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ATOMS AND MOLECULES class IX

LAW OF CHEMICAL COMBINATION : The  chemical reaction between two or more substances giving rise to   products is governed by certain laws. These laws are called laws of chemical reaction ,

They are a) law of conservation of mass  b) Law of constant  proportion c)Law of multiple proportion  d)Law of reciprocal proportion  e) Gay Lussac’s law of combing volume

LAWOF CONSERVATION OF MASS: Mass can neither be created nor can be destroyed in yht chemical reaction this law can be understood ” During the chemical reaction total mass of reactant will be equal  to the total mass of the product ”

A+ B ——-> AB  ie mass of A + mass of B = mass of AB

2 Na + Cl2 ——-> 2 NaCl  ( Mass of Na + Mass of Cl = Mass of NaCl) let us suppose  5gm of Na  react with  6gm of Cl it gives 11gm of NaCl .

LAW OF CONSTANT PROPORTION : A  pure chemical compound always contain the same element combine together in same proportion by mass .

Example  18 gm of H2O = 16 gm of O2 + 2 gm of H2  ratio of mass of H2 ; mass of  O2 = 2:16=1:8 by mass ,if oxygen is 32 gm than it requires 4 gm of hydrogen to form 36 gm of water.

DALTONS ATOMIC THEORY : Itis based on the laws of chemical combination, It provides the explanation  for law of conservation of mass  and law of constant composition  The postulates of the theory are a) All matter is made up of very small tiny particles called atoms b) Atoms are indivisible particles which  can neither be created nor can be destroyed in chemical reaction (law of conservation of mass) c) Atoms of the element have identical mass and chemical properties d) Atoms of different elements have different mass and properties e)Atoms combine in the ratio of small whole number to form compound (Law of constant proportion ) f) Relative number and kinds of atoms are constant in given compound .

ATOM ; Itis the smallest particle of an element which takes part in chemical reaction such that it maintain it’s identity through the  chemical or physical changes . They are so small that cannot be seen through naked eye/ powerful microscope. Radius of smallest atom hydrogen is .037 nm

ATOMIC MASS : The mass of the atom of an element is called its  atomic mass. It is defined as atomic mass unit which is defined as the quantity of mass equal to an atom of mass of carbon -12  1amu = 1/12 mass of carbon atom  C-12

1amu = 1.66 x10-27 kg

Element name/ symbol / atomic mass / valency : i) Hydrogen/ H / 1/+ 1  ii) Helium /He /4 / zero iii) Lithium / Li/ 7/+1 iv) Beryllium/ Be/9/+2   v)Boron/B/11/+3   vi)carbon/C/12/+4  vii) Nitrogen/N/ 14/ -3  viii) oxygen / O/16/ -2  ix) Fluorine /F/19 /-1 x) Neon/Ne/2o/ zero. xi) sodium / Na/ 23/+1  xii) Magnesium/Mg/ 24/ +2 xiii) Aluminium/Al/27/+3  xiv) Silicon/Si/28/ +4  xv) Phosphorous/P/ 31/+5 or -3  xvi) Sulphur/S/32 /-2  xvii) Chlorine /Cl/35/ -1  xviii) Potassium/K/39/+1   xix) Calcium /Ca/ 40/ +2 xx) Iron /Fe/ 56/ +3

HOW DO ATOM EXIST : Atom of most of the elements are very reactive  and does not exist in free state only the atoms of noble gases ( He, Ne, Ar , Kr ,Xe and Rn) are chemically inactive therefore exists in free state as  monotonic atoms of all other elements combine together to form molecules or ions . Mostly atom are electrically neutral in which number of electron is equal to number of proton . It transfer of electron takes place it form the ions when atom loses the electron it is +ive charge known as cation and if atom gains the electrons becomes -ive charge called anions.

MOLECULE  : A molecule is group of two or more atoms which are chemically bonded with each other  ,it is the smallest particles of matter ( expect element ) which is capable of an independent existence and show all properties of substance  eg  H,O is the smallest particle of water which show all the properties of water ,a  molecules may have atom of same /different elements depending  upon this , molecule can be categorized into two categories: Homoatomic molecule (having  atoms of same element )  H2, O2,N2, S8 Heteroatomic molecules compounds ( compounds contain different atoms ) H2O, NH3, CO2.

ATOMICITY :  The number of atoms present in one molecule of an element is called its atomicity .

Monoatomic have single atoms : He, Ar , Ne

Diatomic having two atom  O2  , H2 ,

Polyatomic : P4  , O3

CHEMICAL FORMULA : It is a symbolic representation of the composition of compounds.To form the chemical formula  the valencies/charge on ions must be balance  when compound is formed  metal and non metal comes first  as Cao . when poly atomic ions are formed are used , the ions are enclosed  writing the number to show the ratio  Ca (OH)2.

MOLECULAR MASS : It is the sum of atomic masses of all atoms in one mole  example  H2O molecular mass = 2x  atomic mass of hydrogen + atomic mass of oxygen =2×1 + 16 = 2+16= 18u

FORMULA MASS UNIT :  It is the sum of atomic masses of ions and atoms present in formula for a compound . NaCl =  Atomic mass of Na + atomic mass of chlorine = 23 + 35.5= 58.5

formation of formula : i) Hydrogen sulphide H  : S = Valency 1;2 = formula H2S      ii)  for carbon di oxide  C;O= valency 4; 2 =valency 2:1 = formula CO2iii)   for  Hydrochloric acid H: Cl= valency 1:1 = formula HCl  iv) for carbon tetra chloride C;Cl = valency 4:1 = formula CCl4   v) For magnesium chloride  Mg: Cl =valency 2:1 = formula Mg Cl2

IONS : An ion may be defined as an atom or group of atoms having positive or negative  charge

positive charge = Na^+, K^+ , Ca^2+ negative charge Cl^- , S^2- , SO4^2-

Chemical formula :Na : CO3 = valency 1 :-2=  formula  Na2CO3 (Sodium carbonate)

Aluminium  Sulphate = Al: SO4 = valency 3 : -2 = Al2(SO4)3

MOLAR  MASS : The molar mass of substance  is the mass of one mole of substance it is equal to  6.023×10^23 atoms of that elements.

Atomic mass of hydrogen is 1 u which is equal to molar mass 1g/mol in thxe same way molar mass of nitrogen atom is 14gm /mol ,molar mass of S8 is 32×8 =256gm/mol

MOLE CONCEPT : A group of atoms/ molecule/ions  contain 6.023 x10^23 particles in one mole of that substances.

1 mole atom =6.022x 10^23 atoms/ 1 mole of  molecules = 6.022×10^23 molecules

6.022×10^23 is called Avogadro number ,one mole of substance has mass equal to gram atomic mass of the element .

IMPORTANT  FORMULAE a)  number of mole =  mass of substance / molar mass of the substance= m/M    b) number of moles = given number of particles/ avogadro’s number

example 1 : calculate no of iron atoms in piece of iron of 2.8 gm

first calculate no. of moles  =mass of substance/ molar mass of substance= 2.8/56=1/20=.05

for calculating no. of atom  = no of moles x Avogadro’ s number =.05×6.022×10^23=3.011×10^23

example 2 : Mass of one molecule of a substance is 5.33×10^-23 than for molar mass multiply the mass of one molecule to Avogadro’s number ie 5.33×10^-23 x6.022×10^23=  32gm/mol

 

 

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