The smallest particle, which contains all the main characteristics of an element is called its

atom.Three major parts of an atom:

Proton Neutron Electron

Rutherford fired alpha particles through a piece of gold metal foil and used a zinc sulphide detector to detect the scattered alpha particles and their location.

• 1.Most of the alpha particles passed through the gold metal foil without any deflection from their path and struck the screen.

• 2. A few of them were deflected at some angle after passing through the foil.

• 3. A small number of alpha particles were turned back on their original path .

Most of the alpha particles passed through the gold foil without any deflection, the major part of an atom is empty.

A few of alpha particles were deflected from their path through some angles due to the presence of positively charged alpha particles. He named this body as nucleus, which contains the mass and positive charge of the atom.

A small number of alpha particles turned back on their original path without undergoing any deflection suggested that the nucleus can recoil alpha particles. These observations indicate that the volume occupied by the nucleus is only a minute fraction of the total volume of the atom.

The diameter of the atom is about 10-8 cm, while that of the nucleus is 10-13 cm. It follows that the diameter of an atom is about 105 times greater than that of the nucleus.

This proves conclusively that the major part of the atom is empty.

• 1. All atoms are very small round shaped particle, which have two parts . One is nucleus and another is outer empty space of nucleus.

• 2. An atom consist of a small nucleus which contains the entire mass of the atom. The size of nucleus is very small compared to the size of atom.

    

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3. The entire positive charge of the atom is located on the nucleus while electrons are distributed in vacant space around it.

4. The electrons are in constant motion around the nucleus like the planets which move around the sun .

5. The number of positive charge in the nucleus is equal to the number of negative charge possessed by electrons moving around the nucleus.For this every atom is electrically neutral.

6. Two equal & opposite force act on the moving

electrons. They are the electrostratic force of attraction between the electron and positive nucleus and centrifugal force due to the motion of electrons. These two forces are equal and opposite.

 

1. Rutherford atom model is based on the solar system. But the planets in the solar system are electrically neutral and only the gravitational force act between them. But the electrons which move around the positive nucleus are negatively charged and they repel each other.

2. According to Maxwell the charged electrons are supposed to emit energy continuously and in this way of losing energy they will move in a spiral way and fall to the nucleus. It happens that Ruther ford’s model becomes meaningless.

Limitations of Rutherford atom model3. As this radiation is continuous, the spectrum

of the atom should have continuous lines, which should look like a wide band. But these lines are discontinuous and very bright.

4. Rutherford does not give any idea about shape or size of the orbits pattern in which the electrons move around the nucleus.

5. This model does not mention how electrons move in an atom having large number of electrons.

ORBIT1. There are some stationary shells around

the nucleus with definite quantity of energy along which the electrons move. These are called energy levels or orbits.

2. Orbits are denoted by n. The value of n = 1,2,3…… or K,L,M ….. Shells.2. Orbits represent the two dimensional

motion of electrons.3. All the orbits are circular.4. Orbit contains 2n2 number of electrons

where n is the number of orbits.5. 6. The energy of electrons in different

orbits is different. The order of increasing energy is 1˂ 2˂ 3˂ 4.

1.An atomic orbital is defined as the region in space around the nucleus in which the probability of finding the electrons is maximum.

2. Orbital is expressed by s, p,d ,f.

3. Orbital represents three dimensional electronic motion around the nucleus.

4. Orbital have different shape. ‘S’ orbital is spherical, ‘p’ orbital is dumbbell shaped.

5. Each orbital contains maximum two electrons having opposed spins.

6. The energy of orbital in an orbit is equal. such as: the orbital of ‘p’ sub-orbit Px, Py, Pz contains equal amount of energy.

Electron cloud is a region that surrounds the nucleus that contains most of the space in the atom .

Atoms that have the same number of protons but

different numbers of neutrons

Example:

Isotone

The atoms of different elements which have the same number of neutron but different atomic number are called isotones.

14 15 and 16

6C 7

N 8o

  

Isobar The atoms of different elements which have the same mass number but different atomic number are called isobars.

40 40 40 18Ar 19k 20Ca

   

White light is radiant energy coming from the sun. it is composed of light waves in the range (4000-8000 )A0. Each wave has a characteristic colour. When a beam of white light is passed through a prism , different wavelengths are refracted through different angles. When received on a screen, these form a continous series of colour bands: violet, indigo, blue, green, yellow , orange and red (VIBGYOR). This series of bands that form a continous rainbow of colours , is called a continuous spectrum.

