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Chemistry 9th Class Chemistry

Chapter 2 STRUCTURE OF THE ATOM 9th Class Chemistry Notes

Chapter 2 STRUCTURE OF THE ATOM 9th Class Chemistry Notes

 

Q#1) What is atom? Write in short words about history of atoms?

Answer: In 585 B.C, Ancient Greek philosopher Democritus suggested that matter is composed of tiny indivisible particles called atoms.The name ATOM was derived from the Latin word Atomos meaning indivisible.

Daltons Atomic Theory : In 1808 John Dalton put forward his Atomic Theory. According to this theory 1)All matter is made up of very small,indivisible particles called atoms. 2)Atoms of the same element s.are alike. 3)Atoms combine in different ways to form compound

PLUM PUDDING MODEL: Plum Pudding Model was put forwarded by J.J Thomson.According to this model Atoms were solid structures of positively charge with tiny negative particles stuck inside just like plums in the pudding.

For your information: J.J. Thomson (1856-1940) was a British physicist. He was awarded the 1906 Noble Prize in Physics for the discovery of electron and for his work on the conduction of electricity in gases.

Q#2 ) DISCUSS HOW CATHODE RAYS WERE DISCOVERED OR HOW ELECTRONS WERE DISCOVERED? OR DISCRIBE WORKING OF DISCHARGE TUBE? OR  WRITE NOTE ON CATHODE RAYS AND DISCOVERY OF ELECTRON? How was it proved that electrons are fundamental particles of an atom?

ANSWER: CATHODE RAYS EXPERIMENT: In 1895, Sir William Crooks performed experiments by passing electric current through gases in a discharge tube at a very low pressure. During this experiment crooks observed radiations , that were later named electrons.

J.J. Thomsan performed a large number of experiments and finally prove that electrons are basic particles of atom that are negatively charged and are responsible for all the chemical properties of the atom. So electrons are proved to be the fundamental particles of an atom.

CONSTRUCTION OF APPARATUS : An apparatus consisted of a glass tube fitted with two metallic electrodes which were connected to the high Voltage battery.The pressure inside the tube was kept at 10-4 atm.

When high voltage current was passed through the gas, shiny rays were emitted from the cathode towards the anode.These rays were given the name of Cathode rays as they were originated from cathode.

CHARACTERISTICS OR PROPERTIES OF CATHODE RAYS : The Properties of cathode rays are as follows.

1)These rays travel in a straight line perpendicular to the cathode surface. 2)They can form a sharp shadow of an opaque object if placed in their path, which proved their particle nature.

3)These are defflected towards positive plate in an electric field showing that they are negatively charged. 4)They raised the temperature of a body on which they fall.

5) J-J Thomson discovered their charge/mass (e/m) ratio. 6)Light is produced when these rays hit the sides of a discharge tube. 7)It was found that the same type of rays were emitted no matter which cathode was used in the discharge tube.

All the above properties suggest that the nature of cathode rays was independent of the nature of gas present in the discharge tube or material of cathode. They produce sharp shadow of an object placed in their path which proved that these are fast moving material particles and are named electrons, which are fundamental particles of all atoms.

For your information: Sir William Crooks(1832-1919) was a British chemist and physicist. He was pioneer of vacuum tubes. He worked on spectroscopy.

Q#3 )How canal rays or protons were discovered?  OR    Draw a labeled diagram to show the presence of protons in the discharge tube and explain how canal rays were produced?

ANSWER: DISCOVERY OF PROTON OR DISCOVERY OF CANAL RAYS: In 1886, GOLDSTEIN observed that in addition to cathode rays , other rays were also present in the discharge tube.These rays were trvelling in opposite direction to cathode rays.He used a discharge tube having perforated cathode that is holes were present in cathode. He found that these rays passed through holes present in the cathode and produced a glow on the walls of the discharge tube.He called these rays as Canal rays.

CHARACTERISTICS OR PROPERTIES OF CANAL RAYS: The important properties of canal rays are as follow:

1)These rays travel in a straight line in a direction opposite to cathode rays.

2)Their deflection in electric and magnetic field proved that these are positively charged.

  • The nature of canal rays depends upon the nature of gas present in the discharge tube.
  • These rays do not originate from anode but infact these are produced when the cathode rays or electrons collide with the residual gas molecules present in the discharge tube and ionize them. Example M + e- → M++ 2e

“M” in chemistry stands for any general element (3rd group element).

