# Structure of the Atom| NCERT Solutions| Class 9

NCERT Solutions of  Chapter: Structure of the Atom. NCERT Solutions along with worksheets and notes for Class 9.

Page 47

(1) What are canal rays?

Ans – The positively charged ion or radiation produced in a gas discharge tube is called canal ray (anode ray). In 1886, Eugen Goldstein discovered the canal ray.

(2) If an atom contains one electron and one proton, will it carry any charge or not?

Ans- If an atom contains one electron and one proton it will not carry any charge because equal positive charge and equal negative charge produce neutral charge.

Page 49 A

(1) On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

Ans-Thomson proposed that the negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

(2) On the basis of Rutherford’s model of an atom, which sub – atomic particle is present in the nucleus of an atom?

Ans- On the basis of Rutherford’s model of an atom, positive charge sub – atomic particle is present in the nucleus of an atom.

(3) Draw a sketch of Bohr’s model of an atom with three shells.

Ans- The orbits or shells are presented by letters K,L,M,N… or by numbers 1,2,3….. .

A Bohr’s model of an atom with three shells are sketched below – (4) What do you think would be the observation if the  α – particle scattering experiment is carried out using a foil of a metal other than gold?

Ans- The observations will be the same if α– particles scattering experiment is carried out using a foil of a metal other than gold because gold is a metal and all metals show same properties.

Page 49 B

(1) Name the three sub atomic particles of an atom?

Ans- The name of the three sub – atomic particles of an atom are –

(i) Electron (-) Charged

(ii) Proton (+) Charged

(iii) Neutron (Neutral)

(2) Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Ans- Atomic mass of the helium = 4 u

Number of protons = 2

Mass number of the atom =Number of protons + number of neutrons

4 = 2 + number of neutrons

Number of neutron = 2

Hence, number of the neutron in helium is 2.

Page 50

(1) Write the distribution of electron in carbon and sodium.  (2) If k and L shell of an atom are full, then what would be the total number of electrons in the atom? Page 52 A

(1) How will you find the valency of chlorine, sulphur and magnesium?

Ans-   Page 52 B

(1) If numbers of electron in an atom is 8 and number of protons are also 8, then (i) What is the atomic number of the atoms? and (ii) What is the charge on the atom? (ii) Number of protons (+)ve charged = Number of electrons(-) ve charged

Therefore, the charge on atom will be zero.

(2) With the help of table 4.1, find out the mass number of oxygen and sulphur atom.

Ans- Page 53

(1) For the symbol H, D and T tabulate three sub – atomic particles found in each of them.

Ans- The required table is provide below-

 SN Symbol Sub – atomic particles Electrons Protons Neutrons 1. H 1 1 – 2. D 1 1 1 3. T 1 1 2  Exercises

(1) Compare the properties of electrons, protons and neutrons.

Ans- Properties of the electron, protons and neutrons are tabulated below –

 SN. Property Electron Proton Neutron 1. Charge -1 +1 No charge 2. Mass Negligible 2000 times of the electron Equal to proton 3. Found Outer of the atom Interior of the atom Interior of the atom 4. Removal from atom Easily removed Can not remove Can not remove 5. Symbols e p n

(2) What are the limitations of J.J. Thomson’s model of the atom?

Ans- Limitations of J. J. Thomson

(i) Thomson’s model can not explain stability of an atom. Thomson’s model cannot explain how protons hold on the electrons.

(ii) Thomson’s model cannot explain nucleus.

(iii) Distribution of electrons in the atom is not explained by this model.

(3)What are the limitations of Rutherford’s model of the atom?

Ans- Limitations of Rutherford’s model of the atom

(a) The revolution of the electrons around the nucleus in circular path is not stable. The electrons radiate energy due to acceleration while moving on circular orbits. Therefore, revolving electron will loose energy at fall into the nucleus. As a result atoms will be unstable.

(b) He did not specify orbits and number of electrons in the orbits.

(4)Describe Bohr’s model of the atom.

Ans- Neils Bohr’s postulations for atom structure are discuss below-

(a) Electrons can revolve in certain orbits around the nucleus. These orbits are known as discrete orbits.

(b) Electrons do not radiate energy while moving in discrete orbits.

(c)These orbits or shells are called energy levels. Smaller the size of orbit smaller the energy level of that orbit.  The orbits or shells are presented by letters K,L,M,N… or by numbers 1,2,3….. . (5) Compare all the proposed models of an atom given in this chapter.

Ans-

 SN Thomson’s Model Rutherford’s Model Bohr’s Model 1. Atom consist of positive charge sphere Centre of the atom is positive Centre of the atom is positive 2. Electrons are embedded in the sphere Electrons revolve around the nucleus Electrons revolve in discrete orbits 3. No term of nucleus Nucleus is small in size in comparison of size of atom Nucleus is small in size in comparison of size of atom 4. Atom is not stable Atom is unstable Atom is stable 5. No revolving electrons Revolving electrons would radiate energy Revolving electrons would not radiate energy

(6) Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Ans- The rules for writing of distribution of electrons in various shells for the first eighteen elements are given below – (ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.

