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Question 14 Marks

Rutherford (1871-1937) was known as the ‘Father’ of nuclear physics. He is famous for his work on radioactivity and the discovery of the nucleus of an atom with the gold foil experiment. Ernest Rutherford was interested in knowing how the electrons are arranged within an atom. Rutherford designed an experiment for this. In this experiment, fast moving alpha (α)-particles were made to fall on a thin gold foil. On the basis of his experiment, Rutherford put forward the nuclear model of an atom, which had the following features:

There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
The electrons revolve around the nucleus in circular paths.
The size of the nucleus is very small as compared to the size of the atom.

Drawbacks of Rutherford’s model of the atom: The revolution of the electron in a circular orbit 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. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know. We know that atoms are quite stable.

(1) Which of the following scientist was known as the ‘Father of nuclear physics?

(a) J.J. Thomson

(b) John Dalton

(d) Neilsbhore

(2) Positively charged centre in an atom is termed as

(a) Nucleus

(b) Molecule

(d) Protons

(3) Identify the correct statement

Statement 1 – Positively charged centre in an atom called the nucleus.

Statement 2 – The electrons revolve around the nucleus in circular paths.

Statement 3 – Nearly all the mass of an atom resides in the nucleus.

Statement 4 – The size of the nucleus is very small as compared to the size of the atom.

(a) Only 2

(b) Both 3 & 4

(d) All of the above

(4) Write the features of Rutherford’s nuclear model of an atom?

(5) Define Nucleus.

Answer

(1) c

(2) a

(3) d

(4) Rutherford put forward the nuclear model of an atom, which had the following features:

There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
The electrons revolve around the nucleus in circular paths.
The size of the nucleus is very small as compared to the size of the atom.

(5) There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus

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Question 24 Marks

Protons are present in the nucleus of an atom. It is the number of protons of an atom, which determines its atomic number. It is denoted by ‘Z’. All atoms of an element have the same atomic number, Z. In fact, elements are defined by the number of protons they possess. For hydrogen, Z = 1, because in hydrogen atom have only one proton is present in the nucleus. Therefore, the atomic number is defined as the total number of protons present in the nucleus of an atom.

The mass of an atom is practically due to protons and neutrons alone. These are present in the nucleus of an atom. Hence protons and neutrons are also called nucleons. Therefore, the mass of an atom resides in its nucleus. For example, mass of carbon is 12 u because it has 6 protons and 6 neutrons, 6 u + 6 u = 12 u. Similarly, the mass of aluminium is 27 u (13 protons+14 neutrons). The mass number is defined as the sum of the total number of protons and neutrons present in the nucleus of an atom. It is denoted by ‘A’.

(1) Atomic number is denoted by

(a) X

(b) Y

(d) z

(2) The sum of the total number of protons and neutrons present in the nucleus of an atom.

(a) Atomic number

(b) Mass number

(d) None of the above

(3) Mass number is denoted by

(a) A

(b) a

(d) z

(4) Identify the correct statement

Statement 1 – Protons are present in the nucleus of an atom.

Statement 2 – Atomic number is the number of protons of an atom.

Statement 3 – Atomic number is denoted by ‘Z’.

Statement 4 – The mass of an atom is due to protons and neutrons alone.

(a) Only 2

(b) Both 3 & 4

(d) All of the above

(5) Why mass of carbon is 12 u give the reason?

Answer

(1) c

(2) b

(3) a

(4) d

(5) Mass of carbon is 12 u because it has 6 protons and 6 neutrons, 6 u + 6 u = 12 u.

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Question 34 Marks

Dalton’s atomic theory suggested that the atom was indivisible and indestructible. But the discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton’s atomic theory. It was then considered necessary to know how electrons and protons are arranged within an atom. For explaining this, many scientists proposed various atomic models. J.J. Thomson was the first one to propose a model for the structure of an atom.

J.J. Thomson (1856- 1940) was a British physicist, He was awarded the Nobel Prize in Physics for his work on the discovery of electrons. Thomson proposed the model of an atom to be similar to that of a Christmas pudding. The electrons, in a sphere of positive charge. We can also think of a watermelon, the positive charge in the atom is spread all over like the red edible part of the watermelon, while the electrons are studded in the positively charged sphere, like the seeds in the watermelon. Thomson proposed that: An atom consists of a positively charged sphere and the electrons are embedded in it. The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

(1) Identify the correct statement

Statement 1 – Dalton’s atomic theory suggested that the atom was indivisible and indestructible.

Statement 2 – Electrons and protons are present inside the atom.

Statement 3 – J.J. Thomson was the first one to propose a model for the structure of an atom.

Statement 4 – Protons are positively charged particle.

