Chemistry Podcast

Showing posts with label 09th Class Science Notes. Show all posts
Showing posts with label 09th Class Science Notes. Show all posts

Wednesday, 26 January 2022

9 Class- Periodic Table of Elements

Periodic Table of Elements

More than 115 elements have been discovered which need to be arranged systematically, first elements were classified as metals and non metals.
=> 9th 10th Class Science - Periodic Table of Elements Podcast-

History of classification of elements

1.     Prout’s Hypothesis – 1815
2.     Dobereiner’s Triads – 1829
3.     Newland’s Law of Octaves – 1863
4.     Lother Meyer Curves – 1869
5.     Mendeleev’s Periodic Table – 1869
6.     Modern Periodic Laws of Moseley -1913
7.     Modern Periodic Table ( of 18 columns ) 

 Prout’s Hypothesis

                                    According to this hypothesis atomic mass of elements is multiple of mass of hydrogen atom. So elements are related to each other on the basis of their atomic masses. It fails due to the reason that some elements do not have atomic mass in whole no. (Like chlorine have atomic mass of 35.5).

Dobereiner’s Triads

                                    According to Dobereiner’s triads elements arranged in the group of three elements so that atomic mass of central element is appropriate mean of 1st and 3rd element. These groups of three elements are known as Dobereiner’s triads.
 Li                Na                  K
Atomic Mass                                              
7                 23                  39
Mean of Atomic Masses of 1st and 3rd elements  is (7+39)/2  =   46/2   =  23
Drawback of Dobereiner’s triads : All elements are not arranged in triads.

Newland’s Law of Octaves

According to Newland’s Law of Octaves elements arranged in the group of 8 elements and every 8th element is similar to the 1st element.
Li         Be         B        C          N         O           F
Na        Mg       Al        Si         P          S          Cl 

Drawback of Newland’s Law of Octaves :

Heavy elements do not follow this law, inert gases disturb the order of octave.

Lother Meyer Curves

                                       In 1869, Lother Meyer derived following conclusion by obtaining information from graph between the atomic weight at x-axis and atomic volume at y-axis.
1.     Elements which have similar properties found on the same position on the graph curves.
2.     All alkali metals like Li, Na, K, Rb, Cs, and Fr found at highest peak of curve.
3.     Halogens like F, Cl, Br and I are found on ascending portion of curve.
4.     Alkaline earth metals like Mg, Ca, Sr, Ba and Ra are found on descending portion of curve.
5.     Elements which do not melt easily like Be, B, C, Al, Si, Cu etc. are found at lowest point of curve.

On the basis of above conclusions Lother Meyer give periodic Law:
                                                                                                  According to Lother Meyer periodic law – “Atomic volume of elements are periodic function of their atomic weights”.

Periodic Table

                        To classify elements a chart is prepared in which elements are arranged in rows and columns. These elements are arranged in table in such a way that the elements with similar properties are repeat after some intervals; also elements with similar properties are put in same column below one another. In periodic table vertical columns are called group while horizontal rows are called periods. In periodic table this repetition of properties in regular interval is known as periodicity. Read below chemistry notes of periodic table like Mendeleev periodic table, Mosley periodic table, Modern periodic table etc.

Mendeleev’s Periodic Table

                                                In Mendeleev’s Periodic Table, Mendeleev classify elements according to their atomic masses and arranged these elements in table according to their increasing order of atomic masses.

Mendeleev’s Periodic Law

                                                According to Mendeleev’s Periodic Law – “Physical and chemical properties of elements are periodic function of their atomic masses”.
Mendeleev’s Periodic Table contains seven horizontal rows known as periods and nine vertical columns known as groups.

Groups of Mendeleev’s Periodic Table

1.     In original Mendeleev’s Periodic Table only 8 groups present because Zero group contains noble gases is added later after discovery of noble gases.
2.     So, total vertical column is 9 including group 1 to 8 and Zero group.
3.     Group 1 to 7 are divided into two subgroups (A and B) each.
4.     In eighth group, three elements found together in each period (from fourth period onward) known as transition triplet.