5)Mass of these particles was found equal to that of a proton or simple multiple of it.The mass of a proton is 1840 times more than that of an electron.

RESULTS OR CONCLUSIONS: Following are the major conclusions which can be made from above above discussions about the nature and mass of these canal rays. 1)These rays are made up of positively charged particles called protons. 2)The mass and charge of these particles depend upon the nature of the gas present in the discharge tube. 3) Positive particles produced by the gas will be of the same type that is positive rays produced by the lightest gas hydrogen contain protons.

Q# 4) Describe the discovery of Neutrons?

Answer: DISCOVERY OF NEUTRON: In 1920 Rutherford predicted the existance of neutral particle in an atom. In 1932 James Chadwick bombarded alpha particles on the nucleus of beryllium. It produced neutral particles having mass equal to proton and carry no charge.These particles were named as neutrons.

Nuclear Reaction: 4Be9 +2He4 → 6C12 + 0n1.

PROPERTIES OF NEUTRON: The most important properties of neutron are as follow:

  • Neutrons carry no charge that is they are neutral particles.
  • They are highly penetrating.
  • Neutrons have mass equal to proton.

4) These are present inside the nucleus of an atom.

Q#5) HOW RUTHERFORD PROVE THAT ATOM HAS A NUCLEUS LOCATED AT THE  CENTER OF THE ATOM?

ANSWER: RUTHERFORD’S ATOMIC MODEL OR PLANETRY MODEL OF AN ATOM OR GOLD FOIL EXPERIMENT OR DISCOVERY OF NUCLEUS: In order to determine the structure of an atom, Rutherford carried out Gold Foil experiment in year 1911.He bombarded alpha-particles on 0.00004cm thick gold foil.Alpha-particles are emitted from radioactive elements like Radium and Polonium.

Alpha particles are helium nuclei which can penetrate through matter to some extent. Rutherford observed the effect of alpha particles on the photographic plate or a screen coated with zinc sulphide.He proved that the Plum Pudding Model of the atom was not correct.

OBSERVATIONS: Observations made by Rutherford are as follow: 1)Almost all the particles passed through the foil un-deflected. 2)Out of 20,000 particles, only a few were deflected at fairly large angles and very few bounced back on hitting the gold foil.

RESULTS OF EXPERIMENT: After carrying out an experiment, Rutherford proposed PLANETRY MODEL for an atom and concluded following results. 1)Since most of the particles passed through the foil un-defflected , therefore most of the volume occupied by an atom is empty.

  • The defflection of a few particles proved that there is a “centre of positive charges” in an atom, which is called “nucleus” of an atom.
  • The complete rebounce of a few particles show that the nucleus is very dense and hard.
  • Nucleus is located at the centre of an atom.
  • The whole mass of an atom is concentrated in the nucleus.
  • Since a few particles were deflected it shows that the size of the nucleus is very small as compared to the total volume of an atom.
  • An atom as a whole is neutral, therefore the number of electrons in an atom is equal to the number of protons.
  • Except electrons, all other fundamental particles that lie within the nucleus , are known as Neucleons.
  • The electrons revolve around the nucleus.

DEFECTS IN RUTHERFORD’S MODEL: Following are the main defects found in Rutherford’s atomic model.

  • According to classical theory of radiations, electrons being the charged particles should release or emit energy continuously and they should ultimately fall into the nucleus.
  • If electrons emit energy continuously, they should form a continuous spectrum, but infact line spectrum was observed.

For your information: Rutherford was a british Newzeland chemist. He performed a series of experiments using α particles. He won the 1908 Noble Prize in Chemistry. In 1911, he proposed the nuclear model of the atom and performed the first experiment to split atom. Because of his great contributions, he is considered the father of nuclear science.

Question#6 :    (A)DESCRIBE BOHR’S MODEL?

  • One of postulate of Bohr model is that angular momentum of a moving electron is quantized. Explain its meaning and calculate the angular momentum of third orbit (i,e  n=3)

ANSWER: (A) BOHR’ S ATOMIC THEORY: In 1913, NEIL BOHR presented another model of an atom which was based on MAX PLANK’S QUANTUM THEORY. According to BOHR ‘S model , revolving electron in an atom does not absorb or emit energy continuously. The energy of a revolving electron is Quantized as it revolves only in orbits of fixed energy, called “energy levels” by him.

