons.

Lewis has given symbol method to represent outermost shell of all elements of periodic table for example :-

Lewsis symbols for elements of second period is given below :

significance of lewis symbols :

The dots represent number of valence electrons and it helps to calculate group valence electrons

Group Valence electrons :

The group valence of the elements is generally either equal to the number of dots in Lewis symbols or 8 minus the number of dots or valence electrons.

Kössel, in relation to chemical bonding,drew attention to the following facts:

• In the periodic table, the highly electronegative halogens and the highly electropositive alkali metals are separated by the noble gases.

• The formation of a negative ion from a halogen atom and a positive ion from an alkali metal atom is associated with the gain and loss of an electron by the
respective atoms.

• The negative and positive ions thus formed attain stable noble gas electronic configurations. The noble gases (with the exception of helium which has a duplet
of electrons) have a particularly stable outer shell configuration of eight (octet) electrons, ns2np6.

• The negative and positive ions are stabilized by electrostatic attraction.

Let us take example of formation of Nacl:-

Electrovalent Bond :

The bond formed, as a result of the electrostatic attraction between the positive and negative ions was termed as electrovalent bond.The electrovalence
is thus equal to the number of unit charge(s) on the ion.

Octet Rule :

According to this,atoms can combine either by transfer of valence electrons from one atom to another (gaining or losing) or by sharing of valence electrons in order to have an octet in their valence shells. This is known as octet rule.

Covalent Bond :

Sharing of electron to attain noble gas configuration leads to formation of covalent bond.for example chlorine formed by sharing a pair of electron among two chlorine atoms

The dots represent electrons. Such structures are referred to as Lewis dot structures.

The important conditions Lewis dots are :
• Each bond is formed as a result of sharing of an electron pair between the atoms.

• Each combining atom contributes at least one electron to the shared pair.

• The combining atoms attain the outershell noble gas configurations as a result of the sharing of electrons.

If two atoms share two pairs of electrons, the covalent bond between them is called a double bond

When combining atoms share three electron pairs as in the case of two nitrogen atoms in the N2 molecule and the two carbon atoms in the ethyne molecule, a triple bond is formed.

Previous year question paper

Chemistry Class 11 Previous Year Question Papers

Chemistry Class 12 Previous Year Question Papers

Some basic concepts of chemistry

Chemistry :

chemistry is the branch of science which deals with the composition ,structure and properties of matter.

Branches of chemistry :

Inorganic chemistry :

Inorganic chemistry deals with the synthesis and behavior of inorganic and organometallic compounds or it is the study of

elements other then carbon and its compounds.

organic chemistry:

organic chemistry is the branch of chemistry in which we study about organic compounds or the substances produced by organic compounds.

Physical chemistry :

Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms

of the principles, practices and concepts of physics such as motion, energy, force, time, thermodynamics, quantum

chemistry, statistical mechanics, analytical dynamics and chemical equilibrium.

Analytical chemistry :
It is  the branch of chemistry which deals with qualitative analysis of chemical substances.

In additions to these branches their are many other branches like nuclear chemistry,biochemistry etc.

Matter :
Anything which occupies space and have some specific mass is call matter.

Classification of matter :

Physical classification :

solid :- Solid have particles arranged very closeley so they have definite volume and shape

liquid :- Liquid have closed packed arrangement of particles but particles are free to move anywhere so liquid have definite volume but not definite shape.

Gas :- Gases have particles losely held so they do not have definite shape and definite volume.so gases have no definite shape and definite volume

Chemical Classification :
Chemically their are two types of substances present i.e pure substance and mixtures which can be further classified as

given below :-

Pure substances :-

a)elements  :

It is the simplest form of pure substance which can be neither decomposed, divided or built  by simple chemical or

physical methods and it only one kind of atoms

an element is of three typees i.e metal ,non metal and mettaloids.till now 118 elements are known ,hydrogen is the most

abudant element in the universe and oxygen in the most abudant non metal in the earth crust



b)compounds :
compound is a form of matter which can be formed by combining two or more elements in a definite ration by mass.It can be

divided or separated by suitable chemical reaction.
compound can of two types :-

organic and inoragnic

Organic compounds :

Organic compunds are hydrides of carbon earlier it was considered to be formed by living organism like plant and animals.

Inorganic compounds :

Inorganic compounds are the compound of all element except hydries of carbon ealier it was considered as it is derived from inorganic materials like rock and minerals.

Symbols :

a symbol is an abbreviation or shortened form for the full name of an element,,all the elements have some symbols to

represent them.like

sodium have Na,potassium have k ,copper has cu irin has Fe etc.

Mixtures :

Mixture are made up of two or more substances they can possess variable composition and can be seprated into their components by some physical methods.

it is of two types homogeneous in which composition is uniform throughout and hetrogeneous in which composition is not uniform throughout

Filtration :

filtration is the process of seprating solids which are suspended in liquids with the help of filter funnel when mixture passes through the filter funnel solid remain intact with it and liquid passes.

Distillation :

Distillation is the process of heating liquid to form vapours then collecting vapours and cooling vapours to get back the liquid.

Sublimation :

This is the process of conversion of solid directly into vapours on heating.

Magnetic sepration :

This process is based on the magnetic property of a substance as a magnet attracts magnetic substance of the mixture when it is placed in front of mixture.

Crystallisation :
It is the process of separating solids having different solubilities in a particular solvent.

Atmolysis :

This method based on rates of diffusion of gases and used for their separation of gases

Atoms and molecules :-

Atom is the smallest particle of an element which not be further divided.It may or may not capable of independent existence.

Molecule is the simplest particlenof matter that has independent existence.