Periods of Mendeleev’s Periodic Table

1.     Mendeleev’s Periodic Table has seven periods.
2.     First period is known as very short period because contain only two elements.
3.     Second and Third period is known as short period because contain only 8 elements each.
4.     Fourth and Fifth period is known as long period because contain 18 elements each.
5.     Sixth period is known as very long period because contain 32 elements each, 18 elements in table and remaining 14 elements called Lanthanides (from atomic no. 58 to 71)
6.     Seventh period is known as incomplete period because it contains only 24 elements and some places are left vacant for elements as they are not known yet. 10 elements in the table and remaining 14 elements called Actinides (from atomic no. 90 to 103) placed outside at bottom of periodic table.

Demerits of Mendeleev’s Periodic Table

These below points are some Demerits of Mendeleev’s Periodic Table
1.     Position of Hydrogen: Hydrogen is placed in 1st as well as in 7th group because its properties are similar to both groups, which is not explained well.
2.     Elements which have similar properties are placed apart in different groups.
Example: Copper and Mercury with similar properties placed in group first and second.
3.     Elements with different properties are placed in one group.
Example: Coin metals like Li, Na, K, Rb, Cs.
4.     Atomic weights are not kept in serial order.
Example: Argon with higher atomic weight 39.948 is placed before potassium, which have lower atomic weight of 39.102
5.     Position of isotope is not proper because all isotopes of elements are placed in one group though the atomic weights of isotopes are different.
6.      Eighth group elements position is not appropriate because elements put in triplet form.
7.     Rare earth elements (Lanthanides and Actinides) position is not proper so they placed outside at bottom of periodic table.

Modern Periodic Law and Modern Periodic Table

                                                                                   Moseley in 1913, after doing many experiments comes to a conclusion that the elements should be arranged as per their atomic no. and not according to atomic masses. As atomic no. is fundamental property of all elements of periodic table. So Mosley gives a new law which is called is Modern Periodic Law.

Modern Periodic Law

                                         According to Modern Periodic Law- “Physical and Chemical properties of elements are periodic function of their atomic numbers”.

Long Form of Periodic Table

                                                     Bohr Bury invented new periodic table, which is called as Long Form of Periodic Table. This Long Form of Periodic Table is also known as Modern Periodic Table.

Special features of Modern or Long Form of Periodic Table

These below are Special features of Modern or Long Form of Periodic Table
1.     18 vertical columns known as groups.
2.     Horizontal rows known as periods.
3.     Light metals – These are elements of periodic table of group 1 and 2.
4.     Heavy metals or Transition metals - These are elements of periodic table of group 3, 4, 5, 6, 7, 8, 9, 10, 11 and 12.
5.     Non-Metals – These are elements of periodic table of group 13, 14, 15, 16 and 17.
6.     Zero group – These are elements of periodic table of group 18.

Properties of periods

1.     All periods start with alkali metal and end with noble gases.
2.     If we see table, it is very clear that from 1st to 7th period there is an addition of one shell in the each period like n=1,2,3,4,5………………

Properties of groups

1.     In any group, outermost shell electron are known as valance electrons and these electrons are same so main properties of elements of group is similar.
2.     Elements are divided into four blocks, which is s, p, d, f   according to valance electrons.
3.     s-block elements – elements of 1 and 2 group.
4.     p-block elements – elements of 13 to 18 group.
5.     d-block elements – elements of 3 to 12 group.
6.     f-block elements – elements of the Lanthanide and Actinide series.
7.     Representative Elements – elements of s-block and p-block collectively called as Representative elements also known as Normal elements or Typical elements.
8.     Transition Elements – elements of d-block.
9.     Inner Transition Elements – elements of f-block, also known as Rare Earth Elements.
10.                         Alkali Metals – elements of 1st group.
11.                         Alkaline Earth Metals – elements of 2nd group.

Solution for the demerits of Mendeleev’s Periodic Table in the Long Form of Periodic Table (or Modern Periodic Table)

1.     Mendeleev’s Periodic Table contains some heavy elements placed before lighter elements, this problem is solved automatically in Modern Periodic Table
2.     All isotopes of an element have same atomic number so no need to place them separate.
3.     All inert gases (or noble gases) got their appropriate place in zero group (or 18th group).
4.     Mendeleev’s Periodic Table have problem with 8th group but it is resolved in Modern Periodic Table as it is divided into three groups.

