Study Portal 10 – Chapter 10

Structure of Atoms


Q1.      Fill in the blanks with suitable words.

  1. i) Experiments with Cathode Ray tube helped a lot in understanding the nature of subatomic particles.
  2. ii) According to Bohar the angular momentum is Number of h / 2π.
  • The most stable isotope of C has mass
  1. Rutherford bombarded the gold foil with Alpha Particle.
  2. Cathode rays were given the name of electron by J.Stoney in 1891.
  3. Nucleus was discovered by Rutherford’s.
  • The presence of neutron was confirmed by James Chadwick in 1930.
  • Electrons radiate energy when jumps from Upper To Lower energy levels.
  1. The modern research showed that electron behaves both as a Wave and a particle.
  2. Number of neutrons in the atoms of 17Cl35 are 18.

Q2.      Choose the correct answer.

  1. i) Structure determination helps to determine the:

(a) physical characters

(b) chemical characters

(c) both physical and chemical characters

(d) does not helps in any way.

  1. ii) Recent studies show that atom has an average size of:

(a) 10-10 meters                         (b) 10-8 meters

(c) 10-15 meters                           (d) 10-12 meters.

  • 1 angstrom is equal to the:

(a) 10-10 meters                           (b) 10-12 meters

(c) 10-15 meters                           (d) 1010 meters.

  1. Neil Bohar mainly concentrated upon:

(a) nucleus of atom                     (b) behavior of electrons       

(c) protons                                              (d) neutrons.

  1. Electronic configuration tells about the:

(a) number of sub shells             (b) number of shells

(c) valency                                  (d) all sub shell, shell and valency.

Q3.      Answer the following questions in short.

  1. i) Describe the structure of cathode ray tube:

Ans.  A cathode ray tube is a glass tube linked with high voltage source of current at its two electrodes, the air pressure in the tube is reduced by using vacuum pump.

  1. ii) Do you agree that atoms are indivisible?

Ans.  According to Dalton, an atom is an indivisible but in the late 1800 and early 1900’s scientists observed new sub atomic particles includes electron, proton and neutron. These sub particles go to make the structure of atom.

iii)     Write a note on discovery of neutrons.

Ans.  In 1932 Chadwick discovered neutron when he bombarded alpha particles on a beryllium target. He observed that highly penetrating radiation was produced. These radiations were called neutron.

iii)     Why position of electrons is hard to know?

Ans.  It was observed that when high voltage current is passed through cathode ray tube at a very low pressure then a beam of ray is produced. When an object is placed in the path of these rays, it is observed that they cast a sharp shadow it means they travel in straight line. When fly wheat is placed in their path it is moved showing that these rays are particle and when these rays are subjected to electric or magnetic fields they are showing negative nature. So in these conditions it is hard to know the position of electron.

  1. iv) Discuss the shape of ‘S’ and ‘P’ orbits.

Ans.  All S-orbital are spherical in shape. S-orbital has only one orientation i.e. centered at the nucleus. Their size increase with increase in “n”. all P-orbital have dumbbell shape. The P-orbital are of three types (Px, Py, Pz). Each P-orbital has two lobes. One on each side of the nucleus.

  1. v) What is the role of electronic configuration in determining the chemical nature of an element?

Ans.  The position of electrons in an atom is called its electronic configuration. In order to calculate electronic configuration it is kept in mind that electrons will fill that orbits first whose energy is least. The number of electron that a shell can accommodate is given by 2n2, where ‘n’ is the shell number.

  1. vi) Write electronic configuration of Na and Na+.

Ans.     Na        1S2, 2S2, 2P6, 3S1

Na+         1S2, 2S2, 2P6

vii)       What do you know about shells of atoms?

Ans.  The distribution of electrons around the nucleus in various shells and sub shells. The electrons fill the shell in order of their increasing energy, i.e. lower energy level is occupied first then the higher energy level and so on as indicated earlier.

viii)      Describe division of shells into sub shells?