POSTULATES OF BOHR’S ATOMIC MODEL: Bohr’s atomic mode was based upon the following main postulates.

  • The hydrogen atom consists of a tiny nucleus and electrons are revolving in one of the circular orbits of radius “r” around the nucleus.
  • Each orbit has a fixed energy that is quantized.
  • As long as electron remains in a particular orbit, It does not radiate or absorb energy.The energy is emitted or absorbed only when an electron jumps from one orbit to another.
  • When an electron jumps from lower orbit to higher orbit ,it absorbs energy and when it jumps from higher orbit to lower orbit it radiates energy.This change in energy (∆E)is given by plank’s equation.

∆E = E2 – E1 = hv

Where h= Plank’s constant=6.63 × 10-34Js and V= frequency of light E2= energy of higher orbit E1= energy of lower orbit and ∆E = change in energy.

  • Electron can revolve only in orbits of a fixed angular momentum that is mvr = nh/2π Where n= quantum number or orbit number and n= 1,2,3,  m=mass of electron r= radius of orbit v= velocity of electron.

For your information:Neil Bohr was a Danish physicist who joined Rutherford in 1912 for his post doctoral research. In 1913, Bohr presented his atomic model based upon Quantum theory. He won the 1922 Noble Prize for Physics for his work on the structure of an atom.

(B) ENERGY OF THE PAST MOVING ELECTRONS IS QUANTIZED OR ANGULAR MOMENTUM OF MOVING ELECTRONS IS QUANTIZED:

According to Bohr pastulate of atomic model, angular momentum of electrons is quantized. Quantum means fixed energy. It shows that amount of energy of each and every electron in a specific shell is fixed and remains constant as long as electrons remains in a particular orbit.

Formula to calculate angular momentum of electrons in orbits (shells): The formula used to find the angular momentum of moving electrons in orbits is as follows.

mvr = nh/2π.

In the formula “n” shows the number of shell in which electrons revolve. For first orbit it is 1, for second orbit it is 2, for third orbit it 3, and so on.

  • For first orbit angular momentum may be given as:

mvr = nh/2π.

mvr = 1× 6.63 ×10 -34 JS/2×3.14

mvr =  3.315 ×10-34 / 3.14

mvr =  1.0 ×10-34 JS (kg.m2.S-1)

  • For second orbit angular momentum may be given as:

mvr = nh/2π.

mvr = 2× 6.63 ×10 -34 JS/2×3.14

mvr = 2.12 ×10 -34 JS

  • For Third orbit angular momentum may be given as:

mvr = nh/2π.

mvr = 3× 6.63 ×10 -34 JS/2×3.14

mvr  = 3.0 × 10 -34 JS.

Q#7) Defferentiate between Rutherford atomic model and Bohr atomic model?

Answer: DIFFERENCE RUTHERFORD BETWEEN ATOMIC MODEL AND BOHR’S ATOMIC MODEL:

Rutherford model Bohr model
1) It was based upon classical theory. It was based upon quantum theory.
2) electrons revolve around the nucleus.Electrons revolve around the nucleus in orbits of fixed energy.
3) No idea about orbits was introduced.Orbits had angular momentum.
4) Atom should produce continuous spectrum.Atom should produce line spectrum.
5) Atoms should collapse.Atoms should exist.

 

Question#8) What do you mean by electronic configuration? What are the basic requirements while writing electronic configuration of an element (atom) ?

Answer: ELECTRONIC CONFIGURATION: The distribution or arrangment of electrons around the nucleus in various shells and sub shells according to their increasing energy is called electronic configuration.

Explanation: The electrons fill the shells in order of their increasing energy that is lower energy level is occupied first then the higher energy level and so on.

The electrons revolve around the nucleus in different energy levels or shells according to their increasing energy. Energy levels are represented by “n” values, and n= 1, 2, 3 . . . They are designated by alphabets K, L, M , … . K shell is closest to the nucleus, the energy of the shells increases from K -shell onwards. n = n = n = n =

  • K-shell= 1st energy level.
  • L-shell= 2nd energy.
  • M-shell= 3rd energy level.
  • N-shell= 4th energy level

n ValueShellSubshell
1KOnly s
2Ls, p
3Ms, p, d
4Ns, p, d, f

 

 

A shell or orbit also consists of subshells or arbitals.The number of sub shells in a shell is equal to its n – value. Subshells may be of four types. s ,p ,d and f.