Drawback of Modern Periodic Table

1.     Position of Hydrogen is still not clear in Modern Periodic Table.
2.     Position of f-block elements (Lanthanides and Actinides) is not proper as these placed at bottom, separately from periodic table that is impractical.

Periodic Properties of the Elements

Periodic properties are the properties of the elements which depend on their electronic configuration and these properties changes on moving down in the group and on moving left to right in the period of the periodic table. Some periodic properties are atomic size, electron affinity, ionization energy, metallic and non metallic properties etc. are the main periodic properties.

Atomic Size

                        Size of any atom is explained with the help of atomic radius.

Atomic Radius

                        The distance between outermost shell electrons and centre of nucleus of isolated atom is called atomic radius.

There are two hypotheses for the calculation of atomic radius i.e.

1.     Van der Waals Radii

It is the one half of the distance between the two adjacent atoms nuclei centres, is known as Van der Waals radii.

Van der Waals radii is calculated as follows, first calculate the distance from the centre of nuclei of one atom to centre of nuclei of another adjacent atom in solid state of same substance. Then divide this distance by two. Van der Waals distance is represented by Angstrom A.

Where, 1 A = 10-8 cm.

2.     Covalent Radii

It is the one half of the distance between the diatomic molecule nuclei centres, is known as Covalent Radii.

Covalent radii is calculated as follows, first calculate the distance from the centre of nuclei of one atom to centre of nuclei of another atom in diatomic molecule containing covalent single bond. Then divide this distance by two.

  Isolated Atom

                        Isolated atom is an absolutely alone single atom.

  Periodicity in Atomic Size

  In Period: On moving left to right in any period atomic size decreases because atomic radii decreases, As electrons are attracted towards nucleus due to increasing nuclear charge.

  In Group: On moving down in any group atomic size increases because atomic radii increase, as more electrons shells added.

  An ionic radius of cation is smaller than its normal atom while that of anion is larger than its normal atom.

  Cations are formed by loss of electron or electrons and carry positive charge.

  Anions are formed by gain of electron or electrons and carry negative charge.

  Ionization Energy

                                    The energy required for the removal of one electron from the isolated atom in gaseous state is called ionization energy.

  Ionization Energy is represented by I.E.

  Ionization Energy is expressed in Electron Volts per Atom or Kilo Joules per Mole or Kilo Calorie per Mole. Ionization Energy is also known as Ionization Potential.

  Equation for I.E.

  Atom (g) + Ionization Energy (I.E.) ---à Cation + Electron (e-

  Removal of further electron from cation is difficult so value of Ionization Energy kept on increasing.

  I.E.1 < I.E.2 < I.E.3 < I.E.4 …………………………………..

  Periodicity in I.E. (Ionisation Energy)

  In Period: On moving left to right in any period ionization energy increases because nuclear charge increases with the increasing atomic number, so more energy required to remove electron. Therefore value of I.E. increases on moving left to right in any period.

  In Group: On moving down in any group ionisation energy decreases because atomic size increase, as more electrons shells added.

  Electron Affinity

                                    The amount of energy release as a result of addition of electron in any atom in gaseous state to form anion is known as Electron Affinity.

  Electron Affinity is represented by E.A.

  Electron Affinity is expressed in Electron Volts per Mole or Kilo Joule per Mole

  Values of all electron affinity are negative except first Electron Affinity which is positive.

  Periodicity in E.A. (Electron Affinity)

  In Period: On moving left to right in any period the value of Electron Affinity increases because atomic size decreases so amount of energy released is more.

  In Group: On moving down in any group the value of Electron Affinity decreases with some irregularities.

  Metallic and Non-Metallic characters

 In Period: On moving left to right in any period Metallic character decreases and Non Metallic character increases.

  In Group: On moving down in any group Metallic character increases and Non Metallic character decreases.

  In Periodic Table: Metallic elements are found on left hand side and Non Metallic elements are found on right hand side.


                  The elements which are neither metals nor non metals are known as metalloids.

  Example of Metalloids: B, Si, As, Te etc.

