Structure of atoms questions Class 9 science Chapter 4

Class 9 Science Chapter 4 Structure of atoms Extra questions –

Here is Extra questions for class 9 science Chapter 4 Structure of atoms with detailed answers. Important questions for class 9 science for exam preparations.

class 9 science chapter 4 structure of atoms extra questions

Class 9 Science

Chapter 4 Structure of Atoms

Extra Questions – Topicwise

Topic – Atom

Q- 1 What are canal rays ?

Ans – The positively charged radiations discovered by E. Goldstein in a gas discharge tube are called canal rays. These rays travel in the opposite direction to the cathode rays. Later These rays are called Protons.

Q- 2 When anode rays are produced – ?

Ans – When cathode is perforated.

Q- 3 If an atom contains one electron and one proton, will it carry any charge or not ?

Ans – Electron has negative charge and proton has positive charge both will neutralize each other so this atom will not carry any charge.

Topic – Electron, Proton, Neutron

Q- 1 The first model of an atom was given by – ?

Ans – J. J. Thomson

Q- 2 The relative atomic masses of many elements are not whole numbers because –

Ans – of the existence of isotopes.

Q- 3 Who has discovered electrons ?

Ans – J.J Thomson

Q- 4 Who has discovered proton ?

Ans – Goldstein

Q- 5 Who has discovered neutron ?

Ans – J. Chadwick

Q- 6 One electron is present in the outermost shell of the atom of an element X. What would be the nature and value of charge on the ion formed if this electron is removed from the outermost shell?

Ans – It will be positive ion and charge will be +1 if electron is removed from the outermost shell.

Q- 7 A subatomic particle which has no charge and has mass nearly that of the proton. Name the particle and what is its location in the atom.

Ans – That subatomic particle is neutron and it is present in the nucleus of the atom with almost same mass.

Q- 8 A subatomic particle which has negative charge and has mass very less than positive charge –

Ans – That subatomic particle which has negative charge is electron.

Q- 9 An atom has equal number of particles –

Ans – An atom has equal number of protons and electrons and which is also equal to atomic number of that atom. i.e. Nitrogen has number of proton (p+) 7 and number of electron (e-) 7 and atomic number (Z)is also equal to 7.

Q- 10 What is the location of electron in the atom.

Ans – Electron revolve around the nucleus in orbits.

Q- 11 What constitutes a nucleus of an atom?

Ans – An atom’s nucleus constitutes proton and neutron.

Q- 12 The maximum number of electrons in any shell will be determined by which formula ?

Ans – 2n²

Q- 13 How many electrons can the outermost shell accommodate?

Ans – 8 electrons

Q- 14 What will be the valency of an atom if it contains 3 protons and 4 neutrons?

Ans – 3 Protons means 3 electrons so 2 electrons will be in first shell and remaining 1 electron will be in the outermost shell.

Q- 15 What is the maximum number of electrons which can be accommodated in ‘N’ shell ?

Ans – No of electrons will get by 2n². And n = 4 for N shell . So number of electrons = 2(4)² = 2 x 16 = 32

Q- 16 What are the sub atomic particles of an atom?

Ans – Electron, Proton and Neutron

Q- 17 The whole mass of atom is constituted in which part of an atom?

Ans – The whole mass of atom is constituted in nucleus of an atom as nucleus has proton and neutron which are very higher in mass comparing with electrons. Electrons mass is very very less than proton. Protons mass is approximately 2000 times as that of electrons.

Topic – Thomson model, Rutherford model, Neils Bohr model

Q- 1 Which model of atoms assumed that electrons are uniformly distributed in the positively charged sphere.

Ans – Thomsons model

Q- 2 Whose model had compared the atoms’ structure with the Christmas pudding?

Ans – Thomson’s model

Q- 3 Rutherford’s model of an atom had proposed which sub atomic particle present in the nucleus of an atom?

Ans – Protons

Q- 4 What are the limitations of J.J Thomson’s model of an atom?

