Atomic Structure

Matter :- Anything which occupies space have mass and have fixed volume. Matter is made up of small particles called atoms.

Q:- what is an atom, why does it occur?

Cathode ray experiment:-     Cathode rays are so named because they are emitted by the negative electrode, or cathode, in a vacuum tube.was done by using a high electrical potential between the anode and the cathode to ionize the residual gas in the tube; the ions were accelerated by the electric field and released electrons when they collided with the cathode.Cathode rays are invisible, but their presence was first detected in early vacuum tubes when they struck the glass wall of the tube, exciting the atoms of the glass and causing them to emit light, a glow called fluorescence.

Properties of Cathode Rays

·  They travelled in straight lines

·  Produced a shadow when obstructed by objects

·  could pass through thin metal foils without disturbing them.

SOME FACTS

Thomson model  :- Size of atom ->  10-8                           size of nucleus ->  10-15

 

ATOMIC NUMBERxATOMIC MASS

Isotopes:-atoms of same element having same atomic number but different atomic mass number example  :-₁H¹ (protium),  ₁H²(deuterium), ₁H³(tritium).

Isobar:-atoms of the different element having different atomic number but same atomic mass.example  :-    ₁₈Ar⁴⁰ ,  ₁₉K⁴⁰ , ₂₀Ca⁴⁰.

Isotone:-the species having same number of neutrons are called isotones.

No. of neutrons = atomic mass – atomic number

example  :-    ₆C¹⁴ (no. of neutrons = 14 – 6 = 8),   ₇N¹⁵ (no. of neutrons = 15 – 7=8) ,  ₈O¹⁶(no. of neutrons = 16 – 8 = 8).

isoelectronic species:-The isoelectronic species are those elements, which have the same no. of electrons inside their atoms. Like an example is neon, and sodium ion, both of which consists of 10 electrons each.

“The elements, which are isoelectronic with each other, differ in their chemical and physical properties, like as the positive charge on a species increases, it's reactivity increases due to the increased electropositive nature of that species”.

ATOMIC MODELS

We know the fundamental particles of the atom. Now let us see, how these particles are arranged in an atom to suggest a model of the atom.

Thomson’s Model:

J.J. Thomson, in 1904, proposed that there was an equal and opposite positive charge enveloping the electrons in a matrix. This model is called the plum – pudding model after a type of Victorian dessert in which bits of plums were surrounded by matrix of pudding.

This model could not satisfactorily explain the results of scattering experiment carried out by Rutherford who worked with Thomson.

Rutherford’s Model:

α– particles emitted by radioactive substance were shown to be dipositive Helium ions (He⁺⁺) having a mass of 4 units and 2 units of positive charge.

Rutherford allowed a narrow beam of α–particles to fall on a very thin gold foil of thickness of the order of 0.0004 cm and determined the subsequent path of these particles with the help of a zinc sulphide fluorescent screen. The zinc sulphide screen gives off a visible flash of light when struck by an a particle, as ZnS has the remarkable property of converting kinetic energy of α particle into visible light. [For this experiment, Rutherford specifically used α particles because they are relatively heavy resulting in high momentum].

•  Majority of the a–particles pass straight through the gold strip with little or no deflection.
•  Some α–particles are deflected from their path and diverge.
•  Very few α–particles are deflected backwards through angles greater than 90°.
•  Some were even scattered in the opposite direction at an angle of 180° 

 Conclusions:

•  The fact that most of the α - particles passed straight through the metal foil indicates the most part of the atom is empty.
•  The fact that few α - particles are deflected at large angles indicates the presence of a heavy positively charge body i.e., for such large deflections to occur α - particles must have come closer to or collided with a massive positively charged body.

 The fact that one in 20,000 have deflected at 180° backwards indicates that volume occupied by this heavy positively charged body is very small in comparison to total volume of the atom.

 Conclusions of a-Scattering Experiment:

On the basis of the above observation, and having realized that the rebounding α-particles had met something even more massive than themselves inside the gold atom, Rutherford proposed an atomic model as follows.

