4 - Structure of The Atom



PAGE NO – 47

Q1 – What are canal rays ?
Ans – Canal rays are positively charged radiations. These rays consist of positively charged particles known as protons. They were discovered by Goldstein in 1886.

Q2 – If an atom contains one electron and one proton, will it carry any charge or not ?
Ans – The atom will not contain any charge because one negative charge of single electron and one positive charge of single proton are neutralized by each other.

PAGE NO – 49

Q1 - On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.
Ans – As per Thomson’s model of an atom, the number of electrons are equal to the number of protons in an atom. Hence the positive and negative charge neutralized by each other. And makes atom neutral.

Q2 – On the basis of Rutherford’s model of an atom, which subatomic particles is present in the nucleus of an atom ?
Ans – The sub atomic particle proton is present in the nucleus of an atom according to Rutherford’s model of an atom.

Q3 – Draw a sketch of Borh’s model of an atom with three shells.
Ans – 


Q4 – What do you think would be the observation if the Alpha-particle scattering experiment is carried our using a foil of a metal other than gold ?
Ans – If the alpha-scattering experiment is carried out using a foil of metal rather than gold, there would be no change in the observation.

Q1 – Name the three sub-atomic particles of an atom.
Ans – Electron, Proton and Neutron.

Q2 – Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have ?
Ans – The number of neutrons present in a helium atom = atomic mass – no. Of protons
                = 4 – 2 = 2

PAGE NO – 50

Q1 – Write the distribution of electrons in carbon and sodium atoms.
Ans – Distribution of electrons in carbon atom, atomic number of carbon = 6 = 2, 4
Distribution of electrons in sodium atom , atomic number of sodium = 11 = 2, 8, 1

Q2 – If K and L shells of an atom are full, then what would be the total number of electrons in the atom ?
Ans – If K and L shells of an atom are full, then the total number of electrons in the atom will be 10 because K shell can accommodate total 2 and L shell accommodate maximum 8 electrons that makes a total of 10.

PAGE NO – 52

Q1 – How will you find the valency of chlorine, sulphur and magnesium ?
Ans – If the number of electrons in the outermost shell of the atom of an element is less than or equal to 4, then they valency of the element is equal to the number of electrons in the outermost shell.  On the other hand, if the number of electrons in the outermost shell of the atom of an element is greater than 4, then the valency of that element is determined by subtracting the number of electrons in the outermost shell form 8.

The distribution of electrons in chlorine = 2, 8 , 7
Valency of chlorine = 8 – 7 = 1

The distribution of electrons in sulphur = 2, 8, 6
Valency of sulphur = 8 – 6 = 2

The distribution of electrons in magnesium = 2, 8, 2
Valency of magnesium = 2

Q1 – If the number of electrons in an atom is 8 and number of protons is also 8, then
(i) What is the atomic number of atom? And
(ii) What is the charge on the atom ?
Ans –(i) Atomic number = number of protons = number of electrons = 8
(ii) atom will be neutral because number of protons ( positive charge ) is equal to the number of electrons ( negative charge ).

Q2 – With the help of table 4.1, find out the mass number of oxygen and sulphur atom.
Ans – Mass number of oxygen = number of neutrons + number of protons
                 = 8 + 8 = 16.
Mass number of sulphur = number of neutrons + number of protons
= 16 + 16 = 32.

PAGE NO – 53

Q1 - For the symbol H, D and T tabulate three sub-atomic particles found in each of them.
Ans – 


Q2 – Write the electronic configuration of any one pair of isotopes and isobars.
Ans - Isotopes of carbon : 126C  and 136C. Electronic configuration is 2, 4 because both the atoms have the same number of electrons as 6.
Isobars : 4012Ca - 2, 8, 8, 2 as atomic number is 20 and 4018Ar – 2, 8, 8 as atomic number is 18.

EXERCISE QUESTIONS

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

Q2 – What are the limitations of J.J Thomson’s model of the atom ?
Ans – J.J Thomson’s model of atom could explain the electrical neutrality of the atom but it failed to explain the results of scattering experiments performed by Rutherford. And it was rejected in favour of Rutherford’s model of atom.

Q3
 – What is the limitations of Rutherford’s model of the atom ?
Ans – According to Rutherford’s model of an atom, electrons revolve around the nucleus in fixed orbits. But, an electron revolving in circular orbits will not be stable because during revolution, it experience acceleration. Due to acceleration, the electrons will lose energy in the form of radiation and fall into the nucleus. In such case, the atom would be highly unstable and collapse.

Q4 – Describe Bohr’s model of atom.
Ans – Niels Borh’s model of atom remove the drawbacks of Rutherford’s atomic model. The special features of Bohr’s model are given below : -
(i)  Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.
(ii) While revolving in discrete orbits, the electrons don not radiate energy.
(iii) energy is emitted or absorbed by an atom only when an electron moves from one orbit to another.



