Chapter # 02 [Chemical Combinations]

UNIT# 02

ATOMIC STRUCTURE

Structure of an atom was more complicated to understand for the scientists for many years. From 1808, many scientists explained the structure of the atom but finally, it was accepted in 1913.

Scientists played a functional role in the description of atom. As it includes further particles termed as Electron, Proton and neutron. They are named so, because of the charges they contain.

State Dalton’s atomic theory.

DALTON’S THEORY [1808]

IMPORTANT POSTULATES  

Following are the main postulates of Dalton’s atomic theory:

1.     All elements are made up of small indivisible particles and indestructible particles called atoms.

2.     All atoms in a single element always identical in all respect having same size, mass and chemical properties.

3.     When atoms of more than one element combine in a simple whole ratio, compounds are formed.

4.     Atoms are neither created nor destroyed during chemical reaction.

 

State the defects of Dalton’s atomic theory.

DEFECTS OF DALTON’S THEORY [1808]

●      According to Dalton’s atomic theory, atom is indivisible, indestructible particles but according to modern atomic theory atom can be sub divided into electron, proton and neutron

●      According to Dalton’s atomic theory, different atoms of same element are identical in all respects (having same size, mass and chemical properties) but contrary isotopes of elements differ in mass.

State Modern atomic theory.

MODERN ATOMIC THEORY

Modern atomic theory failed the Dalton’s atomic theory by providing the following facts:

1.     Atom is the complex organization; even composed of smaller particles called electron, proton and neutron.

2.     Atom cannot have the same size, mass and chemical properties; this point was failed by the discovery of isotopes.

3.     Compounds are formed when atoms of more than one element combined in a simple whole number ration.

4.     Atom can neither created nor destroyed during any chemical reaction.

Describe the construction and working of Discharge tube.

DISCHARGE TUBE EXPERIMENT [DISCOVERY OF ELECTRON]

INTRODUCTION

The fundamental negative charged sub particle was discovered by J.J. Thomson in 1897.

CONSTRUCTION

The discharge tube consists of a glass tube, fitted with two metal electrodes connected to a high voltage source and a vacuum pump.

WORKING

When the tube is evacuated and a current of high potential is passed between the electrodes at very low pressure (1mm of Hg), streaks of bluish light extending from negative electrodes (cathode) towards positive electrodes (anode). The rays appear to travel in a straight line. These rays were deflected towards the positive plate in an electric and magnetic field. The high voltage is slowly increased till a charge was observed.

CONCLUSION

The name electron was given to these rays because of containing negative charge. 

 

Give the properties of Cathode rays.

PROPERTIES OF CATHODE RAYS

By performing the discharge tube experiment, following properties of the electron came to observe.

1.     They travel in straight line from cathode to anode.

2.     They are negatively charged particles.

3.     These rays can cause a light paddle wheel to rotate.

4.     They possess kinetic energy.

5.     They produce florescence.

6.     Charge / mass ratio is same as electron.

7.     These rays deflected towards positive plate, when passed through electric and magnetic field.

 

Describe the experiment for the discovery of Proton.

DISCOVERY OF PROTON [GOLDSTEIN EXPERIMENT]

Discovery of proton was made possible by the similar instrument used in the discovery of electron i.e. Discharge tube, Goldstein confirmed the identity of proton but the properties of proton was studied by J.J Thomson.

EXPERIMENT

Discharge tube was filled with hydrogen gas while cathode was perforated. Goldstein observed light when high voltage is applied at low pressure.

CONCLUSION

It was concluded that light was observed due to the ionization of gas. As the rays were moving towards anode, so Canal rays (positive rays) were named.

Give the properties of Canal rays OR positive rays OR Proton.

PROPERTIES OF CANAL RAYS / POSITIVE RAYS / PROTON

1.     These rays travel in straight line from anode to cathode.

2.     They have positive charge.

3.     Charge / mass ratio is smaller than electrons.

4.     The mass of proton is 1836 times greater than electron.

5.     The charge on proton is 1.602 X 10^-19 coulomb.

 

Describe the discovery of neutron.

