CH 2 STRUCTURE OF ATOM

#) Constituents of atom. Atom is no longer considered as indivisible. It is made

up of electrons, protons and neutrons called fundamental particles.

#)Discovery of electron. Electron was discovered by J.J Thomson from study of

cathode rays. These are invisible rays emitted from cathode in the discharge

tube when pressure of the gas inside the discharge tube is about 10-4 atm (0.01

mm) and voltage of about 10,000 volts is applied. Their main characteristics are

(I) they pads through the perforated anode and produce green fluorescence on

opposite glass wall (ii) They produce a sharp shadow of the object placed in

their path showing that they travel in straight line (iii) They can rotate a

mounted mica wheel placed in their path showing that they consist of material

particles (iv)They are defected towards positive plate when external field is

applied on them showing that they carry negative charge. The negatively

charged particles present in cathode rays are called electrons. Charge on

electron = 1.60* 10-19 coulomb = 4.8 * 10-10 esu = 1 unit. Mass of electrons

=9.11 *10-31 kg = 1/1837th of that of H-atom. Hence, electron is that

fundamental particles which has one unit -ve charge and mass nearly 1/1837th

of that of H-atom. Any gas may be taken inside and cathode taken may be of

any material, electrons produced carry same charge and mass. This shows that

electrons are universal constituents of all atoms.

#) Discovery of proton. Proton was discovered by Goldstein from study of

anode rays using a perforated cathode in the discharge tube. On applying high

voltage, positively charged ions are produced from atoms of the gas inside due

to knock out of electrons from them. These travel away from anode and

towards cathode, pass through the canals (holes) of the cathode and strike the

glass wall coated with ZnS producing green fluorescence. These rays are,

therefore, also called canal rays or positive rays. Their main characteristics are:

(I) They have in straight lines. (ii) They are made up of material particles. (iii)

They carry positive charge. However, charge on the particles depend upon

number of electrons knocked out and mass depends upon the nature of gas

taken. The smallest charge =+ 1 unit and smallest mass =mass of H-atom. These

particles are called protons. thus, a proton is that fundamental particle which

has one unit +ve charge and mass nearly equal to that of H-atom.

#) Discovery of neutron. Neutron was discovered by Chadwick by bombarding

Be or B atoms with a-rays. Neutral particles with mass nearly equal to that of

proton were emitted. These were called neutrons.Thus, neutron is defined as

that fundamental particle which carries no charge but has mass nearly equal to

that of H-atom.

#) Thomson’s model of atom. An atom is a sphere of positive electricity in

which sufficient number of electrons was embedded to neutralize the positive

charge just as seeds in a melon or raisins in pudding. It could not explain results

of Rutherford's scattering experiments.

#) Rutherford’s model of atom A thin foil of gold was bombarded with a-

particles. Most of the a-particles passed through the foil undeflected,a few

were deflected through small angle while very few were deflected back. It was

therefore, concluded that there was sufficient empty space within the atom

and small heavy positively charged body at the centre called nucleus. Thus,

atom consists of a heavy positively charged nucleus in the centre containing all

protons and neutrons and the electrons were revolving around the nucleus so

that the centrifugal force balances the force of attraction.

#) Atomic number and mass number. As atom as a whole is electrically neutral,

no. Of protons (p)=No. Of electrons (e). This number is called atomic number

(Z). Further, as mass of atom is mainly due to protons and neutrons, and each

has obe unit mass, sum if number of protons (p) and neutrons (n) is called mass

number (A). We represent them with the symbol as A/Z X.

Z=p=e, A=p+n. :n=A-Z

#) Maxwell’s electromagnetic wave theory. Energy is emitted continuously

from any source in the form of radiation travelling in the form of waves and

associated with electric and magnetic fields, oscillating perpendicular to each

other and to the direction of radiation. All of them travel with the velocity of

light.

#) Limitations of Electromagnetic wave theory. It could not explain the

phenomena of black body radiation and photoelectric effect.

#) Black body radiation. If the substance being heated us a black body (which is

a perfect absorber and prefer radiator of energy) the radiation emitted is called

black body radiation.

#) Photoelectric effect. When radiation of certain minimum frequency

(Vo) strike the surface of a metal, electrons are ejected. This minimum energy

(hv o) is called wave function ( W o ).

