Md. Imran Nur ManikLecture

Department of PharmacyNorthern University

Bangladesh

Structure of Atom

Atom The existence of atoms has been proposed since the time of early Indian and Greek philosophers (400 B.C.) who were of the view that atoms are the fundamental building blocks of matter. 

According to them, the continued subdivisions of matter would ultimately yield atoms which would not be further divisible. The word ‘atom’ has been derived from the Greek word ‘a-tomio’ which means ‘uncutable’ or ‘non-divisible’.

These earlier ideas were mere speculations and there was no way to test them experimentally. These ideas remained dormant for a very long time and were revived again by scientists in the nineteenth century.

Theories of Atomic Structure The atomic theory of matter was first proposed on a firm scientific basis by John Dalton, a British school teacher in 1803. His theory, called Dalton’s atomic theory, regarded the atom as the ultimate particle of matter.

The physical and chemical properties of elements and their compounds are essentially with the structure of atoms. A systemic knowledge of the structure of atoms is, therefore, necessary for the study of modern inorganic chemistry.

In this unit, we shall start with the experimental observations made by scientists towards the end of nineteenth and beginning of twentieth century. These established that, atoms can further be divided into subatomic particles i.e. electrons, protons and neutrons.

Fundamental Particles of AtomAn atom is the smallest particles of an element having its own chemical identity and properties. Atom consists of still smaller particles, such as electron, proton, neutron etc. into which it can be subdivided. These particles are regarded as fundamental particles,

because these are the main constituents of atoms.

Classification of Fundamental Particles:i) Permanent Fundamental Particles: The particles which play an important role for the formation of structure of an atom and are present in all atoms (except hydrogen where neutron is absent) are called permanent fundamental particles. There are three kinds of permanent fundamental particles such as electron, proton and neutron.Electron bears negative charge and revolves around the nucleus in a circular path. The mass of electron is very small.Proton bears the positive charge and stays at the nucleus with neutrons.Neutron has no charge, i.e. it is neutral. It is placed in the nucleus with proton.

ii) Temporary Fundamental Particles:The particles which exist in some

atoms for short time are called temporary fundamental particles. There are about 100 numbers of temporary fundamental particles in different atom like positron, neutrino, antineutrino, meson etc.

Discovery of ElectronProduction of Cathode Rays:

In mid 1850s many scientists mainly Faraday began to study electrical discharge in partially evacuated tubes, known as cathode ray discharge tubes. It is depicted in Fig.

The knowledge about the electron was derived as a result of the electric discharge in a discharge tube. The discharge tube consists of a glass tube with metal electrodes fused in the walls. The pressure inside the tube can be reduced with a pump. When the electrodes are connected to a source of high voltage (10,000 volts), an electric discharge from high potential source is passed through a gas at pressure of 0.001 mm of Hg in the discharge tube and the glass wall opposite to the cathode begins to glow with a very faint greenish light. This glow or fluorescence is formed due to the bombardment on the glass by certain rays which are emitted from the cathode surface and move towards the anode with tremendous velocity. These rays were called cathode rays by Goldstein since these were originated from the cathode.

The results of this experiment are summarized below

(i) The cathode rays start from cathode and move towards the anode.(ii) These rays themselves are not visible but their behavior can be observed with the help of certain kind of materials (fluorescent or phorosphescent) which glow when they are hit by the cathode rays.(iii) In the absence of electrical or magnetic field, these rays travel in straight lines (Fig. 2.2)(iv) In the presence of electrical or magnetic field, the behavior of cathode rays are similar to that expected from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles, called electrons.(v) The characteristics of cathode rays (electrons) do not depend upon the material of electrodes and the nature of the gas present in the cathode ray tube. Thus, we can conclude that electrons are basic constituent of all the atoms.The rays which proceed from the cathode and move away from it right angles in straight lines are called cathode rays. They are deflected in electric and magnetic fields indicating that these rays carry electric charge.

Properties of Cathode Rays1. They travel in straight lines away from the cathode. e.g. Shadow of

aluminium cross.2. They produce fluorescence (a glow) when they strike the glass wall of

discharge tube.3. They produce X-rays when they strike a metallic target.4. Consist of moving particles with definite mass and velocity i.e. Momentum.5. They can ionize gases.6. They can pass through thin metal foil.7. The cathode rays are deflected towards the positive pole of the electric field

which proves that the rays are associated with negative charges. The particles making up the cathode rays were all identical and ultimately given the name electron by the Dutch Physicist H. A. Lorentz.

Discovery of Proton In 1886, German physicist ,

Eugen Goldstein used a discharge tube provided with a cathode perforated with extremely fine holes and observed that when the electrodes are connected to a source of high voltage (10,000 volts), not only cathode rays of electrons were originated from the cathode; moving from cathode towards anode but also new type of colored rays were simultaneously originated from the anode moving from anode towards cathode through the fine holes or canals in the

cathode causing a glow on the wall opposite to anode.• Initially these rays were

called canal rays, since they pass through the canals or holes of the cathode. These are also called anode rays as they seem to originate from the anode.

• Sir J. J. Thomson has shown that these rays consist of particles carrying a positive charge and hence he named them positive rays.

Explanation of the Production of Positive Rays

• When a high voltage is applied across the metallic electrodes of a discharge tube filled with a gas under reduced pressure, the molecules of the gas are dissociated into atoms.

• Now when high speed electrons (i.e. cathode rays) originated from cathode strike these atoms, the atoms are converted into cations which pass through the holes of the cathode and constitute positive rays.

• The electrons obtained in the conversion of the gaseous atoms into cations move towards anode as negative rays.

M + e- → M+ + 2e-

Gaseousatoms

Originated fromcathode

Pass through theperforated cathodeas positive rays

Move towardsanode as negativerays

The characteristics of these positively charged rays are listed below.

1. Unlike cathode rays, the positively charged particles depend upon the nature of gas present in the cathode ray tube. These are simply the positively charged gaseous ions.2. Some of the positively charged particles carry a multiple of the fundamental unit of electrical charge.3. The behaviour of these particles in the magnetic or electrical field is opposite to that observed for

electron or cathode rays.The smallest and lightest positive ion was obtained from hydrogen and was called proton.

This positively charged particle was characterized in 1919.

Thomson Model of AtomJ. J. Thomson, in 1898, proposed that an atom possesses a

spherical shape (radius approximately 10–10 m) in which the positive charge is uniformly distributed. The electrons are embedded into it in such a manner as to give the most stable electrostatic arrangement (Figure). Many different names were given to this model, for example, plum pudding, raisin pudding or watermelon.

This atomic model was given up after some time, since it could not explain the large deflection suffered by α— particles in Rutherford’s experiment.

Discovery of NeutronUp to 1932, it was postulated that an atom is composed of only proton and electron.Mass of an atom is due to proton as mass of electron is negligible.Rutherford noticed that mass of different atoms can not be explained if it is supposed that atom is composed of proton and electron. He predicted, in 1920, that some kind of neutral particles having mass equal to proton must be present in an atom. These particles were discovered by James Chadwick (1932) by bombarding a thin sheet of beryllium by α-particles when electrically neutral particles having a mass slightly greater than that of the protons were emitted. He named these particles as neutrons .

ExperimentA stream of alpha-particles was directed from a polonium source to a Beryllium target.A charge detector was placed behind the Beryllium target.A new radiation was produced which had no charge on them indicated by charge detector.The reaction in Chadwick’s experiment converted an ‘Be’ atom into a Carbon atom through the nuclear reaction.

Summary of Electron, Proton & Neutron