(a) Only 2

(b) Both 3 & 4

(d) All of the above

(2) According to Dalton’s Atomic Theory, matter consists of indivisible _______

(a) Molecules

(b) Atoms

(d) Mixtures

(3) Who was the first to propose atomic theory?

(a) J.J. Thomson

(b) John Dalton

(d) Neilsbhore

(4) “Atom is indivisible and indestructible” why this aspect of Dalton’s atomic theory leds to the failure?

(5) Explain the J.J. Thomson’s model for the structure of an atom?

Answer

(1) d

(2) b

(3) b

(4) Dalton’s atomic theory suggested that the atom was indivisible and indestructible. But the discovery of two fundamental particles (electrons and protons) inside the atom, led to the failure of this aspect of Dalton’s atomic theory.

(5) Thomson was the first one to propose a model for the structure of an atom:

Postulate 1: An atom consists of a positively charged sphere with electrons embedded in it.

Postulate 2: An atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude

Thomson atomic model is compared to watermelon. Where he considered:

Watermelon seeds as negatively charged particles
The red part of the watermelon as positively charged

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Question 44 Marks

Neils Bohr got the Nobel Prize for his work on the structure of atom in 1922. Among Professor Bohr’s numerous writings, three appearing as books are: (i) The Theory of Spectra and Atomic Constitution, (ii) Atomic Theory and, (iii) The Description of Nature.

In order to overcome the objections raised against Rutherford’s model of the atom, Neil’s Bohr put forward the following postulates about the model of an atom:

Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
While revolving in discrete orbits the electrons do not radiate energy. These orbits or shells are called energy levels. Energy levels in an atom are shown in Fig. A few energy levels in an atom These orbits or shells are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,….

(1) The orbits or shells are represented by

(a) Letters

(b) Numbers

(d) Special symbols

(2) These orbits or shells are called

(a) Energy levels

(b) Discrete orbit

(d) None of the above

(3) Which of the following book is written by Professor Bohr’s

(a) The Theory of Spectra and Atomic Constitution

(b) Atomic Theory

(d) All of the above

(4) Identify the correct statement

Statement 1 – The orbits or shells are represented by letters only.

Statement 2 – The orbits or shells are represented by numbers only.

Statement 3 – While revolving in discrete orbits the electrons do not radiate energy.

Statement 4 – Certain special orbits known as discrete orbits of electrons.

(a) Both 1 & 2

(b) Both 3 & 4

(d) All of the above

(5) Write the postulate of Neil’s Bohr model of an atom?

Answer

(1) c

(2) a

(3) d

(4) b

(5) Neil’s Bohr put forward the following postulates about the model of an atom:

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

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Question 54 Marks

A number of atoms of some elements have the same atomic number but different mass numbers. For example, hydrogen atom, it has three atomic species, namely Protium, Deuterium and Tritium. The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively. On the basis of these examples, isotopes are defined as the atoms of the same element, having the same atomic number but different mass numbers. Therefore, we can say that there are three isotopes of hydrogen atom, namely protium, deuterium and tritium.

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.

The mass of an atom of any natural element is taken as the average mass of all the naturally occurring atoms of that element. If an element has no isotopes, then the mass of its atom would be the same as the sum of protons and neutrons in it. But if an element occurs in isotopic forms, then we have to know the percentage of each isotopic form and then the average mass is calculated.

Chemical properties of all the isotopes of an element are the same. Some isotopes have special properties which find them useful in various fields. Such as, an isotope of uranium is used as a fuel in nuclear reactors, isotope of cobalt is used in the treatment of cancer, iodine is used in the treatment of goitre.

(1) The atoms of the same element, having the same atomic number but different mass numbers are termed as __________

(a) Isotopes

(b) Protium

(d) Tritium

(2) Which of the following are the isotopes of hydrogen atom.

(a) Protium

(b) Deuterium

(d) All of the above

(3) Identify the correct statement

Statement 1 – Chemical properties of all the isotopes of an element are the same.

Statement 2 – Physical properties are different.

Statement 3 – Chemical properties of all the isotopes of an element are different.

Statement 4 – Physical properties are same.

(a) Only 2

(b)Both 3 & 4

(d) All of the above

(4) Give any two uses of isotopes.

(5) Define isotopes.

Answer

(1) a

(2) d

(3) c

(4) Isotopes have special properties which find them useful in various fields. Such as,

An isotope of uranium is used as a fuel in nuclear reactors,

Isotope of cobalt is used in the treatment of cancer,
Isotope of iodine is used in the treatment of goitre.

(5) Isotopes are defined as the atoms of the same element, having the same atomic number but different mass numbers

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