1st shell that is K shell has only 1 subshell s, 2nd shell that is L shell has 2 subshells s and p, 3rd shell that is M shell has 3 subshells s,p and d, 4th shell that is N shell has 4 subshells s,p,d and f.

The maximum capacity of shells to accommodate the electrons is as follow K shell can accommodate 2 electrons, L shell can accommodate 8 electrons, M shell can accommodate 18 electrons, N shell can accommodate 32 electrons. The order of filling of electrons in sub shells is given by s,p,d,f.

S subshell can accommodate 2 electrons.

P subshell can accommodate 6 electrons.

d subshell can accommodate 10 electrons.

f subshell can accommodate 14 electrons.

Requirements for electronic configuration : Following are the requirements for electronic configuration.

(1) Find atomic number of the required atom: To configure electrons in an atom’s shells and sub shells. First you have to know the atomic number of the atom which is the number of protons in the nucleus of an atom. It is also the number of neutrons in the shells, if the atom is neutral.

(2) Determine the charge on the atom: uncharged atoms have number of electrons equal to the atomic number. If you have a charged atom. Add one electron for each negative charge and subtract one electron for each positive charge. For exaple a magnesium atom with a +2 charge (MG2+) means that it has lost 2 electrons. Neutral Mg atom has 12 electrons. Now there are 10 electrons in magnesium. So electronic configuration of neutral and charged atoms are different.

(3) Standard electronic configuration: You also have to know about standard electronic configuration that is 1S2, 2S2, 2P6, 3S2, 3P6, 4S2, 3d10, 4p6, 5S2, 4d10, 5p6, 6S2, 4f14, 5d10.

(4) Number of electrons in each orbital: keep in mind that each orbital accumudate fixed number of maximum electrons. Each S orbital accumudate 2 electrons, p accumudate 6, d accumudate 10 while f accumodate 14 electrons.

(5) Number of sub shells (orbitals) ; energy level 1 has only one subshell, energy level has 2 sub shells, energy level 3 has three sub shells, and so on.

Question#9) What is an isotope? Describe the isotopes of hydrogen with diagrams? Give the applications of isotopes in the field of radiotheraphy and medicines?

Answer: ISOTOPES: The atoms of an element that have same atomic number but different mass number are known as isotopes. “OR” The atoms of an element that have same number of protons and electrons but different number of neutrons are called isotopes.

“OR”  Atoms of the same element having same chemical properties but different chemical properties are called isotopes.

***These have same number of electrons so their electronic configuration and chemical properties are same but they differ in physical properties.

Examples: 1) Isotopes of Hydrogen: There are three isotopes of hydrogen.

  1. i) Protium (1H1) :This isotope of hydrogen has 1 proton, 1 electron and no neutrons. Atomic number is 1 and mass number is also 1.
  2. ii) Deutrium (1H2):This isotope of hydrogen has 1 proton, 1 electron and 1 neutron. Atomic number is 1 and mass number is 2.

iii) Tritium (1H3): This isotope of hydrogen has 1 proton, 1 electron and 2 neutrons. It has atomic number 1 and atomic mass 3.

2) Isotopes of Carbon:

There are 3 isotopes of Carbon.

  1. i) Carbon-12 (6C12) : This isotope of Carbon has 6 proton, 6 electron and 6 neutrons. Atomic number is 6 and mass number is also 12.
  2. ii) Carbon-13 (6C13) : This isotope of Carbon has 6 proton, 6 electron and 7 neutrons. Atomic number is 6 and mass number is also 13.

iii) Carbon-14 (6C14) : This isotope of Carbon has 6 proton, 6 electron and 8 neutrons. Atomic number is 6 and mass number is also 14.

 

3) Isotopes of Chorine: There are two isotopes of chlorine.

  1. i) Chlorine-35 (17Cl35) : This isotope of Chlorine has 17 proton, 17 electron and 18 neutrons. Atomic number is 17 and mass number is also 35.
  2. ii) Chlorine-35 (17Cl37)  : This isotope of Chlorine has 17 proton, 17 electron and 20 neutrons. Atomic number is 17 and mass number is also 37.
  • Isotopes of Uranium:

There are three isotopes of uranium.