If atoms and molecules are heated to sufficiently high temperatures they emit light of certain wavelengths shown by a discharge tube containing gaseous element say hydrogen gas.

The observed spectrum consists of a number of coloured lines on a black back ground. This spectrum is called an emission or line spectrum.

When light is passed through a substance then black lines appear in the spectrum on a bright background. This spectrum is called absorption spectrum, where electrons on absorption of energy jump from lower energy state to higher state. It is used to identify a substance.

Waves are characterized by the following properties.

Wavelengths : The wavelength is defined as the distance between two successive crests or troughs

of a wave. Wavelength is denoted by the Greek letter λ. It is

expressed in centimeters or meters or in angstrom units.

1A0= 10-8 cm =10-10m 1 nm = 10-9m

Frequency : The frequency is the number of waves which

pass a given point in one second. Frequency in denoted by the letter ν (nu) and is expressed in hertz (hz).

Speed The speed (or velocity) of a wave is the

distance through which a particular wave travels in one second.

c = v λ speed = Frequency x wavelengthWave number This is reciprocal of the wavelength and

is given by the symbol v (nu bar).Its units are cm-1 or m-1.

  v = 1 /λ

1. The wavelength of a violet light is 400 nm. Calculate its frequency and wave number.

2. The frequency of strong yellow line in the spectrum of sodium is 5.09x1014 see-1 Calculate the wavelength of the light in nanometers.

  

(1) When atoms or molecules absorb or emit radiant energy, they do so in separate units of waves called quanta or photons.

(2) The energy , E, of a quantum or photon is given by the relation.

E = hv ....... (i)[Where v is the frequency of the emitted

radiation, and h the Planck’s constant .h= 6.62 x 10-27 erg sec or 6062x10-34 J sec.

 C, the velocity of radiation is given by the eqn C = v λ........... (2)

From (2) and (1) E = hc/ λ Thus the magnitute of a quantum or photon

of energy is directly proportional to the frequency of the radiant energy or is inversely proportional to its wavelength,

(3) An atom or molecule can emit (or absorb) either one quantum of energy

(hv) or any whole number multiple of this unit.

Thus radiant energy can be emitted as hv, 2hv, 3hv and so on, but never as 1.5 hv, 3.27hv, 5.9 hv.

Planck’s Quantum Theory of light

Math: Calculate the magnitude of the energy of the photon (or quantum) associated with

light of wavelength 6057.8 A0 . 0 . ( 1A0 = 10-8cm) .

 

When more than one orbital with equal energy are available then electrons will first occupy these orbital separately with parallel (same) spins and the pairing of electron with opposite spin will start only after all the orbital of a given energy level are singly occupied.

N(7 ) = 1s2 2s2 2px1 2py1 2pz1

N(7 ) = 1s2 2s2 2px2 2py1 2pz 0 , According to Hund’s rule this is not possible.

Because two electrons with parallel spins tend to be as far apart as possible in order to minimize the electrostatic repulsion. Therefore , the electrons prefer to occupy the orbital singly as far as possible, when all the orbital get singly occupied, then the incoming electron will pair up with the other electron.

Pauli’s exclusion principle

1. No two electrons in the same atom can have the same values for all four quantum number.

2. No two electrons in the same atom can have identical sets of 4 quantum number.

Pauli’s exclusion principle suppose there are two electrons in 1st orbit

For the 1st electron n=1, l=0, m=0 ,s =+1/2 For the 2nd electron n=1, l=0, m=0 ,s=-1/2

It follows therefore ,that two electrons in an atom can have the same principle quantum number, same subsidiary quantum number and same magnetic quantum number, but their spin quantum number will be different. So each atomic orbital can accommodate maximum two electrons with opposite spins.

1 Electrons go to different orbital according to their increasing energy. Electrons first go to the orbital of low energy and then systematically to the orbital of higher energy . It is known as Aufbau’s law.

2. The increasing order of energy of various orbital’s is as follows.

The energy of an orbital is determined by the sum of principal quantum number (n) and the azimuthal quantum number (l) . This rule is called (n + l ) rule .

There are two parts of this rule:

(a) The orbital’s with the lower value of (n+ l) has lower energy than the orbital’s of higher ( n+ l) value.

(b) When two orbital’s have same (n + l) value , the orbital with lower value of n has lower energy.

.