Physical Quantities and Their Measurement :

Units are of two types

1)Basic unit :

Fundamental unit are length(m),mass(kg),time(s),electric current(A).thermodynamic temparature(K),amount of substance(mol) and luminous intensity(cd)

2)Derived units : Derived units are derived from fundmental unit example unit of density derived from units of mass and volume.

Various physical quantities are :

a)CGS System :
It is based on centimetre,gram and second as the units of length,mass,time

b)FPS System :

foot,pound,second used as the fundamental units

c)MRS System :

metre,kilogram and second used for measurements

d)SI system :

International system of units :-

PHYSICAL QUANTITY                  Name of S.I UNIT
Length(l)                                             metre
Mass(m)                                              Kilogram
Time(t)                                                 second
Electric current(i)                                 ampere
Thermodynamic temprature(T)            Kelvin
Amount of substance(n)                        mole
Luminous intensity(ly)                          candella

Significant figures :

significant figures are meaningfull digits which are known with certainity.The uncertainity is indicated by writing

the certain digits and the last uncertain digit.

it means in many calculations in which we get slightly different digits,you can say minute difference digits or result

its very neccessary to get accurate result so to get accrate result we take help of significant figures.for example:

suppose we are measuring something and it comes out to be 11.999999 first time and then again 11.999996 second time,so to get accurate result we have take care of significant figure in this example 11 is certain digit while 999996 or 999999 is uncertain digit.

Rules for determining significant figures :

1)All non zero digits are significant. for example in 365 and .456 their are three significant figures

2)Zeros preceding to first non zero digit are not significant .example 0.98 has two significant figure,0.03 has one significant figure.

3)zeros between two non zero digits are significant.example 2.009 has four significant figure.

4)zeros at the end or right of a number are significant provided they are on the right side of the decimal point. for example 0.300 has three significant figures.

5)Exact numbers have an infinite number of significant figures.for example 3 and 30 have infinite significant figures,as 3 can be written as 3.0000.... and 30 can be writeen as 30.000000...... when we write numbers in scientific notation like 3.02 x 10² the number between 1 to 10 gives number of significant figure for example in 3.02 x 10² has three significant figures.

Addition and subtraction of significant figures :

Result cannot have more digits to the right of the decimal point than either of the original numbers.

for example when we add  13.56
                         19.0
                         --------
                          32.56
                         ---------

19.0 has only one digit after the decimal point and the result should contain only one digit after the decimal point.

Multiplication and division of significant figures :

In this also result should not contain more significant figures then any of the numbers for example

2.5 x 1.25=3.125  since 2.5 has two significant figures so result should be 3.1

Rules for rounding of the numbers :

1)If the rightmost digit to removed is more than 5 then preceding number increased by one.
for example 1.457 can be rounded off to 1.46

2)If the rightmost digit to removed is less than 5 then preceding number decreased by one.
for example 1.457 can be rounded off to 1.45

3)If the rightmost digit to removed is equal to 5 then we check the preceding number that it is odd or even and if preceding number is odd then it is increased by one and if it is even the number remain same.
for example : 1.45  can be rounded off to 1.4 and 1.35 can be rounded to 1.4


Law of chemical combination :
Their five basic laws which govern formation of compound with the help of elements.

Law of conservation of mass (Lavoisier,1774) :

This law states that during any physical or chemical change the total mass of the products is equal to the total mass of reactants.It does not hold good for nuclear

reactions.

Law of definite proportions (Proust,1799) :

According to this law a chemical compound obtained by different sources always contains same percentage of each constituent element.


Law of multiple proportions (Dalton,1803) :


According to this law if two elements can combine to form more then one compound,the masses of one element that combine with a fixed mass of the other element are in

the ratio of small whole numbers


Gay Lussac law of gaseous volumes :

when gases combine or produced in a chemical reaction they do so in a simple ratio by volume provided all gases are at same temperature and pressure.

Avogadro Law :

Equal volumes of gases at the same temperature and pressure should contain equal number of molecules.
for example : Two volume of hydrogen combine with one volume of oxygen to form two volume of water vapour

Dalton atomic theory :-

1. Matter consists of indivisible atoms.

2. All the atoms of a given element have identical properties including identical mass. Atoms of different elements dffer in mass.

3. Compounds are formed when atoms of different elements combine in a fixed ratio.

4. Chemical reactions involve reorganisation of atoms. These are neither created nor destroyed in a chemical reaction.
Dalton’s theory could explain the laws of chemical combination.

Atomic and molecular masses :

One atomic mass unit is defined as a mass exactly equal to one twelfth the mass of one carbon - 12 atom.
And 1 amu = 1.66056×10^–24 g

Today, ‘amu’ has been replaced by ‘u’ which is known as unified mass.

In general for elements we take avaerage atomic mass as an elements can contain many form like carbon has three isotopes c-12,c-13,c-14  so we have to take average of these

Molecular mass :

Molecular mass is the sum of atomic masses of the elements present in a molecule

for example : Molecular mass of methane,
(CH₄) = (12.011 u) + 4 (1.008 u)
= 16.043 u

Formula mass :

The formula such as NaCl is used to calculate the formula mass instead of molecular mass as in the solid state sodium
chloride does not exist as a single entity.

Thus, formula mass of sodium chloride =atomic mass of sodium + atomic mass of chlorine= 23.0 u + 35.5 u = 58.5 u

Mole concept and molar masses :

One mole is the amount of a substance that contains as many particles or entities as there are atoms in exactly 12 g (or 0.012kg) of the 12C isotope.

so 12C carbon atom is taken as base for calculation of mole ,so any substance one mole quantity will be equal to the entities present in 12C atom.

mass of a C-12 carbon atom =1.992648
and we know that one mole of carbon weighs 12g we can calculate number of atoms present in it:

=12/1.992648
=6.0221367

this number is called Avogadro number

Molar masses :-

The mass of one mole of a substance in grams is called its molar mass.