•  All the +ve charge and nearly the total mass of an atom is present in a very small region at the centre of the atom. The atom’s central core is called nucleus.
•  The size of the nucleus is very small in comparison to the size of the atom. Diameter of the nucleus is about 10^–13cm while the atom has a diameter of the order of 10^–8 cm. So, the size of atom is 10⁵ times more than that of nucleus.
•  Most of the space outside the nucleus is empty.
•  The electrons, equal in number to the net nuclear positive charge, revolve around the nucleus with fast speed just like planets around the sun.
•  The centrifugal force arising due to the fast speed of an electron balances the coulombic force of attraction of the nucleus and the electron remains stable in its path. Thus according to him atom consists of two parts (a) nucleus and (b) extra nuclear part.

Defects in Rutherford’s Atomic Model:

• Position of electrons: The exact positions of the electrons from the nucleus are not mentioned.

Stability of the atom: Bohr pointed out that Rutherford’s atom should be highly unstable. According to the law of electro–dynamics, when a charged body moves under the influence of an attractive force, it loses energy continuously in the form of electromagnetic radiation. The electron should therefore, continuously emit radiation and lose energy. As a result of this a moving electron will come closer and closer to the nucleus and after passing through a spiral path, it should ultimately fall into the nucleus.

It was calculated that the electron should fall into the nucleus in less than 10^–8 sec. But it is known that electrons keep moving outside the nucleus.

 CHARACTERISTICS OF A WAVE           A wave is a sort of disturbance which originates from some vibrating source and travels outward as a continuous sequence of alternating crests and troughs. Every wave has five important characteristics, namely, wavelength (λ), frequency (v), velocity (c), wave number(v ¯ )  and amplitude (a).

Electronic Magnetic Radiation: 

Ordinary light rays, X–rays,λ–rays, etc. are called electromagnetic radiations because similar waves can be produced by moving a charged body in a magnetic field or a magnet in an electric field. These radiations have wave characteristics and do not require any medium for their propagation.    

•  Wavelength (λ): The distance between two neighbouring troughs or crests is known as wavelength. It is denoted by l and is expressed in cm, m, nanometers (1 nm =10^–9 m) or Angstrom (1 Å=10^–10 m). 

•  Frequency (v): The frequency of a wave is the number of times a wave passes through a given point in a medium in one second. It is denoted by n(nu) and is expressed in cycles per second (cps) or hertz (Hz) 1Hz = 1cps. The frequency of a wave is inversely proportional to its wave length (λ)  v ∝ 1 / λ  or v =c/λ

Velocity: The distance travelled by the wave in one second is called its velocity. It is denoted by c and is expressed in cm sec^–1.  c = vλ or λ = c / v 

•  Wave number ( v^- ): It is defined as number of wavelengths per cm. It is denoted by v^- and is expressed in cm^–1. v^- = 1 / λ or v^- = v / c

•  Amplitude: It is the height of the crest or depth of the trough of a wave and is denoted by a. It determines the intensity or brightness of the beam of light. 
•  Electromagnetic Spectrum: The arrangement of the various types of electromagnetic radiation in order of increasing or decreasing wavelengths or frequencies is known as electromagnetic spectrum.