For example let us compare the (n + l ) value for 3d and 4s orbital’s.

For 3d orbital n = 3, l=2 and n + l = 5 For 4s orbital n= 4, l=0 and n + l= 4

Therefore 4s, orbital is filled before 3d orbital. For 3d and 4p orbital For 3d n + l = 5 n=3 , l=2

For 4p n + l =5 n=4, p=1 It two orbitals have the same value of (n=l) then

electrons will go to that orbital first, which has lower principal quantum number. so electrons will go to the 3d first.

Ruther ford’s nuclear model simply stated that atom had a nucleus and the negative electrons are presents outside the nucleus . It did not say anything as to how and where those electrons are arranged .It could not explain why electrons did not fall into the nucleus due to electrostatic attraction.

In order to overcome the limitation of Rutherfords atom model Neil’s Bohr in 1913 modified the Ruther ford atom model . The Bohr atomic model is stable and correctly correlates the atomic structure and emission spectrum of hydrogen . Bohr’s theory is based on planck’s quantum theory and is built on the following postulates.

1.Electrons travels around the nucleus in specific permitted circular orbits and in no others.

2.While in these specific orbits , an electron does not radiate energy.

3.An electron can move from one energy level to another by quantum or photon jumps only.

4.The angular momentum of an electron orbiting around the nucleus is an integral multiple of planck’s constant divided by 2π.

angular momentum = mvr = n h/ 2π

1. Though Bohr’s atom model can explain the hydrogen atom spectrum but it can’t explain the spectrum of atoms having several electrons .

2. For the transformation of electrons from one energy level to another one , only one line should be obtained in the spectrum, but after analysis the spectrum by high energy spectrometer several fine lines are obtained Bohr’s atom model can’t explain how these fine multiple lines are formed in the atomic spectra.

3. Bohr’s theory fails to recognize the wave property of electron which was established by De Broglie.

4. According to Bohr’s model electrons revolve in circular orbits but modern research reveals that electrons revolve three dimensionally .

5. By precisely defining the radius of the atomic orbits, the theory violated Heisenberg’s uncertainty principle according to which it is impossible to define simultaneously the position and momentum of a moving sub atomic particle. So Bohr atomic model although was an improvement on atom model, it not only failed but measurably failed to explain the atom satisfactorily.

 

Quantum number: The number , which expresses the size ,

shapes and direction of the orbital from the nucleus and the spins of the electron of their own axis, are called quantum number.

There are four types of quantum number: Principal quantum number (n) Subsidiary quantum number (l) Magnetic quantum number (m) Spin quantum number (s)

.This expresses the orbits or principal energy level to which the electron belongs and represents the average distance of the electron from the nucleus. It is denoted by n (n=1,2 &3) .

This quantum number represents the size of the electron orbit . when n=1, represents the first energy level ,

n=2 , represents the second energy level and so on.

The first energy level is the one nearest the nucleus. Each shell has a definite value of energy ,the energy increases as the value of n increases.

There is a limited number of electrons in an atom which can have the same principal quantum number and is given by 2n2, where n is the principal quantum number concerned.

Principal quantum number (n=).1 2 3 4 Letter designation K L M N Maximum no. electrons ( 2n2) 2 8 18

32

Azimuthally Quantum number This is also called secondary or subsidiary quantum number . It

defines the spatial distribution of the electron cloud about the nucleus and describes the angular momentum of the electron. In other words , this quantum number defines the shape of the orbit occupied by the electron . It denoted by l .

The value of l = 0 to (n-1) when, n=1,then l= 0 and so the 1st energy level has 1 sub

shell. The values of the subsidiary quantum number 0, 1, 2, 3 are

represented as s, p, d, f respectively . n = 1, l = 0 n = 2, l = 0 ,1 n = 3, l = 0 , 1, 2 

The spin quantum Number The spin quantum number represents the direction of the

electron spin and is denoted by s. This is due to the spinning of the electron about of its own axis.

Since an electron can spin clockwise or anti clock wise (in two opposite directions), These are two possible values of s that are equal and opposite.

s =+1/2 and s= –1/2 depending upon whether the electron spins in one direction or the other.

These are often represented by putting two arrows in the opposite direction ↓ and ↑, both are respectively . Two electrons with the same sign of the spin quantum numbers are said to have parallel spins while those having opposite signs of the spin quantum number are said to have opposite spin or anti parallel spin or paired up spin.