UNIT 2 ( PAGE 2)

ATOMIC MODELS

                Dalton’s atomic theory regarded atom as indivisible. Discovery of sub-atomic particles necessiated the rivision of the theory. Atomic models were therefore , proposed to explain the structure of atom. Different atomic models are the pictures of the structure of atom proposed from time to time. Two of the earlier models were proposed by J.J.Thomson and Ernest Rutherford.

Thomson Model of Atom

J.J. Thomson , the discoverer of electron, proposed in 1898 a model in which he assumed an atom to consist of a uniform sphere (radius approximately 10^-10 m) of positive electricity (positive charge) with electrons embedded into it in such a way as to give the most stable electrostatic arrangement (Fig)

Thomson model of atom

In this model, the atom is visualised as a pudding or cake of positive charge with raisins (electrons) embedded in it. This model is therefore , sometimes called ‘raisin pudding’ model. An important feature of this model is that the mass of the atom is considered to be evenly spread over the atom.

            This model of atom could account for the electrical neutrality of atom, but it could not explain the results of gold foil scattering experiment carried out by Rutherford.

RUTHERFORD’S SCATTERING EXPERIMENT

Rutherford and his students (Hans Geiger and Ernest  Marsden) bombarded a thin gold foil ( thickness 4 x 10^-5 cm ) by  a-particles. These particles were obtained in the form of a narrow beam by passing through a slit of lead plate.   A circular screen coated with ZnS  was placed around the foil to detect the deflection suffered by  a-particles as shown in Fig . 

Rutherford's a-particle scattering experiment.

Rutherford's model for deflections

Of a-particles.

Rutherford observed that :-

i)        Most of the a-particles passed through the gold foil undeflected.

ii)      A small fraction of the a-particles was deflected by small angles.

iii)     A very few a-particles ( ~1 in 20,000) bounced back, i.e., were deflected by nearly 180°.

On the basis of these observations , Rutherford drew the following conclusions regarding the structure of atom:

(i)      Most of the space in the atom is empty as most of the             a-particles passed through the foil.

(ii)     A few positively charged a-particles are deflected. The deflection must be due to enormous repulsive force showing that positive charge of the atom is not spread throughout the atom as Thomson had thought. The positive charge has to be concentrated in a very small volume that repelled and deflected the positively charged a-particles. This very small portion of the atom is called nucleus by Rutherford.

(iii)    Calculations by Rutherford showed that the volume occupied by the nucleus is very small as compared to the total volume of the atom.  The radius of the atom is about 10^-10 m while nucleus is 10^-15 m.

Rutherford assumed that the solid gold foil consists of layers of individual atoms which are touching each other so that there is hardly any empty space between them. As such the   a-particles striking the gold foil must pass through the atoms.

RUTHERFORD’S MODEL OF ATOM

On the basis of scattering experiment, Rutherford  put forward Nuclear Model of atom. Main points of this model are :

(i)      An atom consists of a tiny positively charged nucleus at its centre.

(ii)     The positive charge of the nucleus is due to protons. The mass of the nucleus, is due to protons and some other neutral particles each having mass nearly equal to the mass of proton. This neutral particle is called neutron. Protons and neutrons are present in the nucleus are collectively known as nucleons. The total number of nucleons is known as mass number (A) of the atom.

(iii)    The nucleus is surrounded by electrons that move around the nucleus with very high speed in circular paths called orbits. Thus Rutherford’s model of atom resembles the solar system in which sun plays the role of nucleus and planets that are revolving electrons.

(iv)    The number of electrons in an atom is equal to the number of protons in it. Thus the total positive charge of the nucleus  exactly balances the total negative charge in the atom making it electrically neutral. The number of protons in an atom is called its atomic number (Z).

(v)     Electrons and nucleus are held together by electrostatic forces of attraction.

Failure of Rutherford Atom Model

In Rutherford model of atom, there exists a positively  charged nucleus at the centre and it is surrounded by negatively charged  electrons which move in definite paths called orbits. Most of the mass of an atom is concentrated in the nucleus.

            Now according to Maxwell’s theory of electromagnetic radiation, charged particles when accelerated should lose energy in the form of electromagnetic radiation. In Rutherford’s model, negatively charged electrons move in orbits around the positively charged nucleus. In order to maintain the circular motion of an electron in orbit, the electron is subjected to an acceleration. Thus according to Maxwell’s theory the electron should continuously lose energy and spiral into the nucleus with subsequent collapse of the atom. Calculations show that the electron would require only 10^-8 seconds to do so. Hence such an arrangement of the nucleus and electrons will not yield a stable atom.

 Fig. 12. Continuous decrease in energy.

