Introduction:-In previous chapter you learn about nature of matter and formation of compounds by combining of elements.

   The basic idea : Matter is made up of very tiny particles and large types of object can be created from just a few building blocks.

Law of chemical combination:- Also known as indestructibily of matter .

         During any chemical reaction, there is no detectable loss or gain in total mass of substances .

           In other words mass can neither be created nor be destroyed but it can only transform from one form to another.

e.g. 12g carbon combines with 32 g of oxygen to form 44 gram of carbon dioxide.

         C+O₂---->CO₂ 

       12g+32g---->44g.

LAW OF CONSTANT PROPORTION:(Also known as law of definite proportion)

         It states that: all pure specimens of same chemical compound always contains same element combined together in same proportion in weight.

Example water always consists of two elements hydrogen and oxygen in the same proportion of 1 :8 by parts.

It is observed that compound obtained from different method, but the ratio of elements would be same.

DALTON ATOMIC THEORY:-

1. John Dalton provide the basic theory about the nature of matter.

2. Proposed a model of an atom known as Dalton's atomic theory.

3. The main postulates or assumptions of dalton's atomic theory are:-

a. Matter is made up of very small particles. These are called atoms.

b. Atoms are cannot for the broken down i.e indivisible and indestructible ultimate particles.

c. Atoms can neither be created nor be destroyed in any chemical reaction.

d. Atoms are identical of a given element in all respect i.e mass size and chemical properties.

e. Atom of different elements are different in all respect.

f. They join to form compounds i.e two or more elements are to be joined.

g. In chemical reaction, atom of different element join in simple numerical ratio.e.g 1:1, 1:2,2:3.

h. Atom of same element can be join in more than one ratio to produce different types of compound.e.g CO,CO₂ etc.

Merits of Dalton atomic theory:

1. Explain the law of chemical combination.

2. Dalton was the first to recognise a workable distinction between ultimate particles of element (atom)and that of compound [( compound atom), later known as molecule].

         Dalton second postulate: Atoms can neither be created nor destroyed validates the law of conservation of mass.

ATOM (An ultimate particle of Matter)

It is the smallest particle of an element that maintain its chemical identity (all chemical and physical changes).

They are building blocks of all matter.

Atoms of most elements have no free existence. Atoms of noble gas only free existence.

Characteristics of atom:-

Atom of same elements are similar, but they differ with other elements.

They show all the properties of an element.

Size of an Atom: Extremely tiny particles,  spherical in shape cannot be seen by ordinary microscope.

Diameter of atom is about 1 x 10^-10m.

        More than a million of atoms when joined together one on top of other (make layer) value as thick as a sheet of paper.

Drawbacks of Dalton's atomic theory:-

1. Atoms have been found to be made of steel smaller subatomic particles- proton, electron and neutron. Therefore it is no longer indivisible.

2. Existence of isotopes (same atomic number but different mass number).

3. It may be similar in one or more respects of different elements.

Explanation of law of chemical combination by Dalton's atomic theory:-

1. Explanation for the law of conservation of mass: According to Dalton's atomic theory:  Matter is made up of atoms and atoms can neither be created nor be destroyed during any physical or chemical change.

2. Explanation for the law of constant proportion: According to Dalton atomic theory the relative number and types of atoms in a given compound is fixed.

Dalton's atomic theory and the terms, element and compound:-

An element is a pure substance which is made up of only one kind of atom.

An atom is the smallest particle of an element which may or may not have independent existence.

Just as the ultimate particle of an element is atom, the ultimate particle of a particle is called a molecule.

Molecule is the smallest particle of a substance (element or compound), capable of independent existence and under ordinary conditions and shows all the properties of that substances.

Symbols of atoms of the elements:-

A symbol is shorthand representation of an element.

Guidelines for writing the symbols of the elements:-

1. First letter in capital of English name of the element used as abbreviation to represent an element.

Example hydrogen-H oxygen--O etc 

2. Names of two or more elements with same initial letter, the initial  letter followed by second letter is used to symbolize. the element. e.g Barium- Ba, Bismuth-Bi etc.

