Structure of Atom

This theory is based on law of mass conservation and law of definite proportions. The salient feature’s of this theory are :- (1) Every element is composed of extremely small particles called atoms. (2) Atoms of a particular element are like but differ from atoms of other element. (3) Atom of each element is an ultimate particle and it has a characteristic mass but is structureless (4) Atoms are indestructible i.e. they can neither be created nor be destroyed. (5) Atoms of different elements take part in chemical reaction to form molecule.

Particles carrying negative charge were called negatrons by Thomson.The name negatron was changed to ‘electron’ by Stoney.

Nucleus has a diameter of the order of 10–12 to 10–13 cm and the atom has a diameter of the order of 10–8 cm.

R = A1/3 Þ = = R0 A⅟³ R0= 1.33 x 10⁻¹³cm A= no of protons+ no of neutrons ie mass nubmber

There is an empty space around the nucleus called extra nuclear part. In this part electrons are present. The number of electrons in an atom is always equal to number of protons present in the nucleus. As the nuclear part of atom is responsible for the mass of the atom, the extra nuclear part is responsible for its volume. The volume of the atom is about 1015 Volumeof theatom (10 )-8X3 Volumeof thenucleus (10)-13x3 =1015 times the volume of the nucleus.

the energy is transmitted from one body to another in the form of waves and these waves travel in the space with the same speed as light ( 3 × 108 m/s) and these waves are known as Electro magnetic waves or radiant energy. The radiant Energy do not need any medium for propogation. The radiant energy have electric and magnetic fields and travel at right angle to these fields.

Hertz in 1887 observed that when a light of certain frequency strikes the surface of a metal, electrons are ejected from the metal. This phenomenon is known as photoelectric effect and the ejected electrons are called photoelectrons.

hv0 is the threshold energy (or) the work function denoted by f = hv0 (minimum energy of the photon to liberate electron). It is constant for particular metal.

i) 0.529 angs ii) 2.116 angs iii) 4. 761 angs iv) 8. 464 angs

(i) emission -> the wavelengths of the light possessed by the sample element is given in the photographic plate (ii) absorption -> the wavelengths of light of the element is absorbed by the element and the remainig all colours are given in the photographic plat.

In this method, oil droplets in the form of mist, produced by the atomiser, were allowed to enter through a tiny hole in the upper plate of electrical condenser. The downward motion of these droplets was viewed through the telescope, equipped with a micrometer eye piece. By measuring the rate of fall of these droplets, Millikan was able to measure the mass of oil droplets.The air inside the chamber was ionized by passing a beam of X-rays through it. The electrical charge on these oil droplets was acquired by collisions with gaseous ions. The fall of these charged oil droplets can be retarded, accelerated or made stationary depending upon the charge on the droplets and the polarity and strength of the voltage applied to the plate. By carefully measuring the effects of electrical field strength on the motion of oil droplets, Millikan concluded that the magnitude of electrical charge, q, on the droplets is always an integral multiple of the electrical charge, e, that is, q = n e, where n = 1, 2, 3… .

Wilhalm Röentgen (1845-1923) in 1895 showed that when electrons strike a material in the cathode ray tubes, produce rays which can cause fluorescence in the fluorescent materials placed outside the cathode ray tubes. Since Röentgen did not know the nature of the radiation, he named them X-rays and the name is still carried on. It was noticed that X-rays are produced effectively when electrons strike the dense metal anode, called targets. These are not deflected by the electric and magnetic fields and have a very high penetrating power through the matter and that is the reason that these rays are used to study the interior of the objects. These rays are of very short wavelengths (∼0.1 nm) and possess electro-magnetic character (Section 2.3.1)

3.1). Henri Becqueral (1852-1908) observed that there are certain elements which emit radiation on their own and named this phenomenon as radioactivity and the elements known as radioactive elements. This field was developed by Marie Curie, Piere Curie, Rutherford and Fredrick Soddy

It was observed that three kinds of rays i.e., α, β- and γ-rays are emitted. Rutherford found that α-rays consists of high energy particles carrying two units of positive charge and four unit of atomic mass. He concluded that α- particles are helium nuclei as when α- particles combined with two electrons yielded helium gas. β-rays are negatively charged particles similar to electrons. The γ-rays are high energy radiations like X-rays, are neutral in nature and do not consist of particles. As regards penetrating power, α-particles are the least, followed by β-rays (100 times that of α–particles) and γ-rays (1000 times of that α-particles).

(i) Dual character of the electromagnetic radiation which means that radiations possess both wave like and particle like properties, and (ii) Experimental results regarding atomic spectra which can be explained only by assuming quantized (Section 2.4) electronic energy levels in atoms.