Q5 – Compare all the proposed models of an atom given in this chapter.
Ans – THOMSON'S MODEL - An atom consists of a positively charged sphere with electrons embedded in it. 
RUTHERFORD'S MODEL - An atom consists of a positively charged particles concentrated at nucleus and electrons revolve around the nucleus.
BOHR'S  MODEL - As per this model electrons are distributed in different shells with specific energy around nucleus. With complete atomic shells, atom becomes more stable.

Q6 – Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.
Ans – The following rules are followed
(i) The maximum number of electrons present in a shell is given by the formula 2n2, Where ‘n’ is the orbit number.
Therefore the maximum number of electrons in different shells are as follows : - 
First orbit of K – shell will be = 2 x 1 = 2
Second orbit of L – shell will be = 2 x 22 = 8
Third orbit of L – shell will be = 2 x 23 = 18, and so on
(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.
(iii) Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.

Q7 – Define valency by taking examples of silicon and oxygen.
Ans -  The number of electrons ( present in outer most shell ) which an atom gives or takes or shares to complete its octet, or acquire stable configuration.
If the number of valence electrons of the atom of an element is less than or equal to four, then the valency of that element is equal to the  number of valence electrons.
Ex – The atom of silicon has four valence electrons. Thus, the valency of silicon is four.
And if the number of valence electrons of the atom of an element is greater than four, then the valency of that element is greater then four, then the valency of that element is obtained by subtracting the number of valence electrons from eight.
Ex – The atom of oxygen has six valence electrons. Thus, the valency of oxygen is ( 8- 6 ) = 2.

Q8 – Explain with examples
(i) Atomic number
(ii) Mass number
(iii) Isotopes and
(iv) Isobars.
Give any two uses of isotopes.
Ans –
(i) Atomic Number – It is the number of protons present inside nucleus of the atom.
Ex – No, of protons in hydrogen is 1, therefore the atomic number of hydrogen is 1.
(ii) Mass Number – It is the total number of protons and neutrons present inside the nucleus of an atom.
Ex – Mass of carbon is 12 u because it has 6 protons and 6 neutrons, 6 + 6  = 12u.
(iii) Isotopes – They are atoms of same element and have same atomic number but different mass number.
Ex – 126C and 136C, are two isotopes of carbon.
(iv) Isobars – They are atoms of different elements having same mass number but different atomic number.
Ex – 4020Ca and 4018Ar
The two uses of isotopes are:
(i) One isotope of uranium is used as fuel in nuclear reactors.
(ii) One isotope of cobalt is used in the treatment of cancer.

Q9 – Na has completely filled K and L shells. Explain
Ans – Na has atomic number 11, so its electronic configuration is – 2, 8, 1
But Na+ ion has gives one electron from its outermost shell and changes to Na+ = 2, 8
And completely fill K and L shells.

Q10 – If Bromine atom is available in the form of, say two isotopes 7935Br (49.7 %) and 8135Br (50.3 %), calculate the average atomic mass of bromine atom.
Ans – 

Q11 – the average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 168X and 188X in the sample ?
Ans – 

Q12 – If Z = 3, what would be the valency of the element ? Also, name the element.
Ans – Z = 3 it means that the atomic number of the element is 3. Its electronic configuration is 2, 1. Hence the valency of the element is 1.
And the element with Z – 3 is Lithium.
Q13 – Composition of the nuclei of two atomic species X and Y are given as under
Given the mass numbers of X and Y. What is the relation between the two species ?




Ans – The mass number of X = 6 + 6 =12
The mass number of Y = 6 + 8 = 14
Since their number of protons are same but atomic mass are different so they are isotopes.

Q14 – for the following statements, write t for True and F for False.
(a) J.J.Thomson proposed that the nucleus of an atom contains only nucleons.
(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.
(c) The mass of an electron is about 12000 times that of proton.
(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.
Ans –
(a)   F
(b)   F
(c)   T
(d)   F
Put tick against correct choice and cross against wrong choice in questions 15, 16 and 17
Q15 – Rutherford’s alpha-particle scattering experiment was responsible for the discovery of
(a) Atomic mass
(b) Electron
(c) Proton
(d) Neutron
Ans – (a) Atomic nucleus
Q16 – Number of valence electrons in Clion are :
(a)   16
(b)   8
(c)   17
(d)   18
Ans – (b)   8
Q18 – Which one of the following is correct electronic configuration of sodium ?
(a)   2, 8
(b)   8, 2, 1
(c)   2, 1, 8
(d)  2, 8, 1
Ans – (d) 2, 8, 1 is the correct configuration of sodium.
Q19 – Complete the following table : -


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