  1. i) Uranium-234 (92U234) : This isotope of Uranium has 92 proton, 92 electron and 142 neutrons. Atomic number is 92 and mass number is also 234.
  2. ii) Uranium-235 : (92U235) : This isotope of Uranium has 92 proton, 92 electron and 143 neutrons. Atomic number is 92 and mass number is also 235.

iii) Uranium- 238 : (92U238) : This isotope of Uranium has 92 proton, 92 electron and 146 neutrons. Atomic number is 92 and mass number is also 238.

 

SymbolAtomic NumberMass NumberNo. of ProtonNo. of Neutron
1H1110
2H1211
3H1312
12C61266
13C61367
14C61468
35Cl17351718
37Cl17371720
234U9223492142
235U9223592143
238U9223892146

 

APLICATIONS OF ISOTOPES: Isotopes have various applications in science and different fiellds of technologies and medicine. Isotopes are used in diagonosis , radiotherapy and treatment of various diseases.The most important applications of isotopes are as follow

  1. Radiotherapy: Isotopes like P-32 and Sr- 90 emit less penetrating beta radiations which are used for treatment of skin cancer. Co-60 emit strongly penetrating gamma radiations which are used for treatment of internal cancer.

b)Tracer for Diagonosis and Medicine: Radioisotopes are used as tracers in medicine to diagonose the presence of tumour in human body. For example I-131 is used for diagonosis of goitor in thyroid gland. Similarly technitium is used to monitor the growth of bone.

c)Archeological and Geological uses: The radioactive isotopes are used to estimate the age of fossils like dead plants , animals and stones, etc The age determination of very old objects based on the half-lives of the radioactive isotope called radioactive isotope-dating. For example radioactive C- 14 is used for age determination of old carbon containing objects called fossils, this process is known as radioactive dating or carbon-dating.

d) Chemical Reactions and Structure Determination: The radioactive isotopes like C-14 are used in chemical reactionsto follow a radioactive element during a chemical reaction and ultimately to determine the structure of the compound. For example CO2 which is produced by plants during photosynthesis to prepare

e)Application In Power Generation: Radioactive isotopes like U-235 are used to generate electricity by carrying out controlled nuclear fission reactions in nuclear reactors. For example, when U-235 is bombarded with slow moving neutrons , the U-235 nucleus breaks up to produceBa-139 and Kr-94 along with 3 neutrons and large amount of energy. Nuclear reaction is given by.

92U235+ 0n1  → 56 Ba139 + 36 Kr94+ 30 n1 + energy.

This energy is called atomic energy and is used to convert water into steam in boilers. Then the steam drives the turbines to generate electricity.

TEST YOURSELF 2.1

  1. Do you know any element having no neutrons in its atoms?

Answer: yes, the element having no neutron in its atom is protium isotope of the hydrogen.

  1. Who discovered an electron, a proton and a neutron?

Answer: Electron was discovered by william crooks.

Proton was discovered by Goldstein.

Neutron was discovered by James Chadwick.

iii. How does electron differ from a neutron?

Answer:

ElectronNeutron
1) An electron has a negative charge.Neutron has no charge.
2) It revolves around the nucleus.It is present in the nucleus.
3) it is lighter.It is denser.

 

  1. Explain, how anode rays are formed from the gas present in the discharge tube?

Answer: Electrons from cathode are bombarded on molecule of gas present in discharge. These molecules are ionized and form positive ions. These positive ions travels towards cathode in the form of rays. These rays are called anode rays or canal rays or protons.

TEST YOURSELF 2.2.

Question#1. How was it proved that the whole mass of an atom is located at its centre?

Answer: Since most of the alpha particles in Rutherford experiment passed undeflected  through the foil, therefore most of the volume occupied by an atom is empty. The complete rebounce of a few particles show that the nucleus is very hard, dense and small and is located at the center.

Question#2. How was it shown that atomic nuclei are positively charged?

Answer: In Rutherford experiment the deflection of alpha double positively charged particles showed that nucleus is positively charged.

Question#3. Name the particles which determine the mass of an atom.

Answer: The mass of an atom is due to the particles present in the nucleus of an atom that are protons and neutrons.

  1. What is the classical theory of radiation? How does it differ from quantum theory?

Answer: According to classical theory of radiation moving negatively charged particles, electrons in an atom release energy continuously and ultimately falls into the nucleus. According to Max planks  quantum theory  of radiation revolving does not emit or absorb energy continuously. The energy of a revolving electron is quantized as it revolves only in orbits of fixed energy.

 

  1. How can you prove that angular momentum is quantized?

Hint: Let angular momentum (mvr) of 1st orbit is mvr = nh/2π

By putting the values of h and π

 

Answer:  Angular momentum is quantized means that electrons revolve with an angular momentum in orbits of fixed energy .For electrons of each and every orbit angular momentum is fixed for each and every electron.  Formula for finding the angular momentum is  mvr = nh/2π .

In the formula “n” shows the number of shell in which electrons revolve. For first orbit it is 1, for second orbit it is 2, for third orbit it 3, and so on.

1) For first orbit angular momentum may be given as:

mvr = nh/2π.

mvr = 1× 6.63 ×10 -34 JS/2×3.14

mvr =  3.315 ×10-34 / 3.14

mvr =  1.0 ×10-34 JS (kg.m2.S-1)

2) For second orbit angular momentum may be given as:

mvr = nh/2π.

mvr = 2× 6.63 ×10 -34 JS/2×3.14

mvr = 2.12 ×10 -34 JS

3) For Third orbit angular momentum may be given as:

mvr = nh/2π.

mvr = 3× 6.63 ×10 -34 JS/2×3.14

mvr  = 3.0 × 10 -34 JS.

TESTYOURSELF 2.3

1.What is the maximum number of electrons that can be accommodated in a p-subshell?

Answer: Maximum number of electrons that can be accommodated in a p-subshell is 6.

  1. How many subshells are there in second shell?

Answer: Two subshells are there in second shell named s and p.

  1. Why does an electron first fill 2p orbital and then 3s orbital?

Answer: Electrons are accomudated in different shells and subshells according to increasing energy. An electron first fill in 2P and then 3S orbital because 2P orbital is lower in energy than 3S orbital.

  1. If both K and L shells of an atom are completely filled; how many total number of electrons are present in them?

Answer: K shell can accomudate maximum of  2 electrons while L shell can accomudate maximum of 8 electrons. So If both K and L shells of an atom are completely filled then there will be 10 electrons in the atom.

  1. How many electrons can be accommodated in M shell?

Answer: According to 2n2 Formula M shell can accumodate maximum of 18 electrons.

  1. What is the electronic configuration of a hydrogen atom?

Answer: Electronic configuration of hydrogen atom is 1S1.

  1. What is atomic number of phosphorus? Write down its electronic configuration.

Answer: Atomic number of phosphorous is 15. Its electronic configuration is 1S2, 2S2, 2P6, 3S2, 3P3.

  1. If an element has atomic number 13 and atomic mass 27; how many electrons are there in each atom of the element?

Answer: The number of electrons in this atom is 13.

  1. How many electrons will be in M shell of an atom having atomic number 15.

Answer:  there will be 5 electrons in the M shell of an atom having atomic number 15.

  1. What is maximum capacity of a shell?

Answer: The maximum capacity of a shell can be found by 2n2 formula. According to this formula, maximum capacity of first shell (K) is 2, capacity of  second shell(L) is 8, maximum capacity of third shell (M) Is 18, while maximum capacity of fourth shell (N) is 32.

TEST YOURSELF 2.4

  1. Why do the isotopes of an element have different atomic masses?

Answer: Isotopes of an element have different atomic masses because they have different number of neutrons in their nucleus.

  1. How many neutrons are present in C-12 and C-13?

Answer: Carbon-12 have 6 neutrons while carbon 13 has 7 neutrons.

  1. Which of the isotopes of hydrogen contains greater number of neutrons?

Answer: Tritium has greater number of neutrons (2) in its nucleus.

  1. Give one example each of the use of radioactive isotope in medicine and radiotherapy.

Answer: Iodine-131 is used in medicines while cobalt-60 is used in radiotheraphy.

  1. How is the goiter in thyroid gland detected?

Answer:  The isotope-131 is used to  diagnose goiter in thyroid gland.

  1. Define nuclear fission reaction.

Answer: A nuclear reaction in which a heavy nucleus splits into smaller parts is called nuclear fission reaction. During this process energy is released which is used for different purposes.

  1. When U-235 breaks up, it produces a large amount of energy. How is this energy used?

Answer: When U-235 breaks up, it produces a large amount of energy which is used to convert water into steam in boilers. The steam then drives the turbines to generate electricity.

