Structure of the Atom - Test Questions

Question 1

Question:   What is an electron? State its relative mass and charge.

Answer:    The electron is a negatively charged particle found in the atoms of all the elements. Its relative mass is 1/1840 a.m.u and relative charge is 1.

Question 2

Question:   Define the following terms: a) Nucleons b) Atomic number c) Mass number d) Nucleus

Answer:    a) Nucleons: The particles found inside the nucleus i.e., protons and neutrons are called nucleons.

b) Atomic number: The number of protons present in the nucleus of an atom is called the atomic number. It is denoted as Z. The number of protons is equal to the number of electrons.

c) Mass number: Mass number is defined as the sum of the number of protons and neutrons in the nucleus of the atom. It is denoted by the alphabet 'A'.

d) Nucleus: The solid core of an atom made up of protons and neutrons, is called the nucleus

Question 3

Question:   What are cathode rays? How are they formed?

Answer:    Cathode rays are a stream of negatively charged particles. These particles called electrons are shot from the metal cathode of a discharge tube when an electric current is passed through a gas at a very low pressure. A discharge tube is a long glass tube having two metal electrodes. When the pressure of air in the discharge tube is reduced to 0.001 mm of mercury and a high voltage is applied to the electrode, the emission of light by air stops. But it is noticed that the wall of the discharge tube at the end opposite to the cathode begins to glow with greenish light. Since these rays are formed at the cathode they are known as cathode rays.

Question 4

Question:   a) Which atom contains only two fundamental particles? b) Which particle has constant charge to mass ratio for all matter?

Answer:    a) The hydrogen atom contains only two fundamental particles.

b) Cathode ray particles i.e., electrons, have constant charge to mass ratio for all matter.

Question 5

Question:   What are anode rays? State three properties of anode rays.

Answer:    Anode rays are stream of positively charged particles shot out from the anode of a discharge tube when a current is passed through a gas.

• Anode rays travel in straight lines. They cast shadows of the objects placed in their path.
  
  
• Anode rays can produce mechanical effects. This is evident by the fact they can rotate a light paddle.
  
  
• Anode rays are positively charged as they are deflected towards the negative plate in an electric field.

Question 6

Question:   What happens to e/m ratio of positive rays and why?

Answer:    The mass and charge of positively charged particles depends upon the gas, which is taken in the discharge tube. Different gases contain particles having different masses and different charges and consequently give different types of positive rays. In other words the charge to mass ratio e/m is not constant.

Question 7

Question:   Who discovered the nucleus within an atom? And how?

Answer:    Ernest Rutherford discovered the nucleus within an atom in his alpha-ray scattering experiment. The arrangement of the alpha-particle scattering experiment is as follows:



Rutherford produced a narrow beam of particles from a radioactive source (e.g., radium or polonium), which was allowed to strike an extremely thin gold foil. Rutherford proposed that if the spherical model proposed earlier which made for a uniform distribution of positive and negative particles was correct then the alpha particle striking the gold atoms would be uniformly deflected. However the observation were:

• Most of the alpha particles passed straight through the gold foil without suffering any deflection from their original path
  
  
• A few of them were deflected through small angles, while a very few deflected to a large extent
  
  
• A very small percentage (1 in 100000) was deflected through 90o(turned back)

Question 8

Question:   Describe Rutherford's model of an atom.

Answer:   

• The atom of an element consists of a small positively charged nucleus which is situated at the centre of the atom and which carries almost the entire mass of the atom.
  
  
• The electrons are distributed in the empty space of the atom and are revolving around the nucleus at high speed.
  
  
• The number of electrons in an orbit is equal to the number of positive charges (protons) in the nucleus. Hence the atom is electrically neutral.
  
  
• The volume of the nucleus is negligibly small as compared to the volume of the atom.
  
  
• Most of the space in the atom is empty.
  
  
• The arrangement is just like a solar system.

Question 9

Question:   What is the drawback of Rutherford's nuclear model of the atom?

Answer:    Drawback of Rutherford's nuclear model: It could not explain the stability of the atom because according to the electromagnetic theory if the charged particle undergoes accelerated motion it must lose energy continuously and the electrons should collapse in the nucleus.

Question 10

Question:   Name the fundamental particles whose relative charge is a) +1 b) - 1 C) 0

Answer:   

• +1 proton
  
  
• - 1 electron
  
  
• 0 neutron