Chapter 4 – Structure Of The Atom

Class 9 Science · 41 questions · 41 with answers

Questions

Q1Multiple choice

Which of the following correctly represent the electronic distribution in the Mg atom?

(a)3, 8, 1
(b)2, 8, 2
(c)1, 8, 3
(d)8, 2, 2

Show answer

(b) 2, 8, 2

Q2Multiple choice

Rutherford’s ‘alpha (α) particles scattering experiment’ resulted in to discovery of

(a)Electron
(b)Proton
(c)Nucleus in the atom
(d)Atomic mass

Show answer

Q3Multiple choice

The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following is the correct representation of the element?

(a)15 X
(b)16 X
(c)16 15 X
(d)15 16 X

Show answer

(a) 15 X

Q4Multiple choice

Dalton’s atomic theory successfully explained (i) Law of conservation of mass (ii) Law of constant composition (iii) Law of radioactivity (iv) Law of multiple proportion

(a)(i), (ii) and (iii)
(b)(i), (iii) and (iv)
(c)(ii), (iii) and (iv)
(d)(i), (ii) and (iv) 16-04-2018

Show answer

(d) (i), (ii) and (iv) 16-04-2018

Q5Multiple choice

Which of the following statements about Rutherford’s model of atom are correct? (i) considered the nucleus as positively charged (ii) established that the α–particles are four times as heavy as a hydrogen atom (iii) can be compared to solar system (iv) was in agreement with Thomson’s model

(a)(i) and (iii)
(b)(ii) and (iii)
(c)(i) and (iv)
(d)only (i)

Show answer

(a) (i) and (iii)

Q6Multiple choice

Which of the following are true for an element? (i) Atomic number = number of protons + number of electrons (ii) Mass number = number of protons + number of neutrons (iii) Atomic mass = number of protons = number of neutrons (iv) Atomic number = number of protons = number of electrons

(a)(i) and (ii)
(b)(i) and (iii)
(c)(ii) and (iii)
(d)(ii) and (iv)

Show answer

(d) (ii) and (iv)

Q7Multiple choice

In the Thomson’s model of atom, which of the following statments are correct? (i) the mass of the atom is assumed to be uniformaly distributed over the atom (ii) the positive charge is assumed to be uniformaly distributed over the atom (iii) the electrons are uniformaly distributed in the positively charged sphere (iv) the electrons attract each other to stabilise the atom

(a)(i), (ii) and (iii)
(b)(i) and (iii)
(c)(i) and (iv)
(d)(i), (iii) and (iv)

Show answer

(a) (i), (ii) and (iii)

Q8Multiple choice

Rutherford’s α–particle scattering experiment showed that (i) electrons have negative charge (ii) the mass and positive charge of the atom is concentrated in the nucleus (iii) neutron exists in the nucleus (iv) most of the space in atom is empty Which of the above statements are correct?

(a)(i) and (iii)
(b)(ii) and (iv)
(c)(i) and (iv)
(d)(iii) and (iv) STRUCTURE OF THE ATOM 27 16-04-2018

Show answer

(b) (ii) and (iv)

Q9Multiple choice

The ion of an element has 3 positive charges. Mass number of the atom is 27 and the number of neutrons is 14. What is the number of electrons in the ion?

(a)13
(b)10
(c)14
(d)16

Show answer

(b) 10

Q10Multiple choice

Identify the Mg2+ ion from the Fig.4.1 where, n and p represent the number of neutrons and protons respectively

This question refers to a figure in the original PDF.

(a)
(b)
(c)
(d)Fig. 4.1

Show answer

(d) Fig. 4.1

Q11Multiple choice

In a sample of ethyl ethanoate (CH3COOC2H5) the two oxygen atoms have the same number of electrons but different number of neutrons. Which of the following is the correct reason for it?

(a)One of the oxygen atoms has gained electrons
(b)One of the oxygen atoms has gained two neutrons
(c)The two oxygen atoms are isotopes
(d)The two oxygen atoms are isobars. 16-04-2018

Show answer

Q12Multiple choice

Elements with valency 1 are

(a)always metals
(b)always metalloids
(c)either metals or non-metals
(d)always non-metals

Show answer

Q13Multiple choice

The first model of an atom was given by

(a)N. Bohr
(b)E. Goldstein
(c)Rutherford
(d)J.J. Thomson

Show answer

(d) J.J. Thomson

Q14Multiple choice

An atom with 3 protons and 4 neutrons will have a valency of

(a)3
(b)7
(c)1
(d)4

Show answer

Q15Multiple choice

The electron distribution in an aluminium atom is

(a)2, 8, 3
(b)2, 8, 2
(c)8, 2, 3
(d)2, 3, 8

Show answer

(a) 2, 8, 3

Q16Multiple choice

Which of the following in Fig. 4.2 do not represent Bohr’s model of an atom correctly? (i) (ii) (iii) (iv) Fig. 4.2

This question refers to a figure in the original PDF.