Ans – J.J Thomson had not explained the arrangement of electrons in an atom.

Q- 5 If Rutherford would use a foil of other metal other than gold foil for scattering – particle. What could be the observation for that foil of metal?

Ans – Rutherford used a very thin foil for which alpha-particle is even much smaller than gold atoms in the foil. If any other metal is used which is less malleable than gold then thickness could be more than gold foil and particles may not penetrate that metal foil. Gold foil is thinner than other metal’s foil.

Q- 6 Why did Rutherford select gold foil for penetration of alpha-particles in his experiment?

Ans – There are some reasons –

  1. Rutherford wanted a thin metal foil which can be easily penetrated by alpha- particles.
  2. Gold is most malleable metal which can easily be converted into thin sheet.

Q- 7 What were those alpha-particles which had been used in the Rutherford’s experiment?

Ans – Rutherford used alpha – particles which were doubly charged helium ions. Since they have a mass of 4u, the fast moving particles have a good amount of energy to penetrate the gold foil.

Q- 8 When alpha-particles are sent through a thin metal foil, why most of them go straight through the foil ?

Ans – Most of these particles go straight through the foil because most part of the atom is empty.

Q- 9 What are the three observations Rutherford had found after the alpha-particle penetration on the gold foil? And what were the meaning of these observations?

Ans – The observations were-

  1. There were large number of particles which go straight through the foil. Means most of the part of the atom is empty.
  2. There were some particle get deflected by the some angles through the foil. Means that was the heavy positively charged center called nucleus.
  3. There were every 1 out of 12000 rays were rebounding back i.e. coming back. This conclude that very small volume of nucleus than the volume of atom.

Q- 10 What are the postulates of Bohr’s model of an atom?

Ans – These postulates are –

  1. Only certain special orbits known as discrete orbits of electrons are allowed inside the atoms.
  2. When revolving in these orbits electrons do not radiate energy. This means there is no loss of energy. These orbits are represented by these letters K,L,M,N….. .

Q- 11 State the major drawback in Rutherford’s model in an atom. Mention two features of Bohr’s model which helped in compensation of this drawback.

Ans – According to Rutherford, Electrons revolve around the nucleus in the fixed orbit. Electron would undergo in acceleration. During acceleration electron should lose energy and then falling into nucleus. This should nucleus make highly unstable. But we know that atoms are stable.

Neils Bohr had overcome the drawbacks of the Rutherford’s model. And proposed –

  1. Electron revolve around the nucleus in circular orbits. And it does not loose energy. So the energy of an electron remains constant in a fixed orbit. So atom is stable.

Topic – Electronic Configuration, No. of Electron, Proton, Neutron And Valency

Q- 1 Find the valency of Sulphur, Chlorine, Magnesium.

Ans – Valency of Sulphur –

  1. Valency of Sulphur –

Atomic number – 16 , Number of electrons – 16

Distribution of electrons in shells : K = 2, L = 8, M = 6

Valency of sulphur = 8 – 6 = 2

Valency of sulphur atom is 2.

  1. Valency of Chlorine –

Atomic number – 17 , Number of electrons – 17

Distribution of electrons in shells : K = 2, L = 8, M = 7

Valency of Chlorine = 8 – 7 = 1

Valency of Chlorine atom is 1.

  1. Valency of Magnesium –

Atomic number – 12, Number of electrons – 12

Distribution of electrons in shell : K = 2, L = 8, M = 2

Q- 2 An element ‘X’ contains 7 electrons in ‘M’ shell as valence electrons :

  1. What is the atomic number of ‘X’ ?
  2. Identify whether ‘X’ is a metal or non-metal.

Ans – (a) If ‘X’ contains 7 electrons in ‘M’ shell as valence electrons, then distribution of electrons is K = 2, L = 8, M = 7

So atomic number = 17

  1. It is a non-metal because its valency is 8 – 7 = 1 so it can take one electron and it is electro negative.

 

Q- 3 What will be the valency of an atom if it contains 3 protons and 4 neutrons?