3.The symbol board of few elements are derived from their Latin names.

e.g Sodium-Natrium-Na etc.

Indication of symbol:-

Represent a single element (particular).

One atom of the element.

Represent one atomic mass of the element.

e•g• Symbol [Na] stands for 

Element sodium

One atom of sodium

One atomic mass (23 g) of sodium.

Molecule:- Smallest particle of matter (element or compound) which can exist in a free state is called molecule.

      It can so all the properties of the substances.

 Properties :-

In a given substances all molecules are similar.

  Different substances of different molecules.

 Properties of its molecule of a substance is same.

Molecules of element:-

 Some type of atoms combine to form molecules of an element.

      Some atoms exist free nature not as molecule. example Noble gas-Helium,Argon etc.

 Molecules of compound:-

 When two or more different types of atoms combine to form molecules is known as molecules of that compound.

 example: H₂O, H and O are two different atoms combine to form molecules of water.

 Atomicity:-Number of atoms in a molecule of an element or compound.

Types of molecules:-

 Monatomic molecules:- exist in atomic forms. e.g He,Ne etc.

Diatomic molecules:- exist as molecule consists of two atoms.e.g O₂,CO etc.

Triatomic molecule:- consists of three atoms.e.g. H₂S,CO₂ etc.

Tetra atomic molecules:- Consist of four atoms.e.g P₄ etc.

Polyatomic molecules:- consists of more than four atoms e.g S₈,CH₄ etc.

Ions:-

It is a charged particle.

Some compound are formed by this charge to particle.

In other words iron is a charged particle having negative or positive charge.

A group of atom having charge on them are called polyatomic ions.

Example:- sodium ion ( Na +) calcium ions (Ca²+) , sodium chloride (Nacl) [Na+,cl-]

Valency: it is defined as its capacity to combine atoms with other atoms.

   Valence is of different element having same or different i.e.1,2,3 and 4.

It may be monovalent (valency =1) ,divalent (valency =2), trivalent (valency= 3) and tetravalent (valency =4).

Variable valency:-Some elements of different valencies in different compounds.

Element with higher valency is indicated by the suffix --ic, for lower valency   indicated by the suffix--ous.

e.g. black oxide of copper -cupric oxide (higher valency of copper+2), red oxide of copper -cuprous oxide (lower valency of copper + 1).

Valency of ion:- number of unit positive or negative charges on an ion.

e.g. Na+,Ca²+ and SO₄²⁻ ions are 1,2,2 respectively.

Formula:-represent that molecule using symbol for the atoms present in it.

Empirical or simple formula:- Simplest whole number ratio of atoms of the element in a molecule of the compound.

e.g.H₂O₂ hydrogen peroxide is HO and C₆H₆ benzene is CH.

IT DOES NOT REPRESENT THE ACTUAL FORMULA OF THE COMPOUND.

Molecular or chemical formula:- Representation of chemical compounds using a set of symbols for the atoms of elements present in a molecule of the compound.

         It was actual number of atoms of each element of the compound.

Difference between empirical formula and molecular formula:-

Molecular formula of benzene is C6H6 

Ratio of C and H is 6:6.

Simplest ratio =1:1(C&H).

Hence empirical formula is C₁H₁ i.e CH.

N:B:- IN SOME CASES EMPIRICAL AND MOLECULAR FORMULA ARE SAME.

e.g Methane (CH₄),Boron trichloride (Bcl₃).

 Relation between empirical and molecular formula:-

Molecular formula = n X empirical formula.

Where n=1,2,3,4.

i.e. molecular formula of a compound is equal to simple whole number of multiple of its empirical formula.

How to write a formula?

Rules for writing chemical formula:-

On the basis of valency:- number of atoms of the combining elements in the molecule of a compound are in the inverse ratio of their valencies.