(a)(i) and (ii)
(b)(ii) and (iii)
(c)(ii) and (iv)
(d)(i) and (iv)

Show answer

Q17Multiple choice

Which of the following statement is always correct?

(a)An atom has equal number of electrons and protons.
(b)An atom has equal number of electrons and neutrons.
(c)An atom has equal number of protons and neutrons.
(d)An atom has equal number of electrons, protons and neutrons. STRUCTURE OF THE ATOM 29 16-04-2018

Show answer

(a) An atom has equal number of electrons and protons.

Q18Multiple choice

Atomic models have been improved over the years. Arrange the following atomic models in the order of their chronological order (i) Rutherford’s atomic model (ii) Thomson’s atomic model (iii) Bohr’s atomic model

(a)(i), (ii) and (iii)
(b)(ii), (iii) and (i)
(c)(ii), (i) and (iii)
(d)(iii), (ii) and (i)

Show answer

Q19Short answer

Is it possible for the atom of an element to have one electron, one proton and no neutron. If so, name the element.

Show answer

Yes, it is true for hydrogen atom which is represented as 11 H

Q20Short answer

Write any two observations which support the fact that atoms are divisible.

Show answer

Hint— Discovery of electrons and protons

Q21Short answer

Will 35Cl and 37Cl have different valencies? Justify your answer.

Show answer

Hint— No, 35Cl and 37Cl are isotopes of an element.

Q22Short answer

Why did Rutherford select a gold foil in his α–ray scattering experiment?

Show answer

Hint— gold has high malleability

Q23Multiple choice

Find out the valency of the atoms represented by the Fig. 4.3

This question refers to a figure in the original PDF.

(a)and
(b). (a) (b) Fig. 4.3

Show answer

(a) and

(b) . (a) (b) Fig. 4.3

Q24Short answer

One electron is present in the outer most 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 outer most shell?

Show answer

Q25Short answer

Write down the electron distribution of chlorine atom. How many electrons are there in the L shell? (Atomic number of chlorine is 17).

Show answer

2, 8, 7. The L shell has eight electrons

Q26Short answer

In the atom of an element X, 6 electrons are present in the outermost shell. If it acquires noble gas configuration by accepting requisite number of electrons, then what would be the charge on the ion so formed? 16-04-2018

Show answer

–2

Q27Short answer

What information do you get from the Fig. 4.4 about the atomic number, mass number and valency of atoms X, Y and Z? Give your answer in a tabular form. (X) (Y) (Z) Fig. 4.4

This question refers to a figure in the original PDF.

Show answer

Atomic No. Mass No. Valency X 5 11 3 Y 8 18 2 Z 15 31 3,5

Q28Short answer

In response to a question, a student stated that in an atom, the number of protons is greater than the number of neutrons, which in turn is greater than the number of electrons. Do you agree with the statement? Justify your answer.

Show answer

Hint— No, the statement is incorrect. In an atom the number of protons and electrons is always equal.

Q29Short answer

Calculate the number of neutrons present in the nucleus of an element X which is represented as 15 X

Show answer

Mass number = No. of protons + No. of neutrons = 31 ∴ Number of neutrons = 31– number of protons = 31–15 = 16

Q30Multiple choice

Match the names of the Scientists given in column A with their contributions towards the understanding of the atomic structure as given in column B (A) (B) (a) Ernest Rutherford

(i)Indivisibility of atoms (b) J.J.Thomson
(ii)Stationary orbits (c) Dalton
(iii)Concept of nucleus (d) Neils Bohr
(iv)Discovery of electrons (e) James Chadwick (v) Atomic number (f) E. Goldstein (vi) Neutron (g) Mosley (vii) Canal rays

Show answer

(i) Indivisibility of atoms (b) J.J.Thomson

(iii) Concept of nucleus (d) Neils Bohr

(ii) Stationary orbits (c) Dalton

(iv) Discovery of electrons (e) James Chadwick (v) Atomic number (f) E. Goldstein (vi) Neutron (g) Mosley (vii) Canal rays

Q31Short answer

The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of both these elements is 40. What is the name given to such a pair of elements?

