Chapter Review:

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• Atoms and molecules are the fundamental building blocks of matter.
• The existence of different kinds of matter is due to different atoms constituting them.
• The electron is identified by J.J. Thomson.
• Electrons, protons and nutrons are sub atomic particles. 
• E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays.
• Electrons carry Negative (-) charge. 
• Protons carry Positive (+) charge.
• Nutrons carry No any charge.
• J.J. Thomson was the first one to propose a model for the structure of an atom.
• The electrons revolve around the nucleus in well-defined orbits.
• The size of the nucleus is very small as compared to the size of the atom.
• Protons and Nutrons are situated in nucleus of atom.
• Orbits or shells of an atom are represented by the letters K, L, M, and N.
• J. Chadwick discovered another sub atomic particles named nutron. 
• The number of protons present in nucleus of an atom is known as atomic number. i.e Atomic mass = total number of protons in nucleus os an atom. 
• The sum of the masses of protons and neutrons present in the nucleus is known as Atomic Mass.
• The maximum number of electrons present in a shell is given by the formulla 2n².
• The electrons present in the outermost shell of an atom are known as the valence electrons.
• The number of valence electrons present in outer most shell of an atom is known as its valency.
• The mass number is defined as the sum of the total number of protons and
  neutrons present in the nucleus of an atom.
• The atoms of the same element, having the same atomic number but differentmass numbers is called its isotopes.
• Protium, Deuterium and tritium are isotopes of Hydrogen.
• Many elements consist of a mixture of isotopes. Each isotope of an element is a pure substance.
• The chemical properties of isotopes are similar but their physical properties are different.
• An isotope of uranium is used as a fuel in nuclear reactors.
• An isotope of cobalt is used in the treatment of cancer.
• An isotope of iodine is used in the treatment of goitre.
• Atoms of different elements with different atomic numbers, which have the same mass number, are known as isobars.

Exercises

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Q1. Compare the properties of electrons, protons and neutrons.

Answer: 

Q2. What are the limitations of J.J. Thomson’s model of the atom?

Answer: 

Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model.

Q3. What are the limitations of Rutherford’s model of the atom?

Answer: Acording to Rutherford Atomic model : 

The electrons revolve around the nucleus. The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo acceleration. During acceleration, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus. In this case the atom should be highly unstable and hence matter would not exist in the form. But this is not fact we know atom is stable.

Q4. Describe Bohr’s model of the atom.

Answer: Neils Bohr put forward the following postulates about the model of an atom:

(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.

(ii) While revolving in discrete orbits the electrons do not radiate energy.

Q5. Compare all the proposed models of an atom given in this chapter.

Answer: 

Q6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Answer: 

The various shells which are also called orbit are represented by the letters K, L, M, N ....... 

(i) The maximum number of electrons present in a shell is given by theformula 2n², where ‘n’ is the orbit number like n=1,2,3,4,…. or energy level index, 1,2,3,…. and so on. 

For shell K, n = 1 

Therefore, maximum number of electrons = 2(1)² = 2 

For shell L, n = 2  

Therefore, maximum number of electrons = 2(2)² = 8 

For shell M, n = 3

Therefore, maximum number of electron = 2(3)² = 18 

(ii) The outermost orbit can be accommodated by a maximum number of 8 electrons if they do have more than 8 or 18 electrons. 

(iii) Shells are filled with electrons in a step wise manner i.e., the outer shell is not occupied with electrons unless the inner shells are completely filled with electrons.  

Q7. Define valency by taking examples of silicon and oxygen.

Answer: The electrons present in the outermost shell of an atom are known as the valence electrons. This number of valence electrons is called valency.

Example of silicon and oxygen

The number of electrons in Silicon = 14 

Electronic configuration of Silicon = 2, 8, 4 

Therefore, Valance Electrons = 4

Hence Valency = 4, 

Example of oxygen 

The number of electrons in Oxygen = 8 

Electronic configuration = 2, 6

Therefore, Valance electrons = 8 - 6

= 2 

Hence Valency of Oxygen = 2 

Q8. Explain with examples

(i) Atomic number,

(ii) Mass number,
(iii) Isotopes and

(iv) Isobars. Give any two uses of isotopes.

Answer : 

(i) Atomic Number : The number of protons present in the nucleus of an atom is known as atomic number. 

Example : (i) Hydrogen (H) has only 1 proton in its nucleus therefore Atomic Number of Hydrogen is 1. 

(ii) Sodium (Na) has 11 protons in its nucleus therefore Atomic Number of Sodium is 11. 

(ii) Mass Number : The sum of number of protons and neutrons present in nucleus of an atom is known as atomic mass. 

Example : (i) Carbon has 6 protons and 6 neutrons in its nucleus. Therefore Atomic Mass Number of Carbon = 6 + 6 = 12 

(iii) Isotopes: The atoms of an element are called isotopes of that element which has equal atomic number but different atomic mass. 

Example : Hydrogen Isotopes: 

 

(iv) Isobar : The atoms of an element are called isobar of that element which has same atomic mass but different atomic number are called isobar. 

Example : Calcium and argon has same mass number of both these elements is 40. But the atomic number of calcium is 20 and argon is 18. These elements are called isobar. 

Two Uses of Isotopes are : 

(i) An isotope of uranium is used as a fuel in nuclear reactors.

(ii) An isotope of cobalt is used in the treatment of cancer.

Q9. Na+ has completely filled K and L shells. Explain.

Answer : Normally, Electronic configuration of Na is 2, 8, 1. 

But when It forms Na⁺ then its configuration is 2, 8 

In this case Na⁺ completely filled K and L shells. As K and L require maximum number of electron that these hold are 2 and 8. 

 

Additional And Important Questions with solutions

 

Ques.1 – Why does an atom have neutral charge?

Ans. – As an atom have equal protons and electrons. The protons carry positive charge and electrons carry negative charge. Thus an atom has neutral charge.

Ques.2 – What is an alpha particle?

Ans. – Alpha particle is a double charged helium particle which has positive charge.

Ques.3 – Write the name of sub-atomic particles of an atom.

Ans. –

1. Proton – it has positive charge.
2. Electron – it has negative charge.
3. Neutron – it has no charge.

Ques.4 – Name the founder of electron.

Ans. – J. J. Thomson

Ques.5 – What is canal rays?

Ans. – Canal rays were positively charged radiations of a gas discharged from the anode of discharged canal which ultimately led to the discovery of another sub-atomic particle. 

Ques.6 – Whom does credit for discovery of proton go?

Ans – E. Goldstein. 

Ques.7 – What are the limitations of J.J. Thomson’s model of the atom?

Ans. – Thomson’s model explained that atoms are electrically neutral, the results of experiments carried out by other scientists could not be explained by this model,

Ques.8 – What are the limitations of Rutherford’s model of the atom?

Ans. – According to electromagnetic principle Rutherford’s model of the atom makes unstable to an atom while an atom is stable.

Ques.9 – What postuletes Neils Bohr put about the model of an atom?

Ans. – Neils Bohr put forward the following postulates about the model of an atom:

1.  Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
2. While revolving in discrete orbits the electrons do not radiate energy.

Ques.10 – Describe Rutherford’s model of the atom.

Ans. – Rutherford put forward the nuclear model of an atom, which had the following features:

1.  There is a positively charged centre in an atom called the nucleus.
2. Nearly all the mass of an atom resides in the nucleus.
3.  The electrons revolve around the nucleus in well-defined orbits.
4. The size of the nucleus is very small as compared to the size of the atom.

Additional And Important Questions With solutions:

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Q – Write the results of Rutherford’s particle scattering experiment.

A – The following observations were made: Most of the fast moving particles passed straight through the gold foil.

1. Some of the α-particles were deflected by the foil by small angles.
2. Surprisingly one out of every 12000 particles appeared to rebound.

5. Compare all the proposed models of an atom given in this

chapter.

6. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Q – Describe Thomson’s model of the atom.

A – Thomson proposed that:

1.  An atom consists of a positively charged sphere and the electrons are embedded in it.
2. The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

Q – Define valency by taking examples of silicon and oxygen.

A – The electrons present in the outermost shell of an atom are known as the valence electrons. This number of valence electrons is called valency.

Example of silicon and oxygen

Number of electrons in silicon = 14

Electronic configuration = 2, 8, 4

The number of valence electrons = 4

Hence valency = 4   

Example of oxygen

Number of electrons in oxygen = 8

Electronic configuration = 2, 6

The number of valence electrons = 8 – 6 = 2

Hence valency = 2  

Q – Define atomic number.

A – The number of protons presence in nucleus of an atom is known as atomic number.

Q – Define atomic mass.

A – The sum of number of protons and neutrons presence in nucleus of an atom is known as atomic mass.

Q – Define isotopes.

A – The atoms of an element are called isotopes of that element which has equal atomic number but different atomic mass.

Q – Define isobar.

A – The atoms of an element are called isobar of that element which

Q – Differentiate between isotopes and isobars.

A -

isotopes	isobars
It has equal atomic number but different atomic mass. Example Hydrogen has three isotopes : Protium (), deuterium (  or D) and tritium  ( )	It has equal atomic mass but different atomic number. Calcium, atomic number 20, and argon, atomic number 18. The number of electrons in these atoms is different, but the mass number of both these elements is 40.

 

Q – Write three applications of isotopes.

A – Three applications of isotopes.

1.  An isotope of uranium is used as a fuel in nuclear reactors.
2. An isotope of cobalt is used in the treatment of cancer.
3. An isotope of iodine is used in the treatment of goitre.

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