ICSE Class 9 Atomic structure and Chemical Bonding Ex-4A Solution

 

ICSE
Class 9
Atomic Structure and Chemical Bonding
Solutions
Ex-4A

 

Q1. What is the contribution of the following in Atomic Structure.

(a)         Maharshi Kanada

Ans. Maharshi Kanada stated that matter consisted of indestructible minute particles called paramanus, now called atoms. He also stated that a paramanu does not exist in free state, rather it combines with other paramanus to form a bigger particle called anu.

 

(b)         Democritus

Ans. Democritus called the paramanu an atom which comes from the Greek word ‘atomos’ meaning indivisible.

 

Q2. State Dalton’s atomic theory.

Ans. The postulates of Dalton’s atomic theory are:

(i) Matter consists of very small and indivisible particles called atoms.

(ii) Atoms can neither be created nor be destroyed.

(iii) The atoms of an element are alike in all respects but they differ from the atoms of other elements.

(iv) Atoms of an element combine in small numbers to form molecules.

(v) Atoms of one element combine with atoms of another element in a simple ratio to form molecules of compounds.

(vi) Atoms are the smallest units of matter that can take part in a chemical reaction.

 

Q3. What is an α (alpha) particle?

Ans. An α (alpha) particle is a doubly charged helium ion He²⁺, containing two protons and two neutrons. It is formed by removing 2 electrons from helium atom.

 

Q4. What are cathode rays? How are these rays formed?

Ans. The rays which originate from the negative plate i.e. cathode and travel from the cathode towards the anode are called cathode rays.

When electrical discharge of 10,000 volts is passed through gases at very low pressure (0.01 mm), cathode rays are produced.

 

Q5. What is the nature of charge on

(i)                     Cathode rays

Ans. Negative

 

(ii)                   Anode rays

Ans. Positive

 

Q6. How are X-rays formed?

Ans. When a beam of cathode rays is made to fall upon hard metallic targets like tungsten, X-rays are formed.

 

Q7. Why were anode rays also called as ‘canal rays’?

Ans. Anode rays were also called as ‘canal rays’ because they passed through holes or canals in the cathode.

 

Q8. How do cathode rays differ from anode rays?

Ans. Cathode rays are negatively charged whereas anode rays are positively charged.

 

Q9. State one observation which shows that atom is not indivisible.

Ans. Cathode rays and discovery of Electrons

 

Q10. (a) Name an element which does not contain neutron.

Ans. Hydrogen

 

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

Ans. NO, it will not carry any charge, because electron is negatively charged and proton is positively charged, so net charge is zero.

 

Q11. On the basis of Thomson’s model of an atom, explain how an atom as a whole is neutral.

Ans. Since number of electrons is equal to the number of  protons and electrons are negatively charged and protons are positively charged, the total positive charge is equal to the total negative charge. Thus, an atom as a whole is neutral.

 

Q12. Which sub-atomic particle was discovered by

(a)         Thomson

Ans. Electron

 

(b)         Goldstein

Ans. Proton

 

(c)          Chadwick

Neutron

 

Q13. Name the sub-atomic particle whose charge is

(a)         +1

Ans. Proton

 

(b)         -1

Ans. Electron

 

(c)          0

Ans. Neutron

 

Q14. (a) Which metal did Rutherford select for his α particle scattering experiment and why?

Ans. He selected a gold foil because he wanted as thin a layer as possible and gold is the most malleable metal.

 

(b) What do you think would be the observation of α- particle scattering experiment if carried out on

(i) heavy nucleus like platinum

Ans. Heavy nucleus like platinum will show the same observation as gold, i.e. most of the alpha particles will pass through it, some will be deflected from their path and very few will be deflected by very large angles or completely bounce back.

(ii) light nuclei like lithium

Ans. If light nuclei like lithium is used, then fast moving α-particles may even push the light nucleus aside and may not be deflected back.

 

Q15. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?

Ans. Proton

 

Q16. Which part of atom was discovered by Rutherford?

Ans. Nucleus

 

Q17. How was it shown that atom has empty space?

Ans. In Rutherford’s α-particle scattering experiment, most of the α-particles passed straight through the foil. This proved that atom has empty space.

 

Q18. State one major drawback of Rutherford’s model.

Ans. One major drawback of Rutherford’s model was that it failed to explain the stability of an atom.

 

Q19. In the figure given alongside:

(a)         Name the shells denoted by A, B and C. Which shell has least energy?

Ans. A -K shell

B- L shell

C- M shell

K shell has least energy.

 

(b)         Name X and state the charge on it.

Ans. X is the nucleus and it is positively charged.

 

(c)          The above sketch is of ………….. model of an atom.

Ans. The above sketch is of Bohr’s model of an atom.

 

Q20. Give the postulates of Bohr’s atomic model.

Ans. The postulates of Bohr’s atomic model are:
(i) The electrons revolving around the nucleus are confined to certain fixed orbits called shells or energy levels, each of which is associated with a fixed amount of energy.

(ii) While it is revolving around the nucleus in an orbit, an electron neither loses nor gains energy.

(iii) An electron revolving in a particular orbit, on gaining a certain amount of energy, jumps to the next orbit and vice versa.