CBSE Class 9 Chemistry Structure Of Atom Worksheet Set C

CBSE Class 9 Chemistry Worksheet - Structure of Atom (2)

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Q.1. What are Canal rays?

Q.2. On the basis of Thomson's model of an atom explain how the atom is neutral as a whole.

Q.3 Draw a sketch of Bohr's model of an atom with three shells.

Q.4. Helium atom has atomic mass of 4u and has two protons in the nucleus. How many neutrons does it have?

Q.5. If the K and L shells of an atom are full, then what would be the number of el ectrons in the atom?

Q.6. If the number of electrons in an atom is 8 and the number of protons is also 8, then ;

(i) What would be the atomic number of the atom?

(ii) What is the charge on the atom?

Q.7. Na+ ion has completely filled k and L shells.Explain.

Q.8. The average atomic mass of a sample of element X is 16.2 u. What are the percentages of isotopes,

16 8X and 18 8 X in the sample?

Q.9. If Z=3, what would be the valency of the element ? Also, name the element.

Q. 10. Composition of the nuclei of two atomic species X and Y are given as under Protons Neutrons

X 6 6

Y 6 8

Give the mass numbers of X and Y. What is the relation between the two species ?

Q.11. Why do the elements helium, neon and argon have zero valency?

Q.12. Why do elements which exist as isotopes have fractional atomic masses ?

Q.13. Are mass number and atomic mass of an element equal in all respects?

Q.14. The element ALuminium is written by the symbol 27 13 Al. Write the number of protons,electrons and neutrons present in it.

Q.15. The electronic configuration of an element Z is 2, 8, 6. How many electrons does it require to have a stable configuration?

Q.16. Two atoms A and B have the following composition

Exam Questions NCERT Class 9 Science Chapter 4 Structure of the Atom

Question. Write three main features of Rutherford’s nucleus model of an atom.
Ans : On the basis of a -particle scattering experiment, Rutherford proposed a model of atom. According to him :
There is positively charged centre in an atom called the nucleus which contains the whole mass of the atom.
The electrons revolve around the nucleus.
The size of the nucleus is very small as compared to the size of the atom.

Question. What do you think would be the observation if the a -particle scattering experiment is carried out using afoil of a metal other than gold?
Ans :  If experiment is done by using a foil of some other metal, then results would not have been such as most of the particles will be deflected back. Gold can be beaten to an extremely thin sheet as it is extremely malleable metal. Rutherford could predict with the use of extremely thin gold foil that, “Most of the space inside the atom is empty and nucleus is positively charged occupying a small volume within the atom.”

Question. Helium atom has two electrons in its valence shell but its valency is not two. Why?
Ans : Helium atom has two electrons in its valence shell but its valency is not two as its outermost shell is completely filled, its combining capacity or valency is zero.

Question. Write the symbols of two isotopes of uranium.
Ans : Uranium 238 (92U238) and Uranium 235 (92U235).

Question. What kind of elements have a tendency to lose electrons? What are they commonly called?
Ans : The elements having 1, 2 or 3 valence electrons have the tendency to loose electrons. They are commonly called metals.

Question. What is the relation between physical and chemical properties of isobars?
Ans : Isobars have different atomic numbers, so different electronic configuration and have different chemical properties. They are atoms of different elements, hence they have different physical properties.

Question. Is there any relationship between atomic number, mass number, isotopes, isobars and valency of an atom? Explain.
Ans : Atomic number : It tells the number of protons (Z). Atomic Mass : Total number of proton and neutron (A) is called atomic mass.
Isotopes : When atoms of same element have same number of protons but different number of neutrons is called isotopes.
Isobars : When atoms of different element have same atomic mass but different atomic number, such atoms are called isobars.
Valency : It is the combining capacity of an atom or it is defined as number of electrons lost or gained by an atom to acquire noble gas configuration.

Question. State the properties of isotopes.
Ans : Isotopes have the following uses : Treatment of disease : Radioactive isotopes are used for the treatment of dreadful diseases like cancer.
Cobalt-60 is used to kill malignant cells in patients suffering from cancer.
(i) It is used in chemical analysis.
(ii) It is used to detect disease in plants.
(iii) Dating of plants/animals being obtained by using carbon-14 after excavation.
(iv) Thyroid disorders can be treated by I-131.
(v) Leukaemia can be treated by using P-32.
(vi) Uranium-235 can produce electricity.

Question. Name the isotope used for treatment of cancer.
Ans : Isotope of cobalt : Co-60.

Question. Give the mass and charge of an electron.
Ans : (i) The mass of electron is about 9.0 × 10–31 kg.
(ii) An electron is negatively charged particle and has a charge of 1.6 × 10–19 coulomb.

Question. Find the number of neutrons in 31X15.
Ans : 31X15 indicate that no. of proton
                             = 15 and mass number = 31
         Mass number = No. of protons
   + No. of neutrons = 31
Number of neutrons = 31 – number of protons
                              = 31 – 15 = 16

Question. Is C1-35 and C1-37 have different valencies?
Ans : No, it is because these are isotopes of chlorine that have same atomic number but different mass number.

Question. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?
Ans : The positively charged particle in the nucleus of an atom is called ‘proton’.

Question. Where is the mass of an atom concentrated?
Ans : Mass of an atom is concentrated in nucleus.

Question. Why do noble gases show low reactivity?
Ans : The outermost shell of the atoms of noble gases are complete. So, they show little chemical reactivity.

Question. If K and L shells of an atom are full, then what would be the total number of electrons in the atom?
Ans : Number of electrons in full K-shell = 2
Number of electrons in full L-shell = 8
Total number of electrons in an atom is 2 + 8 = 10 electrons.

Question. Why an atom is electrically neutral?
Ans : An atom is electrically neutral because it contains equal number of positively charged particles and negatively charged particles, i.e. protons and electrons.

Question. If Z = 3, what would be the valency of the element? Also, name the element.
Ans : Atomic number Z = 3
Electronic configuration = 2 (K), 1 (L)
Valence shell has 1 electron in the outermost shell, so
valency of element is 1. The element is lithium.

Question. Why is the valency of Na is 1 and not 7?
Ans : It is easy for Na atom to lose one electron instead of gaining seven electrons. So, valency of Na is one and not seven.

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