  1. How many neutrons are produced in the fission reaction of U-235?

Answer: 3 neutrons are produced in the fission reaction of U-235.

  1. U-235 fission produces two atoms of which elements?

Answer:  U-235 fission produces  Barium-139 and krypton-94.

 

EXERCISE:

Short answer questions.

  1. What is the nature of charge on cathode rays?

Answer: The deflection of cathode rays in electric and magnetic field shows that these rays are negatively charged.

  1. Give five characteristics of cathode rays.

Answer: CHARACTERISTICS OR PROPERTIES OF CATHODE RAYS: The Properties of cathode rays are as follows.

1)These rays travel in a straight line perpendicular to the cathode surface. 2)They can form a sharp shadow of an opaque object if placed in their path, which proved their particle nature.

3)These are defflected towards positive plate in an electric field showing that they are negatively charged. 4)They raised the temperature of a body on which they fall. 5)J-J Thomson discovered their charge/mass (e/m) ratio. 6)Light is produced when these rays hit the sides of a discharge tube. 7)It was found that the same type of rays were emitted no matter which cathode was used in the discharge tube.

All the above properties suggest that the nature of cathode rays was independent of the nature of gas present in the discharge tube or material of cathode. They produce sharp shadow of an object placed in their path which proved that these are fast moving material particles and are named electrons, which are fundamental particles of all atoms.

  1. The atomic symbol of a phosphorus ion is given as 1531P3-

 How many protons, electrons and neutrons are there in the ion?

 What is name of the ion?

 Draw the electronic configuration of the ion.

 Name the noble gas which has the same electronic configuration as the phosphorus ion has.

Answer:

ⓐ There are 15 protons, 16 neutrons and 18 electrons in the ion.

ⓑ Name of the ion is phosphide ion ( phosphorous ion).

ⓒ Electronic configuration of the ion is:

For shells: K=2, L=8, M=8.

For subshells: 1S2, 2S2, 2P6, 3S2, 3P6.

ⓓ  Argon has same electronic configuration to that of phosphorous ion.

 

  1. Differentiate between shell and subshell with examples of each.

Answer: The main energy levels  that electrons can occupy are called shells.  OR   The circular path around the nucleus of an atom in which electrons revolve are called shells. The shells are further divided into sub energy levels called sub shells. Examples of shells are K, L, M, N. While that of  subshells are S, P, d and f.

 

 

  1. An element has an atomic number 17. How many electrons are present in K, L and M shells of the atom?

Answer: an element with atomic number 17 has electronic configuration:

K=2,  L=8,  M=7

And the name of the element is  chlorine.

  1. Write down the electronic configuration of Al3+. How many electrons are present in its outermost shell?

Answer: the electronic configuration of Al3+  is :  K =2,  L=8.

In  the outermost shell 8 electrons are present.

Electronic configuration in sub shells are as : 1S2, 2S2, 2P6.

  1. Magnesium has electronic configuration 2, 8, 2, How many electrons are in the outermost shell? In which subshell of the outermost shell electrons are present? Why magnesium tends to lose electrons?

ANSWER: Electronic configuration of Mg: =2, L=8, M=2.

There are 2 electrons in the outermost shell.

Electronic configuration in sub shell: 1S2, 2S2, 2P6, 3S2.

Outermost electrons are present in 3S sub shell.

For completion of its octet and to gain stability magnesium loss its two electrons and attains noble gas electronic configuration.

  1. What will be the nature of charge on an atom when it loses an electron or when it gains an electron?

Answer: An atom attains positive charge when it loss an electron and attains a negative charge when it gains electrons.

  1. For what purpose U-235 is used?

Answer: U-235 is used to generate electricity.

  1. A patient has goiter. How will it be detected?

ANSWER: Goiter can be detected by using Iodine-131 isotope.

  1. Give three properties of positive rays.

Answer: CHARACTERISTICS OR PROPERTIES OF CANAL RAYS OR POSITIVE RAYS OR PROTONS: The important properties of canal rays are as follow:

1)These rays travel in a straight line in a direction opposite to cathode rays.

2)Their deflection in electric and magnetic field proved that these are positively charged.