Ans – Valency of the atom will be 1. Because number of electrons will be 3 and electron distribution in the orbits will K shell = 2 electrons and L shell = 1 electron. Outermost shell has 1 electron so valency is 1.

Q- 4 Helium atom has 2 electrons in its valence shell but its valency is not 2. Explain.

Ans – Helium has 2 electrons in its valence shell where K shell = 2 electrons. In K shell, Maximum 2 electrons can be filled so it does not need to complete its shell, its already complete. So helium has valency zero.

Q- 5 Electrons A,B,C,D and E have atomic numbers 4,9,13,17 and 20 respectively.

  1. Write their electronic distribution
  2. Determine their valency

Ans – (a) Here is the electronic distribution of the A,B,C,D and E

A (4) – – – K shell = 2, L shell = 2 electrons

B (9) – – – K shell = 2, L shell = 7 electrons

C (13) – – – K shell = 2, L shell = 8, M shell = 3 electrons

D (17) – – – K shell = 2, L shell = 8, M shell = 7 electrons

E (20) – – – K shell = 2, L shell = 8, M shell = 8, N shell = 2 electrons

 

(b)Valency of the elements –

A = 2 (outermost shell has 2 electrons)

B = 1 (outermost shell has 7 electrons and valency 8-7 = 1)

C = 3 (Outermost shell has 3 electrons)

D = 1 (Outermost shell has 1 electron)

E = 2 (Outermost shell has 2 electrons)

Q- 6 The given figure depicts the atomic structure of an atom of an element ‘X’

Write the following information about the element ‘X’.

  1. Atomic number of ‘X’
  2. Atomic mass of ‘X’
  3. Valence electrons
  4. Valency o ‘X’
  5. ‘X’ should be metal or non-metal

Ans – (a) Atomic number = 8 as number of proton = 8

(b)Atomic mass of X = number of proton + number of neutron

= 8 + 10 = 18 u

  1. Valence electrons = 6
    (outermost shell)
  2. Valency of X = 8 – 6 = 2
  3. X should be non-metal as there are 6 six electrons in valence shell so it will need 2 electrons to complete its shell then it will be electro negative. And electro negatives are non-metals.

Q- 7 Show by diagram electron distribution in a sodium atom and a sodium ion. Also show electron distribution in a chloride atom and a chloride ion.

Ans – Electron distribution of Sodium atom and sodium ions

As Sodium atom has 11 electrons So K shell = 2, M shell = 8, N shell = 1 electron

Sodium ion has 10 electrons as 1 electron is given by sodium atom so it has become sodium ion. So it has K shell = 2 , M shell = 8 electrons

Chlorine atom has 17 electrons so K shell = 2, M shell = 8, N shell = 7 electrons

Chlorine ion has 18 electrons as it has taken one electron to complete its shell so it becomes chlorine ion. So it has K shell = 2, M shell = 8, N shell = 8 electrons.

Q- 8 Complete the following table –

Atomic number

Mass number

No. of electrons

No. of protons

No. of neutrons

Name of the atomic species

10

11

 

16

32

Sulphur

18

22

2

1

1

0

Ans –

Atomic number

Mass number

No. of electrons

No. of protons

No. of neutrons

Name of the atomic species

10

21

10

10

11

Neon

16

32

16

16

16

Sulphur

18

40

18

18

22

Argon

1

2

1

1

1

Deuterium

1

1

0

1

0

Protium

As Atomic number(Z) = number of proton(p+) = number of electron(e-)

And Atomic mass(A) = number of proton(p+) + number of neutron(n)

We can fill this table.

Topic – Isotopes, Isobars and extra questions

Q- 1 Write down the formulae of

(i)Sodium Oxide (ii) Aluminium chloride

(iii) Sodium sulphide (iv) Magnesium hydroxide

Ans –

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