Chemistry For Class IX (New Book ) - Chapter No. 2- Atomic Structure - Detailed Questions And Answers

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Chapter No.2-Atomic Structure
Detailed Questions And Answers

Q.1: Define Atom? Also describe the discovery of Sub Atomic Particles ( electron, proton and neutron) of an atom.
Ans: ATOM:
Definition:
By Democritus
The word atom is derived from a Greek word 'ATOMOS' means indivisible, which was first describe by Greek philosopher Democritus. Democritus belief that:

"All matter consist of very small indivisible particles which are known as atoms."

By Johan Dalton:
Johan Dalton an English school teacher and chemist suggested the fundamental atomic theory, which explain that:

"All element are made up of indivisible particles called atoms."

Discovery of Sub Atomic (Electron, Proton, Neutron) Particles Of An Atom:
Dalton assumed that no particles smaller then atom exit. Dalton’s atomic theory explains the chemical nature of matter and existence of indivisible atoms, but at the end of 19^th century sub-atomic particles were discovered by different scientists. New experiments show that:
"Atom is composed of even smaller particles which are known as sub-atomic particles.
After that these sub-atomic particles were discovered and named as electron, proton and neutrons.

1. Electron: First sub-atomic particle Electron discovered by M. Farady, William Crooks and J.J. Thomson.
2. Proton: Second sub-atomic particle Proton identified by Goldstein and Ernest Rutherford.
3. Neutron: Third sub-atomic particle Neutron revealed by Chadwick.

All of these findings were milestone in the knowledge of atomic structure which we have now.

Q.2: Describe the Thomson's experiment or discharge tube experiment for the discovery of electrons? Also write down the properties of cathode rays?
Ans: Discovery Of Electron:
Electron is the lightest particle carrying negative charge in an atom discovered by J.J.Thomson and William crooks.

Thomson's Experiment Or Discharge Tube Experiment:
In 1897 J. J. Thomson carried out an experiment for the discovery of an electron.
Discharge Tube:
The apparatus used for this type of experiment is called discharge tube which consists of glass tube fitted with two metal electrodes connected to a high voltage source and a vacuum pump.
Working:
When electrodes inside evacuated, discharge tube are connected with high voltage source at very low pressure (1 mm of Hg), as the high voltage current start passing between electrodes a streak of bluish light originate and travel in straight line from cathode (-ve electrode) to anode (+ve electrode), Which cause glow at the wall of opposite end. These rays ere called cathode rays.
Conclusion:

• J.J.Thomson justified that these rays were deflected towards positive plate in electric and magnetic field which shows that these rays possess negative charge due to this negative charge, particle was named Electron.
• These electrons were obtained from the gas in discharge tube which proves that electrons are constituent of all matter.

Properties of Cathode Rays (Electrons):

1. They travel in straight line from cathode towards Anode.
2. They produce sharp shadow of an opaque object placed in their path.
3. They have negative charge and bend towards positive plate in electric and magnetic field.
4. These rays when strike with glass and other material cause material glow.
5. The (e/m) charge and mass ratio of cathode particles is 1.7588 x 10⁸ coulomb per gram. This is same for all electrons, regardless of any gas in discharge tube.
6. They can produce mechanical pressure indicating they possess kinetic energy (K.E).

Q.3: Describe the discovery of proton? Also write down the properties of canal rays?
Ans: Discovery of Protons
The Proton is positively charge particle discovered by Goldstein in 1886. J. J.Thomson investigate properties of proton in 1897.
Discharge Tube:
Protons were observed in same apparatus of cathode rays tube but with perforated cathode.
Working:
Goldstein discovered that not only negatively charge cathode rays but positively charge rays are moving in opposite direction by perforating cathode. These positive rays passes through the holes of cathode, where they strike cause the glow of tube. These rays named as Canal rays (proton).
The canal rays are not emitted by anode, but they are result of striking of electron with residual gas molecules in discharge tube. Electrons ionize the gas molecules as follows.

M + e^- ⟶ M⁺¹ + 2 e^-

Conclusion:
Goldstein justify that atoms are electrically neutral, while electrons carry negative charge. It mean for each electron there must be one equivalent positive charge to neutralize that electron. This particle is called proton and it is a fundamental particle of all Atoms.