DISCOVERY OF NEUTRON

In 1932, James Chadwick another researcher discovered the third sub-atomic particle i.e. neutron through artificial radioactivity. He bombarded alpha particles on Beryllium which results in penetrating radiations. Chadwick suggested that these radiations were due to material particle with mass comparable to hydrogen atom, having no charge. So, It was found that it has no charge but mass is same as mass of proton.    

₄Be⁹ + ₂He⁴ →        ₆C¹² + ₀n¹

 

Give the properties of Neutron.

PROPERTIES OF NEUTRON

1.     It is neutral particle.

2.     Its mass is 1.76 X 10^-27 Kg.

3.     It has no charge.

4.     They have penetrating ability in matter.

Compare electron, proton and neutron.

Comparison between electron, proton and neutron
Property	Electron	Proton	Neutron
Discovery	J.J. Thomson	Goldstein	Chadwick
Charge	Negative	Positive	Neutral
Mass	9.1 X 10-31Kg	1.6727 X 10-27 Kg	1.6750 X 10-27 Kg
Symbol	e	P	N
Location	Orbit	Nucleus	Nucleus

 

Define radioactivity. Explain the experiment to show the discovery of radioactive rays.

RADIOACTIVITY

The phenomenon in which invisible radiations excrete from the radioactive elements is called radioactivity and the substance is called radioactive element.

FOR EXAMPLE

●      Uranium

●      Polonium

●      Thorium

TYPES OF RADIOACTIVE RAYS

EXPERIMENT

Rutherford took lead box and placed radioactive substance in it. Photographic plate was placed in front of the box. Radioactive substance resolves into three different radiations on the basis of their movement.

Give the properties of alpha, beta and gamma rays.

TYPE I [ALPHA RAYS]

1.     These rays deflected by electric and magnetic field.

2.     They contain double positive charge

3.     They cannot pass from matter

TYPE II [BETA RAYS]

1.     These rays deflected towards the negative plate, thus named beta rays

2.     They contains negative charge

3.     They can pass up to 1-2 m from matter

TYPE III [GAMMA RAYS]

1.     These rays deflected by electric and magnetic field.

2.     They do not contain any charge

3.     They have high penetration ability

 

Give the properties of alpha, beta and gamma rays.

 

Comparison between alpha, beta and gamma rays
Property	Alpha rays	Beta rays	Gamma rays
Ionizing power	Highest Ionization Power	Weaker Ionizing Power Than Alpha	Weakest Ionization Power
Penetration	Lowest Penetration	A Few Cm Penetration Ability	Highest Penetration
Speed	0.1 C	0.9 C	C
Motion in electric and magnetic fields slight deflection	Slight Deflection	Large Deflection	No Deflection

 

Explain Rutherford’s atomic model.

RUTHERFORD ATOMIC MODEL

EXPERIMENT

In 1911, Rutherford passed the beam of the alpha particles (Polonium Po) through a very thin gold metal foil. He founded that most of the alpha particles passed through it without any deflection. However, some of them deflected at large angles and very few of them bounced back.                    

CONCLUSION

1.     According to Rutherford, atom consists of two parts i.e. nucleus and extra nuclear part.

2.     Most of the portion of atom is empty called extra nuclear part.

3.     Positive charge in an atom is present in the center, known as Nucleus.

PREDICTION

He predicted about the structure of atom that atom consists of two major parts:

1.     Nucleus which consists of proton and neutron.

2.     Extra nuclear part. This includes the revolving electron around the nucleus.

3.     Electrons revolving around the nucleus in circular paths with high speed (Velocity).

4.     An atom is electrically neutral i.e. it has equal number of electron and proton.

5.     Size of nucleus is very small as compared to the size of atom.

 

Give the weakness or defects of Rutherford’s atomic model.