Wednesday, 12 January 2022

9 Class- Atomic Structure

Atomic Structure

What is an atom?

In this chapter we learn about atom, atomic structure, discovery of electron proton & neutron and many more interesting facts about atom.

Matter found around us in nature (like iron rod, glass, cup, pen, pencil etc.) is made up from very small particles, which is known as atoms.
we also says that "atoms are very small particles which made matter" so atom is fundamental unit of atom.

Greek Philosopher, Democritus describe atom as very small indivisible particle.

=> Listen This Basic Chemistry Podcast on Atomic Structure

'New System of Chemical Philosophy' is a book about atom written by John Dalton in 1703.

Dalton Atomic Theory

According to this theory atom cannot be divided, but after some years it is proved that atom can be divided further by many scientists like J.J.Thomson, L.Rutherford, N.Bohr, Chadwick etc.

Constituting particles of atom

Atom made up from 3 main particles-
1. Electron 2. Proton 3. Neutron

Discovery of Electron

Sir J.J.Thomson and W.Crooks did many experiments with discharge tube for the discovery of electron.
Discharge tube have tube like shape made from glass with two electrodes (Cathode -ve and Anode +ve) in vacuum created by vacuum pump connected to discharge tube. High electric potential is applied between two electrodes.
9th class chemistry notes of structure of atom - Discharge Tube 
Discharge Tube
Air is bad conductor of electricity so vacuum pump is connected to reduce pressure to 0.02mm inside discharge tube currents starts flowing between electrodes and light is emitted. On further reducing pressure in discharge tube greenish yellow color fluorescence occur. As these rays emerging from cathode, Sir J.J.Thomson named them as cathode rays.
Deflection of cathode rays towards positively charged plate in electric field proves that these rays carry negatively charged particles.
These negatively charged particles are named as electrons.

Properties of Cathode Rays

1. Cathode rays always travel in straight line.
Production of Cathode Rays
2. Velocity of cathode rays and velocity of light are approximately equal.
3. On applying electric field in the path of cathode rays, cathode rays turn towards +vely charged plate that proves cathode rays are made up from negatively charged particles.
4. Cathode rays rotate light wheel placed in their path that proves cathode rays are made from particles having mass.
5. Cathode rays pass through thin metal foil and it gets slightly heated up by action.
6. These rays produce fluorescence at walls of glass tube.
7. Cathode rays ionize gases and also affect photographic plate.
8. When these rays strike any metal with high melting point (like tungsten W) they produces X-Rays.

What is X-rays?

X-rays are electromagnetic radiation , X-rays was discovered by W.K.Roentgen in 1895. X-rays are also known as Roentgen rays.

Why X-rays are used in medical sciences?

X-rays are used in medical sciences because x-rays have high penetrating power.

Determination of charge to mass ratio of electron (e/m)

J.J.Thomson conduct many experiments that charge to mass ratio of an electron remains same, irrespective of nature of gas and nature of cathode electrode material.
value of e/m = -1.76x108 coulombs/gram

Millikan's Oil Drop Experiment

Millikan's Oil Drop Experiment or Determination of charge of electron is conduct by an American Scientist R.A.Millikan, who perform an experiment on the charge on oil drops. R.A.Millikan perform several experiments to calculate charge on oil drops and he gets every time its value equal to -1.6x10-19 coulomb.When these results associated with results of cathode rays then conclude that charge present on particle of cathode rays is -1.6x10-19 coulomb.

Calculation of mass of electron

As we  know e/m = -1.76x108 coulombs/gram
                          e = -1.6x10-19 coulomb
              (e/m)/e = (-1.76x108)/(-1.6x10-19)
                       m = 9.102x10-28 gram
                       m = 9.102x10-31 kilogram

Mass of electron in comparison with atom

Mass of electron in comparison with atom is described below-
Mass of 1 mole of Hydrogen = 1.008gms
Number of hydrogen atom in 1 mole = 6.023x1023
Mass of 1 atom of hydrogen = 1.008/6.023x1023
                                             = 1.67x10-27kg
Mass of electron is 9.109x10-31
                         =  Mass of 1 atom of hydrogen/Mass of electron
                         = (1.67x10-27)/(9.109x10-31) = 1837
     Mass of an electron is 1/1837 th the mass of a hydrogen atom.