            Another serious drawback of Rutherford model is that it says nothing about the electronic structure of atoms i.e., how the electrons are distributed around the nucleus and what are the energies of these electrons.

Rutherford’s model could not explain the discrete spectra exhibited by atoms. By discrete spectra, we mean that an atom on excitation liberates characteristic radiation of fixed wave lengths.

ATOMIC NUMBER ( Z )

Moseley devised an experiment to find out positive charge on the nucleus He calculated the charge on the nucleus from the frequencies  or wave lengths of X-rays emitted by different elements. The number of uni-positive charges  on the nucleus of the atom of the element is called atomic number of the element Since the positive charge on the nucleus is due to protons and each proton carries one unit positive charge, therefore atomic number of an element is equal to the number of protons in the nucleus of its atom. Further in an atom the number of protons is equal to the number of electrons. Hence atomic number is also equal to the number of electrons in an atom of the element. Thus atomic number of an element is equal to the number of protons in the nucleus of its atom or the number of extra-nuclear  electrons. Generally it is denoted by the letter Z.

                 Atomic number Z   =  Number of protons

                                             =  Number of electrons.

MASS NUMBER ( A )

Mass of an atom is mainly concentrated in the nucleus. In the nucleus there are protons and neutrons. The mass of an atom is mainly due to protons and neutrons. The number of protons and neutrons in the nucleus is called mass number of the atom. It is generally represented by the letter A .

 Mass number  = Number of protons + Number of neutrons.

                      =  Number of nucleons.

All the atoms of a particular element have same the number of protons in their nuclei however, the number of neutrons may be different. Such atoms have same atomic number but different mass numbers and are known as isotopes of the element. Thus, isotopes of an element are the atoms of the element with same atomic  number  , but different mass numbers. For example, hydrogen has three isotopes , protium ( H ) Deuterium ( D )  and Tritium ( T ). All the three isotopes have the same atomic number 1, however, their mass numbers are 1, 2 and 3 respectively. The isotopes of other elements do not have special names ; they are  generally indicated  as shown below.

Thus three isotopes of hydrogen can be represented as :

                     It may be noted that atoms of different elements may have the same mass number. The atoms of different elements which have the same mass numbers are called isobars. For example,

are isobars.               

From the knowledge of atomic number and mass number of an element, it is possible to calculate , the number of electrons, protons and neutrons in an atom of the element. For example, atomic number and mass number  of sodium are 11 and 23 respectively. Number of electrons, protons and neutrons in an atom of it can be calculated as under :

Number of protons      =  Atomic number   = 11

Number of electrons    =  Atomic number   = 11

   Number of neutrons   =   Mass number - Atomic number

                                    =    23  -   11  =  12

In case of ions, the number of protons and neutrons remain the same as in atoms, however, the number of electron changes. This is due the reason that  an ion is formed either by addition or by removal of one or more electrons from the neutral atom.

For example, Mg²⁺ ion has two electrons less than the number of electrons in Magnesium atom.

Knowing that the atomic number and mass number of Magnesium as 12 and 24 respectively, the number of electrons , protons and neutrons in Mg²⁺  ions may be calculated as under :

Number of protons =  Atomic number  = 12

                   Number of electrons       =  Atomic number - 2

                                                          =  12  -   2   =  10

                   Number of neutrons       =   24  -  12 = 12

Similarly, a negative ion is formed by a addition of one or more electrons to the neutral atom. For example, P^3-  ion (phosphide ion) is formed by the addition of three electrons to a phosphorus atom.

             P   +   3 e^-    ®    P^3-

Knowing  the atomic number of phosphorus ( Z  =  15 ) , and mass number ( A  = 31 ) , the number of electrons , protons and neutrons in phosphide ion may be calculated as under :

Number of protons    =  Atomic number  =   15

Number of electrons  =  Atomic number + 3 =  18

Number of neutrons  =  Mass number - Atomic number

                               =  31 - 15  = 16

ISOTOPES

            In some cases, the atoms of the same element are found to contain the same number of protons but different number of neutrons. As a result, they have same atomic number but different mass numbers.

            Such atoms of the same element having the same atomic number but different mass numbers are called isotopes

            For example , there are three isotopes of hydrogen having mass numbers , 1, 2 and 3 respectively and each of them having atomic number equal to 1. They are represented as ¹₁H, ²₁H and ³₁H and named as protium(H), deuterium(D) and tritium(T) respectively. Ordinary hydrogen is protium. Isotopes of other elements do not have special names. They are represented by simply indicating the values of A on the symbol. For example, isotopes of chlorine are written as ³⁵Cl and ³⁷Cl.

ISOBARS

            Some atoms of different elements are found to have same mass number.

