NCERT solution for class 9 // Chapter 4 - Structure of the atom

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NCERT solution for class 9 // Chapter 4 - Structure of the atom

Chapter 4 - Structure of the atom

                            Page number 47 

Q1. What are canal rays? 

Ans- Canal rays are positively charged radiation with the help of canal rays the positively charged proton and another subatomic particles were discovered

Q2. If an atom contains one electron and one proton will it carry any charge or not? 

Ans- If that an atom contain same positive and negative charge then  atom is electrically neutral so the atom is neutral in this condition 

                Page number 49 

Q3. On the basis of Thomson model of an atom explain how the atom is neutral as a whole

Ans- According to the Thomson model of atom atom consists of positively charged in a sphere while the negatively charged electron are embedded in that sphere. The number of proton and electron are equal therefore it neutralize to each other charge and overall the atom is neutral

Q4. On the basis of Rutherford model of an atom which subatomic particle is present in the nucleus of an atom ? 

Ans- According to  Rutherford model proton its present in the nucleus of an atom which is centrally located

Q5. Draw a sketch of bohr's model of an atom with three shells?

Ans - 

Q6. What do you think would be the observation if the Alpha particle scattering experiment was carried out using a foil of a metal other than gold?

Ans - If the any other metal foil is used instead of gold then the observation remains same because the structure of atom is similar for all type of matter ( metal)

Q.7. Name the three subatomic particles of an atom? 

Ans - Proton- positively charged particle

 Neutron  - neutral  particle

Electron  - are the three subatomic particles of an atom 

Q8. Helium atom has an atomic mass of 4u a negatively charged particle and 2 proton in its nucleus how many neutrons does it have

Ans - Atomic mass of He = 4u 

Atomic mass = number of proton + neutron 

4 =  2 + number of neutron

 Number of neutron = 4  -  2 = 2 

So Helium atom has 2 neutron

            Page number 50 

Q9 . Write the distribution of electrons in carbon and sodium atom? 

Ans- The distribution of electrons in an atom is called electronic configuration

the atomic number of Carbon is=  6 

Carbon has  6 proton and 6 electron 

K = 2 , L= 4

The electronic configuration of carbon is = ( 2, 4)

the atomic number of sodium is = 11 

Sodium has 11 electron and 11 proton

K= 2 , L = 8 , M= 1

The electronic configuration of sodium (Na) is  =( 2,8,1 ) 

Q10 . If K and L shell of an atom are full then what would be the total number of electron in an atom ? .

Ans - The number of electron  K shell can hold= 2

The number of electron L shell can hold = 8 

Total number of electron found in both shell= 2+8 = 10 electron.

                Page number 52 

Q11. How will you find the valency of chlorine Sulphur and magnesium.

Ans- An atom of each element has a definite combining capacity is called its valency  

Atomic number of chlorine = 17 

Number of proton = Number of electron= 17 

Distribution in different shell K - 2, L - 8 , M- 7

Chlorine needs one electron to complete its outermost shell so it has 1 valency

Atomic number of Sulphur =16 

Number of protons= Number of electrons = 16

Distribution of electrons in different shells

K -   2     L-8 ,    M - 6 

Sulphur needs    2 electron to complete its outermost shell so it has two valency 

 The atomic Number of Magnesium= 12 

Number of proton = Number of electron  - 12 distribution K- 2 ,L- 8 , M - 2 

 Magnesium needs to denote two electrons from its outermost shell to become a stable so its valency is 2 ⁺ 

Q12. If the number of electron in an atom is 8 and the number of protons is also 8 then what is the atomic number of atom and what is the charge on an atom .

Ans- The atomic number of atom is 8 because I am number is nothing but the number of proton or number of electron present in an atom

 the electronic configuration of atom is -  (2,6 )

so ,it has 2^- charge on it.

Page number 53 

Q13. For the symbol H, D and T tabulate three subatomic particles found in each of them 

Ans- 

Protium.	Deuterium.	Tritium
Atomic no 1	Atomic no 1	Atomic no 1
Mass no 1	Mass no 2	Mass no 3
No of proton1	No of proton 1.	No of proton 1
No electron 1.	No electron 1	No electron 1
No neutron nil	No. neutron 1.	No neutron 2

Q14 . Write the electronic configuration of any one pair of isotopes and isobars.

Ans - Isotopes 

Isotopes are atoms which have the same number of protons but the number of neutron is different. 

Eg. Carbon has two isotopes ⁶C₁₂  and ⁶C₁₄

Isobars

Isobars of an atom which have same mass number but different atomic number

Eg. ⁴⁰ Ca _{20  ,}  ^4o Ar₁₈

            Exercise 

Q1. Compare the properties of electron proton and neutron.

Ans- 

Q2. What are the limitations of JJ Thomson model of the atom? 