(iii)The shells are filled in a step – wise manner. Therefore, inner hells or orbits should be filled first.

(7) Define valency by taking examples of silicon and oxygen.

Ans- Electrons present in outermost shell of the any atom is known as valency electrons.

Valency of silicon-

Atomic number of silicon = 14

Electronic distribution of silicon = 2, 8, 4 (By Bohr – burry)

Hence, valency of silicon is 4.

Valency of oxygen

Atomic number of oxygen = 8

Electronic distribution of oxygen = 2, 6 (By Bohr – burry)

Therefore, oxygen atom can leave 6 electrons or can gain 2 electrons. But, it will be easy for oxygen atom to gain 2 electrons and full fill requirement of 8 electrons in outermost shell.

Hence, valency of oxygen is 2.

(8) Explain with examples (i) Atomic number (ii) Mas number, (iii) Isotopes and (iv) Isobars. Give any two uses of isotopes.

Ans-

(i) Atomic number is the number of protons present in the nucleus of an atom.

All atoms of an element have same atomic number.

Atomic number is represented by “Z”.

Eg:- Hydrogen has one proton in its nucleus.

Hence, its atomic number Z = 1.

(ii) The mass number is defined as the sum of the total number of protons and neutrons in the nucleus of an atom.

Mass number is represented by “A”.

Eg:- Find the mass number of carbon atom.

Number of protons in carbon nucleus = 6 p

Number of neutrons in carbon nucleus = 6 n

Mass number of carbon atom = 6 + 6

= 12

(iii) Atoms of the same element that have same atomic number but different mass number are called isotopes of the element.

Eg:- Isotopes of the hydrogen atom are protium, deuterium and tritium.

(i) Many elements consists of mixture of isotopes.

(ii) Each isotope of an element is a pure substance.

(iii) The chemical properties of the isotopes are similar.

(iv) The physical properties of the isotopes are different.

(v) If an element occurs in isotopic form then average mass can be calculated with the percentage of each isotopic form.

Average atomic mass of element

=(Fractional abundance of isotope 1  mass of isotope 1) + (Fractional abundance of isotope 2  mass of isotope 2)…

If abundance in percentage, divide it by 100.

Uses of Isotopes –

Some isotopes have special properties. Some of them are given below –

(i) An isotope of uranium is used as a fuel in nuclear reactors.

(ii) An isotope of cobalt is used in the treatment of cancer.

(9) Na+ has completely filled K and L shells. Explain.

Ans-

Atomic number of sodium is 11.

Hence, distribution of sodium is 2, 8, 1.

Therefore, maximum number of electron could be 1 in third shell.

Sodium atom will lose 1 electron to gain maximum number of 8 electrons in its outermost shell. When sodium atom will leave one electron it will gain positive charge.

Hence, Na+ has completely filled K and L shells.   (12) If Z = 3, what would be the valency of the element? Also, name the element.

Ans-

Atomic number Z = 3

Distribution of electron = 2, 1

Therefore, electron will easily leave one electron.

Hence, valency of the element is 1.

Name of the element is lithium. Ans- (14) 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 neutrons.

(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 1/2000  times that of proton.

(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.

Ans-

(a) J.J.Thomson proposed that the nucleus of an atom contains only neutrons. (F)

(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.  (F)

(c) The mass of an electron is about 1/2000 times that of proton. (T)

(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine. (T)

Put tick (√ ) against correct choice and cross (×) against wrong choice in questions 15, 16 and 17.

(15)Rutherford’s alpha – particle scattering experiment was responsible for the discovery of

(a) Atom Nucleus

(b) Electron

(c) Proton

(d) Neutron

Ans- (a) Atom Nucleus  (√ )

(b) Electron (×)

(c) Proton (×)

(d) Neutron (×)

(16)  Isotopes of an element have

(a) the same physical properties

(b) different chemical properties

(c) different number of neutrons

(d) different atomic numbers.

Ans-

(a) the same physical properties (×)

(b) different chemical properties(×)

(c) different number of neutrons (√ )

(d) different atomic numbers.(×)

(17) Number of valence electrons in Cl ion are :

(a) 16    (b) 8    (c) 17    (d) 18

Ans-

(a) 16 (×)

(b) 8 (√ )

(c) 17 (×)

(d) 18 (×)

(18) Which one of the following is a correct electronic configuration of sodium?

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

Ans- Atomic number of sodium is 11.

Electric configuration of sodium is 2, 8, 1.

(19) Complete the following table.

 Atomic number Mass number Number of Neutrons Number of Protons Number of Electrons Name of the atomic species 9 – 10 – – – 16 32 – – – Sulphur – 24 – 12 – – – 2 – 1 – – – 1 0 1 0 –

Ans-

 Atomic number Mass number Number of Neutrons Number of Protons Number of Electrons Name of the atomic species 9 19 10 9 9 Fluorine 16 32 16 16 16 Sulphur 12 24 12 12 12 Magnesium 1 2 1 1 1 Deuterium 1 1 0 1 0 Hydrogen Ion

Helping Topics

Structure of the Atom

Worksheet Class 9

Atoms and Molecules