Valency of element A=3

Valency of element B=2

Hence formula of A2B3.


Formula of Binary Compounds:- Binary compound is one which is made up of two different elements.

e.g. HCl,CO₂ etc.

Radicals:- an atom or a group of atom act as a single unit, it may be positive negative.

Positive radicals:-radical in a compound is either a metal ion NH₄ + ion or H + ion.

Negative radicals:- carry  negative charge,OH,NO₃,cl etc.

Chemical equation:-Shorthand representation of a chemical reaction using symbols and formula of the substances involved in the chemical reaction.

How the atoms and molecules are weighed and counted:-

Atomic mass unit:- it is mass unit (a.m.u) is equal to one twelfth (1/12) the mass of an atom of carbon-- 12.

Relative atomic mass:-It is a relative quantity and it is the mass of one atom of the element relative to one twelfth (1/12) the mass of one carbon-- 12 atom.

Relative atomic mass= mass of one atom of the element / (1/12) X mass of Carbon--12 atom.

N:B 1/12 mass of carbon 12 atom = 1a.m.u.

1 a.m.u= 1.66 X 10 ^-24 g =1.66 X 10^-27 kg.

1 a.m.u is also called one Dalton (Da).

1 a.m.u is also called unified mass(u).or

United atomic mass constant (Mu).

Gram atomic mass:-when atomic mass is expressed in grams is called the gram atomic mass.

   It  is also called gram -- atom.

Number of gram atom= mass of the element in g / atomic mass of the element.

Relative molecular mass:- relative molecular mass of a substance as compared to one twelfth the mass of one carbon --12 atom.

Relative molecular mass= mass of one molecule of the substance /one twelfth (1/12) X  mass of one C -- 12 atom.

It is thus represent number of times it is heavier than one  twelfth (1/12) mass of an atom of C -- 12 isotopes.

Gram molecular mass :- when molecular mass expressed in gram is called gram molecular mass.

Number of gram molecules= mass of substance in grams/ molecular mass of the Substance.

Formula mass:-It is used for ionic compounds and other where discrete molecules do not exist.

Sodium chloride (Na+,cl-) but for simplicity as NaCl or Na+cl-.

It is it is the sum of the masses of all the atoms in the formula.

e.g. formula mass of NaCl= (atomic mass ofNa)+ (atomic mass of cl) 

                       =23+35.5

                         58.5.

Mole concept:-

A mole represent 6.022 × 10^23 particles.

It is denoted by n.

One mole (of anything) =6.022 × 10^23 in number.

For e.g 1 mole of atom/ molecules/ ions= 6.022 × 10^23 molecules / ions/ atoms.

One mole is fixed for any substance i.e 6.022×10^23.

6.022×10^23 is called Avagadro  constant or Avagadro number.

Moles in terms of mass:- mass of 1 mole of a particular substance is also fixed.

Mole and gram atomic mass:- molar mass of atom is also known as gram atomic mass.

One mole of atom of an element =gram atomic mass of elements or molar mass of elements.

                     =6.022×10^23.

Mole and Molar mass:-

       1 Mole Molecules of a substance= Gram molecular mass of substance or Molar mass of substance.

                    =6.022×10^23 molecules of substance.

One mole of a gas at STP ( Standard temperature pressure) occupies 22.4 litres i.e a mole represent 22.4 litre of a gas at STP (standard pressure=76 mm of Hg) & standard temperature=0°C=273°K.

Thus a mole is the chemist counting unit.

Percentage composition:-

It is calculated from the Molecular formula of the compound.

   It is calculated for element in a compound.

Percentage mass of the element in the compound=total mass of the element/Molecular Mass × 100.

Key Formula:-

Number of moles (n) = Mass of element (m) / Molar mass (M).

Number of Moles (n) = Given number of atoms (N) / Avogadro Number (No/NA).

Mass of element;m = n × M.

Number of particles of element,N = n × No.