Show answer

Isobars

Q32Short answer

Complete the Table 4.1 on the basis of information available in the symbols given below

Show answer

Element np nn Cl 17 18 C 6 6 Br 35 46

Q35Multiple choice

12 81

(a)17 Cl
(b)6 C
(c)35 Br Table 4.1 Element np nn STRUCTURE OF THE ATOM 31 16-04-2018 33. Helium atom has 2 electrons in its valence shell but its valency is not 2, Explain. 34. Fill in the blanks in the following statements (a) Rutherford’s α-particle scattering experiment led to the discovery of the ——— (b) Isotopes have same ———but different———. (c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be———and———respectively.
(d)The electronic configuration of silicon is ———and that of sulphur is ———. 35. An element X has a mass number 4 and atomic number 2. Write the valency of this element?

Show answer

Valency is zero as K shell is completely filled. Long Answer Questions

Q36Long answer

Why do Helium, Neon and Argon have a zero valency?

Show answer

Helium has two electrons in its only energy shell, while Argon and Neon have 8 electrons in their valence shells. As these have maximum number of electrons in their valence shells, they do not have any tendency to combine with other elements. Hence, they have a valency equal to zero. 4 3

Q37Multiple choice

The ratio of the radii of hydrogen atom and its nucleus is ~ 105. Assuming the atom and the nucleus to be spherical,

(i)what will be the ratio of their sizes?
(ii)If atom is represented by planet earth ‘Re’ = 6.4 ×106 m, estimate the size of the nucleus.

Show answer

(i) what will be the ratio of their sizes?

(ii) If atom is represented by planet earth ‘Re’ = 6.4 ×106 m, estimate the size of the nucleus.

Q38Long answer

Enlist the conclusions drawn by Rutherford from his α-ray scattering experiment.

Show answer

Rutherford concluded from the α-particle scattering experiment that– (i) Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected. (ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space. (iii) A very small fraction of α-particles were deflected by 1800, indicating that all the positive charges and mass of the gold atom were concentrated in a very small volume within the atom. From the data he also calculated that the radius of the nucleus is about 105 times less than the radius of the atom.

Q39Long answer

In what way is the Rutherford’s atomic model different from that of Thomson’s atomic model?

Show answer

Rutherford proposed a model in which electrons revolve around the nucleus in well-defined orbits. There is a positively charged centre in an atom called the nucleus. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centred in the nucleus. Whereas, Thomson proposed the model of an atom to be similar to a christmas pudding. The electrons are studded like currants in a positively charged sphere like christmas pudding and the mass of the atom was supposed to be uniformly distributed.

Q40Long answer

What were the drawbacks of Rutherford’s model of an atom?

Show answer

The orbital revolution of the electron is not expected to be stable. Any particle in a circular orbit would undergo a acceleration and the 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.

Q41Long answer

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

Show answer

The postulates put forth by Neils Bohr’s 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. These orbits are called energy levels. Energy levels in an atom are shown by circles. These orbits are represented by the letters K,L,M,N,… or the numbers, n=1,2,3,4,…. ANSWERS 113

Q42Long answer

Show diagramatically the electron distributions in a sodium atom and a sodium ion and also give their atomic number.

Show answer

Sodium atom Sodium ion Since the atomic number of sodium atom is 11, it has 11 electrons. A positively charged sodium ion (Na+ ) is formed by the removal of one electron from a sodium atom. So, a sodium ion has 11–1 = 10 electrons in it. Thus, electronic distribution of sodium ion will be 2, 8. The atomic number of an element is equal to the number of protons in its atom. Since, sodium atom and sodium ion contain the same number of protons, therefore, the atomic number of both is 11.

Q43Long answer

In the Gold foil experiment of Geiger and Marsden, that paved the way for Rutherford’s model of an atom, ~ 1.00% of the α-particles were found to deflect at angles > 50º. If one mole of α-particles were bombarded on the gold foil, compute the number of α-particles that would deflect at angles less than 500. 16-04-2018

Show answer

% of α-particles deflected more than 500 =1% of α-particles. % of α-particles deflected less than 500 =100–1 = 99% Number of α-particles bombarded = 1 mole = 6.022×1023 particles Number of particles that deflected at an angle less than 500 = × 6.022 ×1023 SWTLOUV = ×1023 = 5.96 ×10 23 C hapter 5 _lqucpq Multiple Choice Questions