Properties of Canal Rays (Protons):

1. They travel in straight line from Anode towards Cathode.
2. They produce sharp shadow of object placed in their path.
3. They have positive charge and bend towards negative plate in electric and magnetic field.
4. The (e/m) charge and mass ratio of positive particles is much smaller than electron. It varies according to nature of gas present in tube.
5. The mass of proton is 1836 times more than electron.

Q.4: Describe the discovery of neutron? Also write down the properties of neutrons?
Ans: Discovery of Neutrons:
In 1920 Rutherford predict that atom must possess another neutral particle with equivalent mass of proton. Different scientists started working on this neutral particle. later on 1932 Chadwick become successful to discover Neutron.
Experiment:
Chadwick found that when alpha (μ)particles bombarded on Beryllium some penetrating radiations were given out. Chadwick suggested that these radiations were due to material particle with mass comparable to hydrogen atom but have no charge. These radiations (particle) are called Neutron. It can expressed in equation as follows.

⁹₄Be + ⁴₂He ⟶ ¹²₆C + ₀n¹

Conclusion:
The neutron is fundamental part of an Atom, present in nucleus with proton and include in atomic mass.

Properties of Neutrons:

1. The Neutrons are neutral particles.
2. They have no charge.
3. The mass of neutron is almost equal to that of proton.
4. These particles are most penetrating in matter.

Q.5: How Atomic Number (Z) and Mass Number (A) are related with number of proton and neutron?
Ans: Atom consist of three particles Electron, Proton and Neutron. But if all atoms have same fundamental particles, then none of the atoms of one element can be different from the atoms of another element. Therefore, all atoms can be identified by their atomic number i.e the number of protons they contain. No two elements have the same number of protons. In neutral atoms number of protons are equal to number of electrons.
While the total number (sum) of proton and neutrons in the nucleus of an atom is called Mass Number.
For example:
Atom of Carbon (C) is different from an atom of Nitrogen, because atomic number of carbon is 6 while the atomic number on nitrogen is 7, and the mass number of of carbon is 12 while the atomic number on nitrogen is 14.

Q.6: Define Atomic Number (Z) and Mass Number (A) in term of number of proton and/or neutron?
Ans: Atomic Number (Z):

"The number of protons in the nucleus of an atom is called Atomic Number."

Atomic number is represented by Z.
The elements are identify by their atomic number. Different elements have different atomic numbers because of different number of protons.
In neutral atoms number of protons are equal to number of electrons, so the atomic number also indicate total number of electrons outside the nucleus.
For example:
Atomic number of Carbon(C) is 6. It mean that each carbon atom has 6 protons and 6 electrons in it.
Formula:
Atomic number= Z = Number of proton in nucleus = Total number of electron around nucleus
Atomic number (Z) is written as subscript on the left hand side of the chemical symbol.
e.g

• Carbon: C₆
• Lithium: Li₃
• Oxygen: O₈
• Hydrogen: H₁ etc.

Mass Number (A):

"The total sum of proton and neutrons in the nucleus of an atom is called Mass Number."

Mass number is represented by A.
For example:
The sodium (Na) atom has atomic number 11 and mass number 23. It indicates that sodium atom has 11 protons and 12 neutrons. The mass number (A) is written as superscript on left hand side of chemical symbol.
e.g
²³₁₁Na
Formula:

• Mass number = A = Number of protons (Z) + Number of neutrons (N) OR
  Mass number A = Z + N
• Number of neutron = Mass number - Atomic Number. OR
  Number of neutron N = A – Z

Q.7: Describe the contributions Rutherford made to the development of the atomic theory.

OR

Describe Rutherford's experiment which led him to the discovery of the electron.
Ans: Rutherford Atomic Model
EXPERIMENT:
Gold Foil experiment
Rutherford took a thin sheet of gold and bombarded it with alpha (α) particles obtained from a radioactive element (Like Polonium). These rays scattered from the atom and examined on a zinc sulphide (ZnS) screen.

Observations:

1. Most of the particles passed straight and undeflected through the sheet and produced illumination on the zinc sulphide screen.
2. Very few alpha (α) particles undergo small and strong deflection after passing through gold sheet.
3. A very few alpha (α) particles (one of 8000) retraced their path.