  • The nature of canal rays depends upon the nature of gas present in the discharge tube.
  • These rays do not originate from anode but infact these are produced when the cathode rays or electrons collide with the residual gas molecules present in the discharge tube and ionize them. Example M + e- → M++ 2e-

“M” in chemistry stands for any general element (3rd group element).

5)Mass of these particles was found equal to that of a proton or simple multiple of it.The mass of a proton is 1840 times more than that of an electron.

  1. What are the defects of Rutherford’s atomic model?

Answer: DEFECTS IN RUTHERFORD’S MODEL: Following are the main defects found in Rutherford’s atomic model.

  • According to classical theory of radiations, electrons being the charged particles should release or emit energy continuously and they should ultimately fall into the nucleus.
  • If electrons emit energy continuously, they should form a continuous spectrum, but infact line spectrum was observed.
  1. As long as electron remains in an orbit, it does not emit or absorb energy. When does it emit or absorb energy?Justify the statement.

 Answer: As long as electron remains in an orbit, it does not emit or absorb energy. The energy is emitted or absorbed only when an electron jumps from one orbit to the other. When an electron jumps from higher to lower energy level it emit energy and when it jumps from lower to higher orbit it absorb energy.

Long Answer Questions.

  1. How are cathode rays produced? What are its five major characteristics?

Answer: See answer of question number 2.

Question#2. How was it proved that electrons are fundamental particles of an atom?

Answer: please see answer of question number 2.

Question#3. Draw a labeled diagram to show the presence of protons in the discharge tube and explain how canal rays were produced.

Answer: Please see answer of question number 3.

Question#4. How Rutherford discovered that atom has a nucleus located at the centre of the atom?

Answer: Please see answer of question number 5.

Question#5. One of the postulates of Bohr’s atomic model is that angular momentum of a moving electron is quantized. Explain its meaning and calculate the angular momentum of third orbit (i.e. n=3)

Answer: Please see answer of question number 6.

Question#6. How did Bohr prove that an atom must exist?

Answer: Doctor Neil Bohr in 1913 put forward the Bohr model of atom. This model fully explains the structure of atom and prove that atom must exist and it consists of basic fundamental particles. According to doctor Bohr model revolving electrons in an atom does not emit or absorb energy continuously and cannot fall into the nucleus.

Electrons can only emit or absorb energy, when they jumps from one orbit to the other. When they jumps from higher orbit to lower they emits energy and if jumps from lower to higher they absorbs energy. According to this model nucleus being the central point of the atom rounded in shape , attract the electrons in the circular shells of radius “r”  and give each orbit and specifically to the atom a fixed shape. So an atom should exist and nor destroyed until any of the external factor (force) act on it. This is how Doctor Bohr prove that an atom should exist.

 

Question#7. What do you mean by electronic configuration? What are basic requirements while writing electronic configuration of an element (atom)?

Answer: please see answer of question number  8.

 

Question#8. Describe the electronic configuration of Na+, Mg2+ and Al3+ ions. Do they have the same number of electrons in the outermost shell?

Answer: (A) Sodium has 11 electrons. By the loss of 1 electron Na+ ion is formed. It means that Na+ ion has 10 electrons.

Electronic configuration of Na+ ion in shells:

K=2,  L=8.

Electronic configuration of Na+ ion in sub shells:

1S2,  2S2,  2P6.

(B)  Magnesium  has 12 electrons. By the loss of 2 electron Mg2+ ion is formed. It means that Mg2+ ion has 10 electrons.

Electronic configuration of Mg2+ ion in shells:

K=2,  L=8.

Electronic configuration of Mg2+ ion in sub shells:

1S2,  2S2,  2P6.

(C)  Aluminum has 13 electrons. By the loss of 3 electron Al3+ ion is formed. It means that Al3+ ion has 10 electrons.

Electronic configuration of Al3+ ion in shells:

K=2,  L=8.

Electronic configuration of Al3+ ion in sub shells:

1S2,  2S2,  2P6.

(D)  Yes, all the ions have same number of electrons, that is each having 10 electrons.

 

Question#9. Give the applications of isotopes in the field of radiotherapy and medicines.

Answer:  Please see answer of question number 9.

Read Also > Chapter 1 FUNDAMENTALS OF CHEMISTRY – 9th Class Chemistry Notes

Question#10) What is an isotope? Describe the isotopes of hydrogen with diagrams.

Answer:  Please see answer of question number 9.

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