#)Planck's quantum theory. This theory was put forward to explain the

limitations of Electromagnetic wave theory. It suggests that radiant energy is

emitted or absorbed discontinuously in the form of small packets of energy

called quanta (called photons in case of light ). Energy of each quantum (E)=hv

where 'h' is Planck's constant (=6.626*10-34 Js). Total energy emitted or

absorbed =nhv where n is an integer. If n = N 0 energy is called one Einstein.

#) Explanation of photoelectric effect. When light falls on the metal surface,

energy of photon is absorbed by the electron. Electron is dislodged from the

metal only when energy absorbed (hv) is more than the minimum energy,I.e,

threshold energy ( hv 0) required to overcome its force of attraction. The

difference of energy is imparted to the electron as its kinetic energy. Thus,

K.E. of ejected electrons, 1/2 mv2 = hv -hv 0

As energy of photon depends upon frequency and not on intensity, increasing

the intensity of light of a particular frequency v, increases the number of

electrons ejected but not their kinetic energy.

#) Dual nature of Electromagnetic radiation. Interference, diffraction, etc.can

be explained if light has wave nature. Photoelectric effect can be explained if

light has particle nature. Hence, light is said to have dual nature. This concept

was put forward by Einstein

#) Electron volt (eV) of energy. Energy acquired by an electron under a

potential difference of one volt is called electron volt (eV). (1 eV =1.602* 10-19 J

).

#) Emission and Absorption spectra. When light emitted from any source is

directly passed on to prism and resolved, the spectrum obtained is called

emission spectrum. In case of white light, e.g., from sun, it is resolved into

seven colors (VIBGYOR).The spectrum obtained is called continuous spectrum.

If light emitted from a discharge tube is resolved, some colored lines are

obtained. The spectrum obtained is called line spectrum. If white light is first

passed through the solution of a compound or vapors of a substance and then

resolved, the spectrum obtained is calked absorption spectrum. It has dark lines

in the continuous spectrum.

#) Drawback of Rutherford's model. Revolving electron will lose energy

continuously. Hence, its orbit will become smaller and smaller following a spiral

path and ultimately fall into the nucleus, I.e,atom should collapse. Also, it could

not explain line spectra of H-atom.

#) Bohr's model of atom.To explain the above drawbacks, Bohr postulated that

electrons revolve around the nucleus only in those orbits which have fixed

values of energy. These are called stationary states. They are numbered as

1,2,3,4.......etc. or K,L,M,N....etc.

#)Electronic energy as negative energy. When electron is at infinite distance

from nucleus, there is no attraction on it. Its energy is taken as zero. When it

comes closer to nucleus ,attraction takes place. Energy is released. Hence, it

becomes negative.

#)de Broglie concept of dual nature of matter. Just as light has dual nature,

energy material particle in motion has dual nature. Relation between

them,called de Broglie relation is where v is wavelength , m os mass , v is

velocity and p is momentum of the material particle. Although it is applicable to

all material objects but it has significance only for microscopic particles. The

wave character has been verified by Davisson and Germer's experiment

whereas particle nature by scintillating experiment.

#)Heisenberg's uncertainty principle. It states "It is impossible to measure

simultaneously the position and momentum of a small microscopic particle

with absolute accuracy. If one of them is measured with greater accuracy, the

other becomes less accurate. If the uncertainty in position is uncertainty in

momentum. Like de Broglie concept, it has significance only for microscopic

particles.

#)Quantum mechanical model of atom. Similar to concept of 'orbit ‘in Bohr's

model, it leads to concept of orbital.An orbital is the three dimensional space

around the nucleus within which the probability of finding the electron is

maximum.

#)Pauli exclusion principle. No two electrons in an atom can have the same set

of four quantum numbers or an orbital can have maximum two electrons and

these must have opposite spin.

#)Shapes of atomic orbital’s. The shape of an orbital is found by finding the

probability of the electron in that orbital at different points around the nucleus

and representing by the density of points. The shape of the electron cloud thus

obtained gives the shape of the orbital. Some orbital are found to have a region

of space within it where probability is zero. This is called a node. It may be

spherical/radial or planar/angular.

#) Formulae giving number of nodes:

Number of spherical/radial nodes in any orbital =n -l-1

Number of planar/angular nodes in any orbital =l

: Total number of nodes in any orbital =n -1

#)Energy level diagrams. When orbitals are arranged vertically in order of

increasing energy, the diagram obtained is called energy level diagram. For H-

atom, orbitals of the same sub shell (e.g.,2s, 2p ) have same energy. However in

case of multielectrom atoms, they differ in energy (e.g.,2s < 2p, 3s < 3p < 3d

and so on ).