Conclusion:

1. According to Rutherford an atom consist of two parts nucleus and extra nuclear part.
2. Majority of the alpha particles passed straight line and un-deflected, shows that most volume occupied by atom is empty.
3. Alpha particles are positively charged and their deflection indicates that the spelling of atom has a positive charge, which is named as nucleus.
4. The mass is concentrated in the nucleus and the electrons are distributed outside the positively charge nucleus.
5. The electrons are revolving around the nucleus in extra nuclear part in orbits.

Q.8: Write down the postulates Rutherford's atomic model? Also describe the drawbacks of Rutherford's atomic model?
Ans: Rutherford postulates:

1. An atom consist of positively charged, dense and very small nucleus containing protons and neutron. The entire mass is concentrated in the nucleus of an atom.
2. The nucleus is surrounded by large empty space which is called extra nuclear part where probability of finding electron is maximum.
3. The electrons are revolving around the nucleus in circular paths with high speed (Velocity).
4. These circular paths were known as orbits (Shells).
5. An atom is electrically neutral because it has equal number of protons and electrons.
6. The size of the nucleus is very small as compared to the size of its original atom.

Defects of Rutherford atomic model:

1. Rutherford did not explain the stability of an atom.
2. In Rutherford atomic model the negatively charged electrons revolve around the nucleus in circular path and emits energy continuously. Due to continuous loss of energy ultimately falls into the nucleus.
3. If the revolving electron continuous emits energy, then there would be a continuous spectrum but in contrast to it we get line spectrum from the atoms of elements.

Q.9: Explain how Bohr's atomic model is different? Write down salient features or postulates of Neil Bohr's atomic model?
Ans: Neil Bohr’s Atomic model:
In 1913 Neil Bohr proposed another atomic model. This atomic model was different in this manner that it shows two folds:

1. First to remove the Rutherford atomic model.
2. Second, it explain the line spectrum of Hydrogen atom based on quantum theory of Max Planck.

Postulates of Neil Bohr's Atomic Model:
Neil Bohr proposed the following postulates for atomic structure.

1.  The atom has fixed orbits in which negatively charged electron is revolving around the positively charged nucleus.
2. These orbits possess certain amount of energy which are called shells and named as K, L, M, N shells.
3. The energy levels are represented by an integer (n = 1, 2, 3….) known as quantum number, this quantum range starts from nucleus side, where n = 1 is lowest energy level.
4. Electrons are revolving in particular orbits continuously, but they are not emits or absorb energy.
5. When electron jumps from lower energy level (E₁) to higher energy level (E₂), it absorb energy.
6. When electrons jumps from higher energy level (E₂) to lower energy level (E₁), it emits energy.
7. The emission or absorption is discontinuous in the form of energy packet called Quantum or Photon.
8. The ΔE is the difference in energy of higher (E₁) and lower (E₂) energy level.
   
   ΔE =E₂ – E₁
   ΔE = νh = 1 photon
   Here h is planks constant, its value is 6.63 x 10^-34 Js and ν is a frequency of light.
9. Stationary state were present in those orbits in which angular moment of electron would be integral multiple of h/2pπ.
   
   mνr = nh / 2pπ
   where,
   n = no of orbits
   h = planks constant
   m = mass of electron

Q.10: Write down the limitation or the drawbacks of Bohr's atomic model?
Ans: Limitations Or Drawback of Bohr's Atomic Model:

• Bohr's model of an atom failed to explain the Zeeman Effect (effect of magnetic field on the spectra of atoms).
• It 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 explain the monoelectronic species like H⁺¹, Li⁺², B⁺³.

Q.11: Describe De Broglie Hypothesis atomic model? OR Describe wave particle duality of electron of De Broglie Hypothesis?
Ans: De Broglie Hypothesis:
In 1923 Lois De Broglie extend the wave particle duality to electron, and propose a hypothesis that:

"All matter has particle as well as wave nature at the submicroscopic level."