Ans.  Shells are the main energy levels that electrons occupy. Shells are represented by circles around the nucleus. A shell also consists of sub shell. Each sub shell is designated by a small alphabetical letter S,P,d etc.

No. of Shells Shell Name Sub shell
1 K S
2 L S, P
3 M S, P, d
4 N S, P, d, f
  1. ix) Why Rutherford used alpha rays and not beta rays.

Ans.  Rutherford studied in detail the properties of alpha rays that these rays are:

  1. Travel in a straight line perpendicular to the cathode surface.
  2. They can cast a sharp shadow of an opaque object it placed in their path.
  3. They are deflected towards positive plate in an electric field showing that they are negatively charged. (iv) They raise temperature of the body on which they fall so he used alpha rays not beta rays.

Q4.      Answer the following question with reasoning.

  1. i) Can you explain the experiment that lead to the development that electron has negative charge and proton has positive charge?

Ans.  In 1895 Sir William Crooks performed experiment by passing electric current through gases in a discharge tube at very low pressure. When high voltage current was passed through the gas, shiny rays were emitted from the cathode towards the anode named cathode rays, negative charge. But in 1886 Goldstein observed that in addition to cathode rays, other rays were also present in the discharge tube. These rays were traveling in opposite direction to cathode rays. These rays passed through holes present in the cathode and produced a glow on the wall. He called these rays as “Canal Rays” their deflection in electric and magnetic field proved that these were positively charged.

  1. ii) Why Rutherford used gold foil instead of any other metal foil?

Ans.  Metals can be drawn into foils and beaten into thin sheets instead of breaking up on striking as ionic crystals would do the Malleability of Metals vary from one metal to another. Thus gold is highly malleable than the other metals therefore Ruther ford used gold foil instead of any other metal foil.

iii)     Why scientist concluded that electrons revolve around nucleus and not neutron?

Ans.  Scientist keeping in view the experiments, concluded following results.

  1. Since most of the particles passed through the foil un-deflected, therefore most of the volume occupied by an atom is empty.
  2. The reflection of a few particles proved that there is a centre of positive charges in an atom, which is called nucleus of an atom.
  3. The complete rebounce of a few particles show that the nucleus is very dense and hard.
  4. The electrons revolve around the nucleus.
  5. iv) Why tritium is not found in large quantities in nature.

Ans.  Hydrogen has three isotopic forms i.e. protium (1H1) deuterium (1D2 / 1H2) and Tritium (1T3 / 1H3) protium or common hydrogen, deuterium or heavy hydrogen and tritium or heaviest hydrogen. Due to heaviest hydrogen tritium is not found in large quantities in nature.

  1. v) Can you explain why in the cathode ray tube pressure is reduced?

Ans.  William Crook observed that at ordinary pressure, when electricity is passed through the tube nothing happens, but at reduced pressure high voltage produces a beam of radiation which causes fluorescence on striking with some fluorescent material. So later on he kept the pressure inside the tube at 10-4 atom.

Q5.   Discuss various experiments that lead to establish the atomic structure?

Ans.  In 1895 Sir William crooks performed experiments by passing electric current through gases in a discharge tube at very low pressure. He took a glass tube fitted without metallic electrode, which were connected to a high voltage battery. When high voltage current was passed through the gas, rays were emitted. These rays were given the name of cathode rays. His experiment proved that cathode rays are actually tiny particles having negative charge and are emitted from cathode and travel toward his anode. These particles were named as electron.

In 1886 Goldstein observed that in addition to cathode rays, other rays were also present in the discharge tube. These rays were traveling in opposite directions to cathode rays. In his experiment he found that these rays passed through holes present in the cathode and produced a glow on the wall. He called these rays as ‘canal rays’. Further investigations showed that these rays have positive charge, have momentum. They were named proton.

In 1920, Ernest Rutherford postulated that there were neutral, massive particles in the nucleus of atoms. This conclusion arose from the disparity between an elements atomic number and its atomic mass. James Chadwick proved the existence of these particles. Thus all the basic fundamental sub atomic particles were discovered.

Q6.   Can you describe the experiment and result deduced by Rutherford for explaining atomic structure?

Ans.  In 1910 at this time only proton and electrons were known Rutherford bombarded a thin gold foil (4 ´ 10-7) and observed that Most of the alpha ray particles deflected (one in 20,000) at various angles. Very few deflected back Rutherford concluded that as most of the alpha rays particles passed through foil without any resistance, so most of area in atom is empty. Some particles deflected so in the atom there is a very small area of positive charge with the mass of whole atom in their. He named the positive centre of atom as nucleus of atom. On the basis of this experiment, Rutherford developed a model for atomic structure, which has following important postulates.

  • An atom has a positive centre called nucleus.
  • Around this nucleus the electrons revolve.
  • The numbers of electrons are equal to positive particles so atom as a whole is neutral.

Rutherford’s model gave the basic idea for structure of atom.

Q7.   Explain how Bohar helped in understanding the structure of atoms?

Ans.  Neil Bohar in 1913 proposed a model of atom, the important postulates of which are following.

  • The electron in an atom revolves around nucleus in definite circular paths called orbit or energy levels.
  • The electrons do not radiate energy as long as they remain in an orbit.
  • For electrons only that orbit is allowed for which the angular momentum (mvr) of electron is equal to integral multiple of h / 2π.
  • The electron jumps to higher energy level or emits energy coming to lower energy level by absorbing or emitting energy respectively.
  • This energy is quantized and is given by E=hv, where h= planks constant and v (spelled as nu) is the frequency of radiation.

The Bohar model has following merits over classical models.

  • Bohar model was a successful approach in explaining many basic characteristics of atomic structure.
  • His history was conflicting with classical mechanics and thus he started modern mechanics.
  • He told that the principles at macroscopic level do not apply on microscopic level.
  • Bohar explained the special lines of ‘H’ atom successfully.
  • He calculated radius and energy of electron in an orbit successfully.

Q8.   What do you understand by the terms “isotopes”? Explain with examples.

Ans.  The term isotope has been originated from two words “Iso” that means same and topos (from topes) which means place.

Isotopes are defined as the atom of an element that have same atomic number but different mass number.

They have same number of protons but they differ in the number of neutrons. Isotopes have similar chemical properties but different physical properties because these depend upon atomic masses. Examples:

  • Hydrogen has three isotopic form i.e. protium (1H1) deuterium (1D2, 1H2) and Tritium (1T3, 1H3)
  • Carbon also has 3 isotopic forms

Carbon- 12               6C12

Carbon- 13               6C13

Carbon- 14               6C14

Q9.      Discuss use of isotopes in medical science.

Ans.  With the advancement of the scientific knowledge the isotopes have found many applications in medical science i.e.

  • Isotopes used for the treatment of different types of cancers in the body, the cancerous cells are killed by using radio active rays of radio isotopes. This process is called radio therapy.
  • Isotopes are also used for the investigation of different diseases, e.g. in order to determine the efficiency of kidneys heavy water is past in the body and its traveling is monitored. The malfunctioning kidney does not pass water so the relative functioning of each kidney is determined.

Example:- Write down the electronic configuration of ‘O’ and ‘Cl’.

‘O’ having 8 electron has electronic configuration

= 1S2, 2S2, 2P4

‘Cl’ having 17 atomic number so it has 17 electron

= 1S2,2S2,2P6,3S2, 3P5.

Activity 2.1:- Draw the electronic configuration of ‘N’ and ‘P’.

‘N’ has 7 atomic number so        = 1S2, 2S2, 2P3

‘P’ has 15 atomic number so      = 1S2, 2S2, 2P6 , 3S2, 3P3

Activity 2.2:- Using data given in table 2.3, draw the structure of chlorine isotopes.

‘Cl’      = 1S2, 2S2, 2P6, 3S2, 3P5.

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