[Chemistry Class Notes] on Atomic Structure Pdf for Exam

Atomic structure consisting of a center called nucleus and electron revolve around the nucleus in a fixed circular orbit with fixed energy and fixed velocity. It also has protons and the number of electrons are equal to the number of protons. While revolving around the nucleus electrostatic field is present between the nucleus and electron which provides centripetal force also. While revolution there is neither gain nor loss of energy. There are so many assumptions regarding the model of atoms and there are so many discovered models on atomic structure. The first scientific theory of atomic structure was proposed by John Dalton in the 1800s.

Dalton Atomic Theory

Smallest fundamental particle is atom and atom can not be further subdivided. The Atoms of different elements are different. 

Postulates of Dalton Atomic Theory

1. Every matter is made up of a fundamental unit known as matter.

2. Atoms of different elements are different.

3. Each atom has its specific mass which varies from atom to atom.

4. Atoms undergo rearrangement during a chemical reaction.

5. Atoms cannot be further subdivided.

Drawback of Dalton Atomic Theory

1. This theory was not able to explain the existence of isotopes.

2. There is no well defined definition of the structure of an atom.

3. Particles of atoms are divisible which was proved later.

J.J Thomson Model

1. It is a subatomic model of an atom.

2. It tells us about different atomic particles.

3. Atom is made up of positive charge and electrons are embedded like plum pudding or watermelon model.

4. Atoms are neutral in nature.

Bohr’s Atomic Model

Postulates of Bohr atomic model

1. Atom has a center known as a nucleus.

2. Electrons revolve around the nucleus in  a fixed orbit with fixed energy and fixed velocity.

3. Electrons revolve only in those angular momentum which is integral multiple of nh/2 pi.

Here n is the shell number.

4. During revolution there is electrostatic force between the nucleus and electron which gives centripetal force.

5. While revolution, electrons neither gain nor lose energy.

Demerits of Bohr’s Model

1. It is applicable only for hydrogen and hydrogen-like atoms.

2. Other scientists discovered that electrons can gain energy or lose energy. In case they gain energy they jump to a higher energy level and if they lose energy then they jump to a lower energy level.

3. Amount of energy gain or loss is equal to the difference in two energy levels.

Cathode Ray Tube Experiment

(Image to be added soon)

A cathode-ray tube (CRT) is a vacuum tube in which an electron beam, deflected by applied electric or magnetic fields, produces a trace on a fluorescent screen. It was invented in 1897 by the german physicist Karl Ferdinand. When an electron beam is deflected then the image is on the projector.  As, electrons are accelerated from one end of the tube to the other using an electric field. When the electrons hit the far end of the tube they give up all the energy they carry due to their speed and this is changed to other forms such as heat. A small amount of energy is transformed into X-rays.

Properties of Cathode Rays

1. They are positive charges.

2. They travel in a straight line.

3. They possess kinetic energy.

4. They are made up of small particles.

5. They are deflected towards the positive charged plate in the electric field.

6. They are deflected in a magnetic field.

7. They produce fluorescence.

8. Cathode ray is made up of electrons.

9. On changing the gas and electrode rod material. Also properties of cathode rays remain unchanged.

Rutherford Model

It is also known as the alpha scattering experiment. 

Construction of Alpha Scattering Experiment

1. A very thin gold foil of 1000 atoms thick is taken.

2. Alpha rays (doubly charged Helium He²+) were made to bombard the gold foil.

3. The ZnS screen is placed behind the gold foil.

Observation of Alpha Scattering Experiment

1. Nearly 99.9% of the alpha particle passed undeviated.

2. Some of the alpha particles were deflected by small angels.

3. Very few i.e 1 out of 2000 returns back i.e deflected at 180 degree.

4. Rutherford discovered nucleus.

Conclusion

1. Positive charges are heavy charges known as nucleus.

2. Most of the atoms are hollow.

3. Positive charge resides in very small space i.e nucleus.

4. Number of protons is the same as the number of electrons.

5. Electrons revolve around the nucleus in a closed circular orbit.

Failures

1. Maxwell Theory: Any accelerated charge particle emits radiation and loses energy. So according to this electron will collide with the nucleus and the atom will destroy. So, Rutherford cannot explain the stability of atoms.

2. He cannot explain the line spectrum / discrete spectrum of atom
   s.

Subatomic Particle of Atoms

Protons

1. Protons are positively charged subatomic particles. The charge of a proton is 1e, which corresponds to approximately 1.602 × 10-19

2. The mass of a proton is approximately 1.672 × 10-24

3. Protons are over 1800 times heavier than electrons.

4. The total number of protons in the atoms of an element is always equal to the atomic number of the element.

Neutrons

1. The mass of a neutron is almost the same as that of a proton i.e. 1.674×10-24

2. Neutrons are electrically neutral particles and carry no charge.

3. Different isotopes of an element have the same number of protons but vary in the number of neutrons present in their respective nuclei.

Electrons

1. The charge of an electron is -1e, which approximates to -1.602 × 10-19

       2. The mass of an electron is approximately 9.1 × 10-31.

3. Due to the relatively negligible mass of electrons, they are ignored when calculating the mass of an atom.