De Broglie combine the Einstein and Planck equations and argued that if

E = hγ (Plank equation) ....... (i)

where,
E = energy,
h = plank 'constant,
γ = frequency of light

And

E = mc² (Einstein equation) ....... (ii)

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

Then From eq. (i) and (ii)

hγ = mc²

OR γ= mc²/h ......... (iii)

According to De-Broglie wave particles duality hypothesis

λ = h /mγ ..... (iv)

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.
So,
According to De-Broglie

"A light, or any other electromagnetic wave, can also exhibit the properties of a particle, similarly a particle should also exhibit the properties of a wave, and those two nature are interchangeable."

Q.12: Describe Schrodinger atomic model?
Ans: Schrodinger Atomic Model hypothesis
In 1926 Erwin Schrödinger, an Austrian physicist, took the Bohr’s atomic model one step forward. Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This atomic model is known as the quantum mechanical model of the atom.
Schrodinger model is just an improvement of Bohr’s atomic model. He took an atom of hydrogen because it has one proton and one electron. He proved mathematically that electron can be find in different position around the nucleus and determined by probability.

Postulates Of Schrodinger Atomic Model

• The quantum mechanical model determines that electron can be find in various location around the nucleus. He found electrons are in orbit as an electron cloud.
• Each energy sub-shell in an orbit have different shapes which determine the presence of electron.
• Different sub-shells of orbitals are orbitals named as s, p, d and f with different shapes as 's' is spherical and 'p' is dumbbell shaped.
• The numbers and kind of atomic orbitals depends on the energy sub-shell.

Probability:

• 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.
• The cloud is denser the probability of finding electron is high which are called atomic orbitals.

Q.13: Describe shells Or orbits Or energy level of an atom?
Ans: ENERGY LEVELS OR ORBITS OR SHELLS OF AN ATOM:
Nucleus is present in the centre of an atom and around the nucleus electrons are revolving. These electrons are revolving around nucleus in different levels according to their potential energy.
Definition:

"The Energy levels or Shell or Orbit are all possible paths on which electrons are revolving around nucleus."

Which are shown by 'n'.
These shells are named as K, L, M, N, O, P with quantum numbers n = 1, 2, 3, 4, 5, 6 respectively.

Order Of Energy In Shells:
These shells have definite amount of energy by means of decreasing order as they become away from nucleus. Such as:

• First energy level is K shell has less energy.
• Second energy level is L shell has more energy than K shell.
• Third energy level is M shell has more energy than K and L shells.
• Fourth energy level is N shell has more energy than K, L and M shells.
• Fifth energy level is O shell has more energy than K, L, M and N shells.

Number of Electrons In Shells:
Formula:
The maximum number of electrons that can be accommodated in a shell is represented by the formula:

2n²

where 'n' is the shell number.
The distribution of electrons in different orbits are as follows:

• K-shell/ 1^st orbit (n = 1) = 2(1)² = 2
• L-shell/ 2^nd orbit (n = 2) = 2(2)² = 8
• M-shell/ 3^rd orbit (n = 3) = 2(3)² =18
• N-shell/ 4^th orbit (n = 4) = 2(4)² = 32 and so on

Q.14: Describe sub shells Or sub energy level Or orbitals? OR Describe the presence of sub shells in a shell.
Ans: SUB SHELLS OR SUB ENERGY LEVELS Or ORBITALS:
When atomic spectra of substances were observed in a high powered spectroscope, it was found that they consist of two or more lines closely packed with each other as discussed in zemen and stark effects. These lines means that electrons in the same shell may differ in energy by small amount. Thus:
Definition:

"Main energy level are divided into sub energy levels and known as sub shells. When electrons are many in numbers in a shell they show repulsion and main shell splits into sub shell which named as s, p, d and f sub shells."

Number Of Sub Shells In Shells
The number of subshells in a shell is according to value of that shell, which are as follows:

Value of ‘n’	Shell	Sub shell
1	K	s
2	L	s, p
3	M	s, p, d
4	N	s, p, d, f

Number Of Electron In Sub Shells:
There are slight difference in Energy levels of subshells, that way subshell 's' filled first then subshell 'p' and onward. The distribution of maximum electrons in subshells is as follows:

• 2 electrons in's' subshell
• 6 electrons in 'p' subshell
• 10 electrons in 'd' subshell
• 14 electrons in 'f' subshell

SECTION- C:  QUESTIONS: