Atomic Structure notes, Schemes and Mind Maps of Chemistry

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Discharge Tube Experiment

Explain the construction and working of the discharge tube with reference to the discovery of electron and proton;

DISCHARGE TUBE EXPERIMENT:

All gases air are bad conductors of electricity at normal pressure. But on high voltage and low pressure, they become good conductor. The conduction of electricity was first studied by WILLIAM COOKS. The apparatus used for this purpose is called “DISCHARGE TUBE”. Discharge tube CONSTRUCTION: Cathode Anode (-) (+) Discharge tube consists of a cylindrical glass tube closed at both ends and fitted with two metallic electrodes. These electrodes are connected to the positive and negative terminals of a battery. The discharge tube also possesses a side tube which is connected to a Cathode rays vacuum pump in order to remove the gas or air from it. The removal of air or gas reduced pressure inside the High Voltage To vacuum pump tube.

- +

WORKING:

When a high voltage is applied to a discharge tube at normal pressure, no phenomenon is observed. But when the vacuum pump is started and most of the gas inside the discharge tube is pumped out and the pressure is reduced to 1 torr , the tube soon begins to emit a soft glow. This gives an indication that the gas in the tube begins to conduct electricity. As the pressure is further reduced, the glowing rays move towards anode. Since these rays are produced at the surface of the cathode therefore these are called “CATHODE RAYS”. At still lower pressure about 0.01 torr , the flow from within the tube fades out and dark spaces appears in the discharge tube. At this stage the glass tube at the anode end begins to emit a greenish fluorescence.

2.1.2 PROPERTIES OF CATHODE RAYS:

Explain the properties of: a. cathode rays, b. positive/ anode/ canal rays; The various experiments performed by different scientists indicate that the cathode rays possess the following characteristics:

1. These rays travel in a straight line perpendicular to the cathode surface.
2. These rays produce sharp shadows if an opaque object is placed in their path. 3. These rays emerge from cathode and can be focused by using a concave cathode. 4. These rays can penetrate small thickness of meter e.g., Aluminum or Gold foils without producing any perforation in it.
3. These rays can exert mechanical pressure, showing they possess Kinetic Energy. 6. These rays are deflected in a magnetic field. This behavior indicates that they consist of charged particles.
4. These rays are deflected towards positively charge plate in an electrical field, indicating that they consist of negatively charged particles called ELECTRONS.
5. The charge to mass ratio (e/m) of these rays is 1.76 x 10^11 coulomb / Kg.
6. The charge and mass of these rays are 1.6 x 10-19^ coulomb & 9.1 x 10-31^ Kg respectively.
7. The rays were seen neither to depend on the material of which the electrodes were made nor upon the gas which is filled in the tube.

PROPERTIES OF POSITIVE RAYS:

1.These rays travel in a straight line. 2.These rays are deflected by electric and magnetic fields opposite to the cathode rays. Therefore, these are positively charged rays. 3.These rays are not emitted from anode but are produced from the ionization of gas as a result of bombardment of electrons. 4.Their positive charge was found to be equal to that of electron or simple multiple of it. 5.These particles were found to be much heavier than electrons. The mass of these particles were found to depend on the kind of gas taken in the discharge tube but it is never less than that of an atom or hydrogen.

1. Positive rays unlike cathode rays have different value of e/m depending on the gas present in the discharge tube.

2.3 Bohr’s Atomic Theory

Explain Bohr’s atomic theory and its defects;

BOHR’S ATOMIC THEORY:

The weakness in the Rutherford’s model and the formation of line spectrum were improved by Neil Bohr, who proposed a new theory to explain the electronic structure of the atom in 1913. This theory is based upon the following assumptions.

ASSUMPTIONS:

1. Electrons in an atom revolve around the nucleus in fixed circular which he called orbits or energy levels. 2. As long as an electron revolves in a particular energy level it does not emit or absorb energy. 3. When an electron absorbs energy, it moves to a higher energy level, further away from the nucleus. When it loss energy, it returns to a lower energy level, closer to the nucleus and the energy is emitted as light.
2. The electron loses a definite quantity of energy called “ Quantum ”, when it jumps from an orbit of higher energy level to lower energy level.
3. The energy is emitted in the form of radiations. The frequency of the energy emitted is directly proportional to the difference in energy between two levels. i.e. E 2 – E 1 α υ E 2 – E 1 = hυ

Δ E = hυ where, E 1 = Energy of electron in 1st^ orbit. E 2 = Energy of electron in 2nd^ orbit. ΔE = Energy difference b/w two levels. υ = Frequency of the energy emitted. h = Plank’s Constant

h .

6. The angular momentum (mvr) of an electron in any orbit is integral multiple of 2 π

where, m = mass of electron v = velocity of electron r = Radius of the orbit n = Quantum number = 1, 2, 3, …………… 2.3. Calculate the radius and energy of revolving electrons in orbits with reference to Bohr’s atomic theory;

RADIUS OF AN ORBIT:

Suppose an atom of Hydrogen with atomic number ‘Z’ and electro with mass ‘m’ , charge ‘e’ is revolving around its nucleus at the distance ‘r’. The electrostatic force (centripetal force) between the nucleus and the electron would be 2

Ze e

×

Ze

K = 2 2 r r 2

mv

The centrifugal force which keeps the electron away from the nucleus would be

. Since the electron is in

2 2

n h

2

v

= -------------- (ii)

2 2 2

4 m r

Put this value of ‘v^2 ’ in equation (i) we have 2 2

m

n h

Ze

K ⎜ ⎟ = ⎟ ⇒ ⎜

2 2 2 2

r

4 r

m r

2 2 2

m π

Ze

K

m r

= ×

2

r

r

n h

2 2 2

m π

m Ze

K

2 2

r

n h

2 2

r

n h

m

2 2 2

4 m KZe

2 2

n h

r

= -------------- (iii)

r

Now P.E. = Fcoul X Distance

2

Ze

P E K ×

−. = r ⇒ 2 r 2

Ze

P E K

r

The total energy of electro will be the sum of its K.E. & P.E. ∴ E = K.E + P.E 2 2

Ze

E K

Ze

K

r

r

2

Ze

E K

= -------------- (v)

= − ×

n h

2 4 2

Z e K m

E

= -------------- (vi)

2 2

n h

2.3.3explain spectral lines of hydrogen atom;

SPECTRUM OF HYDROGEN ATOM:

Although Hydrogen atoms contain only one electron, its spectrum gives a large number of series. Balmer in 1885, studied the spectrum of Hydrogen. He found that, when energy is supplied to the sample of hydrogen gas, individual atoms absorb different amounts of energy. The electrons in higher energy levels are unstable &

drop back to the lower energy levels &during this process energy is emitted in the form of line spectrum containing various lines of frequency & wavelength. Balmer observed that a series of lines appeared in the visible region when the electrons dropped from 3rd^ , 4 th,----nth energy levels to the second orbit. These spectral lines are known as “BALMER SERIES”. He proposed an empirical formula to find wave no.(υ) of each line.

υ RH Z

⎢ ⎥ 2 2 n ⎣ ⎦ 2

where n 2 = 3,4,5,……… Lyman later on discovered another series in uv-region. wave no. of each line was sound by similar formula.

υ RH Z

2

n

2

2 where n 2 = 5,6,7,…………… Pfund also found another series far-infrared region. Wave no. of each line was given

by:- ⎡

υ RH Z

⇒ ⎢ ⎥ 2 5 n ⎣ ⎦ 2

where n 2 = 6,7,8,…………… 2.3. calculate wave numbers of photons of various spectral series with reference to

Bohr’s atomic theory;

X-Rays and Atomic Numbers

Explain the production and uses of X-rays;

X – RAYS:

Professor William Roentgen in 1895 discovered that when cathode rays (electrons) collide with a metal anode, a very penetrating radiation is produced, which were named “X – Rays”. They were also called Roentgen rays ”. Types of X-rays: PRODUCTION OF X-RAYS : X-rays can be produced in a special type of discharge tube in which cathode is a heated filament. Under high voltage (5000v) and very low pressure (0.001mm of Hg)cathode rays are emitted from cathode and travel towards anode where they strike with high speed. The transition of electron occurs in the anode atoms which cause the production of X-rays photon. These rays have the following properties. ➢ They are invisible rays. ➢ They can penetrate paper, rubber, glass, metal and human flash. ➢ They have very short wavelength. ➢ They have high energy and are electromagnetic in nature. ➢ Different metals produce X – Rays of different wave lengths. 2.4. Relate the X ray frequency to the atomic number of different elements, with reference to

MOSELEY'S EXPERIMENT:

1. X-ray Spectroscopy: Moseley measured the frequencies of X-rays emitted by different elements. 2.

Relationship: He discovered a linear relationship between the square root of The X-ray frequency and

the atomic number (Z) of the elements.

Key Findings

1. Atomic Number: Moseley's work led to the realization that atomic number (Z) is a more

fundamental property of an element than atomic mass.

2. X-ray Frequency: The frequency of X-rays emitted by an element is directly related to its atomic