DEFECTS OR WEAKNESS IN RUTHERFORD ATOMIC MODEL

On the atomic model of Rutherford, few scientists claimed that:

1.     If electron is revolving around the nucleus constantly so it should falls in the nucleus.

2.     If electron emits energy than it should form continuous spectrum but in actual line spectrum is produced.

Explain Neil Bohr’s atomic model.

NEIL BOHR’S ATOMIC MODEL

In 1913, Neil Bohr proposed that the theoretical explanation of line spectra. His assumptions are mentioned below:

1.     Electron in an atom move only in certain energy levels so an electron in an allowed energy state will not radiate energy continuously and therefore will not fall in the nucleus.

2.     Electron only release and gain energy when it jumps from higher to lower energy level or vice versa.

·        When electron jumps from higher energy level to lower energy level, it emits energy.

·        When electron jumps from lower energy level to higher energy level, it absorbs energy.

3.     When electron release (dissipate) energy, it jumps back to lower energy level because of such movement, color is observed on screen called line spectrum

4.     Quantum energy is directly proportional to the frequency of radiation i.e. ∆E = E₂ – E₁

 

LIMITATIONS OF BOHR’S ATOMIC MODEL

·        Bohr’s atomic model failed to explain the Zeeman Effect (effect of magnetic field on the spectra of atoms)

·        Bohr’s model also failed to explain the Stark effect (effect of electric field on the spectra of atoms)

·        It deviates the Heisenberg Uncertainty principle

·        It could not explain the spectra obtained from larger atoms.

·        It explained the mono electronic species like H⁺¹, Li⁺², B⁺³.

 

Describe Debroglie Hypothesis.

DEBROGLIE HYPOTHESIS

In 1923 Lois De Broglie proposed a hypothesis that all matters have particle as well as wave nature at the submicroscopic level.

De Broglie combined the Einstein and Planck equations and suggested that if;

E=h ט

Where E = energy, h = Plank’s constant, ט = frequency of light

And E = mc²

Where E = energy, m = mass, c = speed of light

Then

h ט = mc² OR γ = mc²/h

ג= h/mγ

The wave nature of a particle is quantified by De Broglie wavelength defined as λ=h/p where p is the momentum of the particle.

Explain Schrödinger Atomic Model.

SCHRODINGER ATOMIC MODEL [QUANTUM MECHANICAL MODEL]

Schrodinger or quantum mechanical model determines that electron position of electron around the nucleus.

Schrodinger found electrons are in orbit as an electron cloud. Each energy subshell in an orbit has different shapes which determine the presence of electron. Different subshells of orbitals are named as s, p, d and f with different shapes as ‘s’ is spherical and 'p' is dumbbell shaped.

 

What is Fuzzy cloud?

FUZZY CLOUD

According to quantum mechanical model probability of finding an electron within certain volume of space surrounding the nucleus can be represented as a fuzzy cloud.

What do you know about electronic configuration?

ELECTRONIC CONFIGURATION

Electron revolves around the nucleus in various circular orbits called shell or energy level or orbit.

The distribution of electron in different energy level is called electronic configuration.

Shells of an atom are as following with quantum number 1,2,3,4,5,6 respectively;

K= 2

L =8

M= 18

N= 32

O=50

P= 72

S, p, d and f are sub shell which contains number of electrons as follows:

s=2,

p=6,

d=10,

f=14

 

K shell	S orbital
L shell	S and p orbital
M shell	S, p and d orbital
N shell	S, p, d and f orbital

 

Describe the electronic configuration of first 18 elements.

ELECTRONIC CONFIGURATION OF FIRST 18 ELEMENTS

The distribution of electrons among the different orbits/shells and subshells according to some rules is known as the electronic configuration of an atom'.

The maximum number of electrons in a shell is represented by the formula 2n², where 'n' is the shell number.

The distribution of electrons in different orbits is as follows:

K-shell/ 1 orbit (n=1) = 2(1)² = 2

L-shell/ 2 orbit (n=2) = 2(2)² = 8

2 M-shell/ 3 orbit (n=3) = 2(3)² =18

2 N-shell/4 orbit (n=4) =2(4)² =32 and so on

 

Element	Symbol	Atomic number	Electronic configuration
Hydrogen	H	1	1s1
Helium	He	2	1s2
Lithium	Li	3	1s2, 2s1
Beryllium	Be	4	1s2, 2s2
Boron	B	5	1s2, 2s2, 2p1
Carbon	C	6	1s2, 2s2, 2p2
Nitrogen	N	7	1s2, 2s2, 2p3
Oxygen	O	8	1s2, 2s2, 2p4
Fluorine	F	9	1s2, 2s2, 2p5
Neon	Ne	10	1s2, 2s2, 2p6
Sodium	Na	11	1s2, 2s2, 2p6, 3s1
Magnesium	Mg	12	1s2, 2s2, 2p6, 3s2
Aluminum	Al	13	1s2, 2s2, 2p6, 3s2, 3p1
Silicon	Si	14	1s2, 2s2, 2p6, 3s2, 3p2
Phosphorous	P	15	1s2, 2s2, 2p6, 3s2, 3p4
Sulphur	S	16	1s2, 2s2, 2p6, 3s2, 3p4
Chlorine	Cl	17	1s2, 2s2, 2p6, 3s2, 3p5
Argon	Ar	18	1s2, 2s2, 2p6, 3s2, 3p6

 

What is Isotopes? Write note on Isotopes of Hydrogen.

ISOTOPES

Those elements which have same charge number but different mass number are called Isotopes.

OR

Atoms of an element having same number of protons but different number of neutrons are called isotopes.

 

ISOTOPES OF HYDROGEN

There are three isotopes of hydrogen.

1.     Protium

2.     Deuterium

3.     Tritium

PROTIUM

It has one proton and no neutron in the nucleus.

It is used in ordinary water.

DEUTERIUM

It has one proton and one neutron in the nucleus.

It is used in heavy water.

TRITIUM

It has one proton and two neutrons in the nucleus.

It is used in crackers. It is also known as radioactive isotope of Hydrogen.

 

Describe the isotopes of Oxygen.

ISOTOPES OF OXYGEN

ISOTOPE	ELECTRON	PROTON	NEUTRON
8O16	8	8	8
8O17	8	8	9
8O18	8	8	10

 

Describe the isotopes of Uranium.

ISOTOPES OF URANIUM

ISOTOPE	ELECTRON	PROTON	NEUTRON
92U234	92	92	142
92U235	92	92	143
92U236	92	92	144

 

Describe the isotopes of Carbon.

ISOTOPES OF CARBON

ISOTOPE	ELECTRON	PROTON	NEUTRON
6C12	6	6	6
6C13	6	6	7
6C14	6	6	8

Describe the isotopes of Chlorine.

ISOTOPES OF CHLORINE

ISOTOPE	ELECTRON	PROTON	NEUTRON
17Cl35	17	17	18
17Cl37	17	17	20

 

Write applications of following isotopes.

APPLICATIONS OF ISOTOPES

 

S#	Radioactive Isotope	Field	Uses
1	Phosphorous- 32 Strontium- 90	Radiotherapy	Treatment of skin cancer
2	Cobalt-60	Radiotherapy	Treatment of body cancer
3	Iodine isotopes	Radiotherapy	Detestations of thyroid glands
4	Technetium radiotherapy	Radiotherapy	To monitor bone growth in fracture healing
5	Gamma ray of Cobalt-60	Medical instrumentation	To sterilization of medical instruments and dressings from harmful bacteria.
6	Americium-241	Safety measures and industries	Used in black scatter gauges, smoke detectors
7	Gold-198 and Technetium-99	Sewage and liquid waste movement for water pollution	Tracing factory waste causing ocean pollution and tracing sand movement in rivers and oceans.
8	Uranium 235	Power generation	Conversion of water energy from steam to generate electricity.
9	Plutonium-238	Medicine	Used to stimulate a regular heart beat in heart pace maker.
10	Carbon-14	Archeology and geology	Used to estimate the age of fossils.