            Such atoms of different elements have same mass numbers are called isobars.

            e.g. ⁴⁰₁₈Ar  ,   ⁴⁰₁₉K,   ⁴⁰₂₀Ca

ISOTONES

            It may be noted that isotopes differ in the number of neutrons only, whereas isobars differ in the number of neutrons as well as protons. However, some atoms of different elements are found to have the same number of neutrons.

            Such atoms of different elements which contain the same number of neutrons are called isotones. e.g. ¹⁴₆C, ¹⁵₇N, ¹⁶₈O.

ISOMERS

            There are some atoms of the same radiactive elements which have same atomic number and same mass number and hence contain the same number of protons and neutrons in their nuclei, yet they differ in their radiactive properties due to difference in their nuclear energy levels (on account of difference in the arrangement of protons in the nuclear shells)

            Such atoms of radioactive elements having the same atomic number and same mass number but different radioactive properties are called isomers or nuclear isomers to distinguish from isomers in organic chemistry. For example, uranium-X₂ and uranium-Z found in the uranium-radium decay series are nuclear isomers.

ISOSTERS

            These are molecules of different substances which contain the same number of atoms and same total number of electrons which leads to similarity in their physical properties. The phenomenon is called isosterism. For example, carbon dioxoide CO₂ and nitrous oxide, N₂O are isosters.

Isodiaphers

            Isodiapheres are the atoms having the same difference of neutron and proton or same isotopic number. Nucleides and its decay product after the emission of an a-particle are called isodiaphers, Example,

Nature and Characteristics of Light

Light is an important form of energy. According to Newton’s corpuscular theory, it was considered as a stream of particles called corpuscles. This theory could successfully explain phenomenon of reflection and refraction, but failed to explain phenomena of diffraction and interference. Therefore, corpuscular theory was discarded and it was replaced by wave theory of light. According to this theory, light is transmitted in the form of electromagnetic waves. These waves are associated with oscillating electric and magnetic field.

RADIANT ENERGY

J.C Maxwell (1864 ) suggested that an alternating current of high frequency is capable of radiating energy in the form of waves which travel in space with the same speed as light. He called these waves as Electromagnetic Radiation. The term electromagnetic arose from his observation that similar waves can be produced by moving a charged body or a magnet to and fro in an oscillating manner. An electromagnetic wave comprises of oscillating electric and magnetic fields directed perpendicular to each other and perpendicular to the direction of propagation of the wave as illustrated in Fig13.

Fig 13.  Electric and magnetic fields associated  with an  electromagnetic wave.These components have the same wave length, frequency, speed and amplitude , but they vibrate in two mutually perpendicular planes.

Thus electromagnetic radiation is a form of radiant energy which propagates through space in the form of waves which are associated with electric and magnetic fields. The velocity ( c ) of radiation is given by n l where n  is the frequency and l is the wave length of the radiation. All types of electromagnetic radiation travel with the same velocity. They differ from one another in their wave lengths  (l) and therefore in frequencies (n). As the velocity is constant, it is evident that the greater the frequency  of the radiation, the shorter the wave length. The energy of an electromagnetic radiation is directly proportional to its frequency.

Characteristics of Wave Motion

In order to characterise the waves , the following parameters are used.

i)  Wave length (l) :  The distance between two successive crests is called wave length. It is represented by Greek letter ( lamda ) and generally measured  in Angstrom units ( Å ) or nanometers ( nm ).

             1 A   =  10^{- 10} m

             1 nm =  10^-9 m

In addition, the following units are also some times used.

   1 picometer, pm     = 10^-12m

  1 micrometer, m m  = 10^-6 m

Fig  14.  Wave motion.

ii) Frequency ( n ) : Frequency is the number of waves which pass through a particular point in one second. It is represented by the Greek letter  n . Its unit are : cycles per second ( cps ) or Hertz ( Hz )

1 c p s   =  1 Hz

1 k Hz    =  10³  Hz

1 M Hz   =  10⁶   Hz

iii) Velocity ( c) : The distance travelled by a wave in a second is called velocity of the wave. It is denoted by the letter c. The frequency (n) and wave length (l)) are related to velocity (c ) by the relation :

                          c  = l n

                          n  = c / l

Velocity of electromagnetic radiation in space or in vaccum is the same and is equal to 3 x 10⁸ m s^-1 or 3 x 10¹⁰ cm s^-1

iv) Wave number (`n ) : Wave number is the number of wave lengths per meter or centimetre and is equal to the reciprocal of wave length expressed in meters or centimetres.

                   `n =  (1/  l )  =  n/ c

v) Amplitude ( a ) : It is the height of crest or depth of the trough of a wave. It is generally expressed by the letter ‘a’ . The amplitude of a wave determines the intensity of radiation.

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