Discovery of Proton

As we know electron is negatively (-vely) charged particle but atom is electrically neutral so there should be some particles which have positive (+ve) charge to neutralize negative (-ve) charge electron.
In 1886, a German scientist E.Goldstein established the presence of +vely charged particles. These positively charged rays travel from anode to cathode so called as anode rays or positive rays.

Why anode rays are called as canal rays?

Anode rays passes through canals or perforation in cathode so called as canal rays.

Properties of Anode Rays

1. Anode rays always travel in straight line.
2. Anode rays rotate light wheel placed in their path that proves anode rays are made from particles having mass.Which produces mechanical action.
 3. On applying electric field in the path of anode rays, anode rays turn towards -vely charged plate that proves that anode rays are made up from positively charged particles.
Production of Anode Rays

Determination of charge to mass ratio of proton (e/m)

On the basis of many experiments performed for anode rays, scientist Wein concluded that charge to mass ratio of proton changes with change in nature of gases present in discharge tube. This value (e/m) is maximum for hydrogen gas = 9.58x104 coulomb per gram. "Positively charged particle of hydrogen is fundamental particle of matter that is called proton."

Charge on proton

Charge on proton is equal to charge on electron but is of opposite nature.
Charge on proton = 1.602x10-19 coulomb.
Mass of the proton
For hydrogen gas,
                       e/m = 9.58x104 coulomb per gram
charge of electron = 1.602x10-19 coulomb
    m = (e)/(e/m) = (1.602x10-19)/(9.58x104)
      m = 1.67x10-24 gram
      m = 1.67x10-27 kg
     mass of proton is 1837 times more than mass of electron and is equal to mass of an hydrogen atom.

Discovery of Neutron

Mass of atom is more than the mass of total proton and electron present in atom, which suggest the presence of another particle in atom which lead the discovery of neutron.
 Chadwick in 1932 discover neutral particles of mass equal to mass of proton by bombarding beryllium metal with stream of fast moving particles through cyclotron. These particle are neutral in nature so named as neutron.
Mass of neutron = 1.6748x10-27 kg

There is three fundamental particles in atom

These fundamental particles are 1. Electron 2. Proton 3. Neutron

Many atomic models are proposed from time to time to show the actual structure of atom, some of them are described below-
1. Thomson's atomic model
2. Rutherford's atomic model
3. Bohr's atomic model

Thomson's Atomic Model

Thomson's Atomic Model
Thomson's atomic model is first atomic model proposed in 1898 related to atomic structure. This model is similar to water-melon in which positive charge protons is like pulp in which negative charge electrons like seeds in water-melon, also known as pudding model. This model is called Thomson's atomic model.

Drawback of Thomson's atomic model

Thomson's atomic model is unable to explain scattering experiment of Rutherford.

Rutherford Model of Atom

This experiment is also known as alpa () particle scattering experiment. Rutherford bombard alpha particle on gold foil of thickness 0.0004cm and found that-
1. So many particles passes through the gold foil without any deflection.
2. Some alpha particle are deflected at different angles.
3. Very few about 1 in 20,000 are bounced back (at 360 degree) from gold foil.

Rutherford Atomic Model
Rutherford concluded nuclear model of atom as-
1. Most of the part of the atom is hollow and neutral so alpha particles passes straight without any deflection.
2. All positive charge is present in center of atom so alpha particle are deflected at different angles by repulsion from positively charged center known as nucleus.
3. Only very few particles bounced back so size of nucleus is very small as compared to size of atom.
4. Electron moves around nucleus like planet moves around sun in orbits.
5. Mass of electron is negligible so all mass of atom is present in nucleus.
6. Atom is electrically neutral so number of electron is equal to number of proton present in atom.

Defects of Rutherford Atomic Model

1. Stability of atom is not explained on the basis of this model.
2. This model could not explain discrete spectrum.

Bohr's Atomic Model

Neils Bohr in 1913 gives a simple model for atomic structure based on the quantum theory.
Main assumption of Bohr atomic model-
1. All atoms consist of dense, very small, positively charged nucleus that have all protons and neutrons in it.
2. Electron revolve around nucleus in definite energy paths known as orbits, shell or energy levels.
3. Orbits denoted by (n). value of n is whole number 1,2,3,4.......... etc. which are represented as K,L,M,N............. etc. respectively.
Bohr Atomic Model
4. As we increase the value of n the orbit move farther from nucleus (means distance between nucleus and orbit of higher n is more than smaller n) and their energy also increases so n=1 or K shell have lowest energy.
5. If an electron revolve in same energy level then their is no change in its energy level.
6. As electron absorb energy from outside, it gets exited and move to higher energy level and come back after emitting energy to lower energy level.
9th class chemistry notes of atomic structure
Absorption and Emission of Electron Energy

Atomic Number

Number of protons present in nucleus of atom of an element is known as atomic number.
It is denoted by (z)
example : Carbon have 6 protons so its atomic number is 6 (z = 6)
For neutral atom,
 atomic number (z) = number of protons (p) = number of electron (e)

Number of protons in cation > Number of electrons in cation
Number of protons in anion < Number of electrons in anion
Number of protons in neutral atom = Number of electron in neutral atom

Mass Number

Mass number is also known as atomic mass. Atomic number or mass number is equal to number of protons and neutrons in atom.
Atomic mass = number of protons + number of neutrons
                 A = p+n
example : Helium have two protons and two neutrons so its atomic mass is 2+2=4


Atoms of same elements which have same atomic number but different mass number is called isotopes.
example : Protium, Deutrium, Tritium are isotopes of hydrogen.

Types of Isotopes

1. Non-radioactive isotope example C(12), C(13)
2. Radioactive isotopes example U(235), U(233), U(238), U(239)


Atoms of different elements which have similar atomic mass and different atomic number are called isobars.
example : Ca(40), Ar(40), Kr(40)
                 50Cr24, 50Ti22
Isobars do not similar in physical and chemical properties

Distribution of electrons in various orbits : ( Bohr Bury  Scheme )

In 1921, Bohr and Bury gives some laws for distribution of electrons in various orbits, which are called as Bohr-Bury Scheme.
Rules of Bohr Bury scheme are as follows-
1. Maximum number of electron in any shell should be 2n2 , where n is serial number of shell.
2. Maximum number of electron that exist in outermost shell or orbit of any atom is eight while maximum number is eighteen for penultimate shell.
3. According to this law, it is not necessary that another shell is formed after completion of 1st shell. A new shell is formed as number of electrons reaches eight in any orbit or shell.

Valence Electrons and Valency

 Outermost incomplete shell of any atom is known as Valence Shell and electron present in incomplete outermost shell are known as Valence Electrons , which participate in formation of bond.
  • If atoms have 1 to 4 valence electrons then VALENCY = Number of valence electron
  • If atoms have 5 to 8 valence electrons then VALENCY = 8 - Number of valence electron
  • But these above rule is not applicable to atoms which show variable valency


Radioactivity is discovered by French scientist Becquerel in 1896. Substances which emits radiation is known as radioactive substances and this property of substances to emit radiation is called as radioactivity.
example : Uranium exhibit radioactivity.

Rutherford conduct some experiments with radioactive substances and his observations are as follows-
1. Rays which turned toward negative electrode is known as alpha rays, mass of alpha particles is 4 a.m.u.  and charge is 2 units of positive charge.
2. Rays which turned toward positive electrode is known as beta  rays, mass and charge of beta particle is equal to mass and charge of electron.
3. The rays which do not turned towards any electrode is known as gamma  rays, gamma rays are electromagnetic rays and not made from matter particle.

Penetrating Power

Gamma > Beta > Alpha
Gamma rays have maximum penetrating power while Alpha has lowest.

Tracer technique of radioactive isotopes

As per requirement, small amount of radioactive isotopes is added to its element then many physical and chemical complex reaction are studied.

Various fields of tracer techniques

  1. In the field of chemical science.
  2. In the field of medical science.
  3. In the field of agriculture.
  4. In the industrial fields.
  5. And in other scientific fields.

Carbon Dating

In this technique carbon isotope is used to determine the age of rocks, minerals, fossils, dead animals and dead plants.


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