ATOMIC SPECTRUM   If the atom gains energy the electron passes from a lower energy level to a higher energy level, energy is absorbed that means a specific wave length is absorbed. Consequently, a dark line will appear in the spectrum. This dark line constitutes the absorption spectrum.  If the atom loses energy, the electron passes from higher to a lower energy level, energy is released and a spectral line of specific wavelength is emitted. This line constitutes the emissionSPECTRAs          Continuous spectra: When white light from any source such as sun or bulb is analysed by passing through a prism, it splits up into seven different wide bands of colour from violet to red (like rainbow). These colour are so continuous that each of them merges into the next. Hence the spectrum is called as continuous spectrum.   Line spectra: When an electric discharge is passed through a gas at low pressure light is emitted. If this light is resolved by a spectroscope, It is found that some isolated colored lines are obtained on a photographic plate separated from each other by dark spaces. This spectrum is called line spectrum. Each line in the spectrum corresponds to a particular wavelength. Each element gives its own characteristic spectrum.  PLANCK’s QUANTUM THEORY When a black body is heated, it emits thermal radiations of different wavelengths or frequency. To explain these radiations, Max Planck put forward a theory known as Planck’s quantum theory. The main points of quantum theory are   Substances radiate or absorb energy discontinuously in the form of small packets or bundles of energy.   The smallest packet of energy is called quantum. In case of light the quantum is known as photon.    The energy of a quantum is directly proportional to the frequency of the radiation. E µ n (or) E = hn where n is the frequency of radiation and h is Planck’s constant having the value 6.626 X 10–27 erg – sec or 6.626 X 10–34 J–sec.   A body can radiate or absorb energy in whole number multiples of a quantum hn, 2hn,3hn………..nhn. Where ‘n' is the positive integer.   HYDROGEN ATOM                                                                                                               If an electric discharge is passed through hydrogen gas taken in a discharge tube under low pressure, and the emitted radiation is analyzed with the help of spectrograph, it is found to consist of a series of sharp lines in the UV, visible and IR regions. This series of lineknown as line or atomic spectrum of hydrogen. The lines in the visible region can be directly seen on the photographic film. Each line of the spectrum corresponds to a light of definite wavelength. The entire spectrum consists of six series of lines, each series, known after their discoverer as the Balmer, Paschen, Lyman, Brackett, Pfund and Humphrey series. The wavelength of all these series can be expressed by a single formula .[     1/ λ  v- = R  (1/ n12  - 1/n22 )    ]  Where, v- = wave number λ = wave length  R = Rydberg constant (109678 cm–1)n1 and n2 have integral values as follows Series n1 n2 Main spectral lines Lyman Balmer Paschen Brackett Pfund 1 2 3 4 5 2, 3, 4, etc         3, 4, 5 etc          4, 5, 6 etc         5, 6, 7 etc          6, 7, etc Ultra – violet               Visible                                    Infra – red                            Infra – red                                Infra – red  All lines in the visible region are of Balmer series but reverse is not true. i.e., all Balmer lines will not fall in visible region.The pattern of lines in atomic spectrum is characteristic of hydrogen. Merits of Bohr’s Theory:   * The experimental value of radii and energies in hydrogen atom are in good agreement with that calculated on the basis of Bohr’s theory.     *  Bohr’s concept of stationary state of electron explains the emission and absorption spectra of hydrogen like atoms.     *  The experimental values of the spectral lines of the hydrogen spectrum are in close agreement with that calculated by Bohr’s theory. Limitations of Bohr’s Theory  * It does not explain the spectra of atoms having more than one electron.   *  Bohr’s atomic model failed to account for the effect of magnetic field (Zeeman effect) or electric field (Stark effect) on the spectra of atoms or ions. It was observed that when the source of a spectrum is placed in a strong magnetic or electric field, each spectral line further splits into a number of lines. This observation could not be explained on the basis of Bohr’s model.     *  De Broglie suggested that electrons like light have dual character. It has particle and wave character. Bohr treated the electron only as particle.     *  Another objection to Bohr’s theory came from Heisenberg’s Uncertainty Principle. According to this principle “It is impossible to determine simultaneously the exact position and momentum of a small moving particle like an electron”. The postulate of Bohr that electrons revolve in well defined orbits around the nucleus with well defined velocities is thus not tenable .DUAL CHARACTER (PARTICLE AND WAVE CHARACTER OF MATTER AND RADIATION) In case of light some phenomenon like diffraction and interference can be explained on the basis of its wave character. However, the certain other phenomenon such as black body radiation and photoelectric effect can be explained only on the basis of its particle nature. Thus, light is said to have a dual character. Louis de Broglie, in 1924 extended the idea of photons to material particles such as electron and he proposed that matter also has a dual character-as wave and as particle. Derivation of de-Broglie Equation: The wavelength of the wave associated with any material particle was calculated by analogy with photon.In case of photon, if it is assumed to have wave character, its energy is given by           E = hv                                             …(i)(According to the Planck’s quantum theory) Where nth frequency of the wave and ‘h’ is is Planck’s constant If the photon is supposed to have particle character, its energy is given by E = mc2                                                                    … (ii)(according to Einstein’s equation) where ‘m’ is the mass of photon, ‘c’ is the velocity of light. By equating (i) and (ii) hv = mc2 But v = c/λ h c/λ   = mc2           (or)  λ = h /mc The above equation is applicable to material particle if the mass and velocity of photon is replaced by the mass and velocity of material particle. Thus for any material particle like electron .λ = h/mv               or λ = h/p where  mv = p is the momentum of the particle.