Ans- According to JJ Thomson an atom is A positively charged sphere while the negatively charged particles are suspended in that sphere but according to the others scientist the proton present only in the center of the atom and electron distributed around it

Q3. What are the limitations of Rutherford model of an atom

Ans- According to Rutherford model the revolution of the electron in a circular orbit is not expected to be stable because the particles in a circular orbit undergoing the acceleration and it radiate the energy so the revolving electrons would lose the energy and finally fall down into the nucleus so the atom should be highly unstable and hence the matter would not be exist but we know that the atom is quite stable

Q4. Describe bohr's model of an atom?

Ans- According to Niels bohr's model

•  The nucleus of an atom is centrally located. around this nucleus the negatively charged electron are found 
• Electron are present inside the atom in a definite orbits and in this orbit the electrons do not radiate energy
•  This orbit is represented by K, L, M,N orbits and the number of the numbers is 1,2,3,4

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

Ans- 

Q6 . Summarise the rules for the writing of distribution of electrons in various shells for the first 18 elements.

Ans- The electrons are accommodated in the orbit for sale with the help of given formula 2n²

 where n is the number of shell 1,2,3,4 

The maximum number of electrons in different orbits 

 K orbit n = 1 , 2n² = 2(1)2 = 2

L orbit n =2 , 2n²=  2(2)2 = 8 

M orbit n= 3 ,2n²=  2(3)2= 18

N orbit n= 4 , 2n²= 2(4)2 = 32 

The highest 18 element has K2 ,L-8. M - 8

Q7. Define valency by taking example of silicon and oxygen? 

Ans- Valency is the combining capacity of an atom

In oxygen atomic number =8 

electronic configuration=  K-2,  L- 6 

valence electron = 6

 valency  8-6 = 2 

In silicon atomic number 14 

electronic configuration = K-2 , L- 8, M- 4 

valence electrons= 4

 valency  = 8-4=  4 

Q8. Explain with example atomic number mass number isotopes isobars give any two uses of isotopes 

Ans- Atomic number - 

Atomic number of an element is the total number of proton or electron present in an atom

Eg . Atomic number of Nitrogen is 7 because it has 7 proton in it.

Mass number-  

The mass number of an element is the addition of number of proton and number of neutrons present in the nucleus of an atom 

Eg. Lithium has 3 proton and 4 neutron in its atom

Isotopes

 The atoms of the same element having same atomic number but different mass number

Eg . Isotopes of hydrogen protium ( H¹_{1 )} , deuterium  (  H ²₁ ) ,tritium (  H³₁)

Isobars

 There are atoms having same mass number but different atomic number is called isobars the atoms of different elements

Eg.- Calcium has atomic number 20 argon has atomic number 18 but both have the same mass number 40

Q9 . Na+  is completely filled K and L shells explain ? 

Ans- Atomic number of sodium is 11 electronic configuration is (2,8,1 )but Na ⁺ has 10 electron K -  2 and L -8 and it has  1⁺  charged means one electron is donate to other atom so that Na ⁺ has completely filled K and L shells

Q10. If bromine atom is available in the earth in the form of say two isotopes of ⁷⁹Br₃₅ (49.7%) and ⁸¹Br₃₅ (50.3%)calculate the average atomic mass of bromine atom 

Ans-  The atomic mass of two isotopes are 

Thus the total mass = (79  x 49.7/100) + (81 x 50.3/100)

= 39.263+ 40.743= 80.006u

Q11. The average atomic mass of a sample of an element X is 16.2 u what are the percentage of isotopes dash and dash in the sample? 

Ans- 

Q12.  If z is equal to 3 what would be the valency of element also name the element 

Ans- Z= 3 

Electronic configuration K-2 , L -1 

Valency = 1 

The element with atomic number 3 is lithium

Q13. Composition of the nuclei of two atomic species x and y are given as under 

                    X.    Y

protons      6     6 

neutrons   6.     8 

Give the mass number of X and y in what is the relation between the two species

Ans- Mass number of x = proton+ neutron =  6+ 6 = 12 

Mass number of Y =  protons + neutrons = 6+8=14

That mass number of both the elements are same and these are different number of neutrons so the mass number is different and they are the isotopes 

Q14. For the following statement write T for true and F for false

a) JJ 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 electron is about 1/ 2000 times that of proton .

d) an isotopes of iodine is used for making tincture iodine which is used as a medicine 

Ans- a) false 

b) false 

c) true

d)  false

put tick the correct choice and cross against the wrong choice in question 15 ,16 and 17 

Q15. Rutherford's Alpha particle scattering experiment was responsible for the discovery of

a)Atomic nucleus b)  proton  c) electron  d) neutron 

Ans- Atomic nucleus 

Q16. Isotopes of an element have

a)The same physical properties    b) different number of neutron     c) different number of proton  d) different atomic number 

Ans - Different number of neutrons 

Q17. Number of valence electrons in Cl- ion are 

a)16   b) 8   c)17   d) 18

Ans -  8 

Q18. Which one of the following is a correct electronic configuration of sodium.

a) 28   b ) 821.  c) 2,1,8. d) 2,8,1

Ans - 2,8,1 

Q19 . Complete the following table