Introduction

• Matter – Anything that occupies space and has mass is called matter.
  For example- Air, water, rocks, bent, chair, paper, and even we people, etc are example of matter.
• Atoms and molecules are the fundamental building blocks of matter. 
• The existence of different kinds of matter is due to different atoms constituting them.
• HOW DO ATOMS LOOKS LIKE?
  First theory in respect of the structure of the atom was given by Sir Dalton

• Dalton Atomic Theory-

1. Matter is made up of very tiny indivisible particles called atom
2. Atoms can neither be created nor destroyed.
3. Atoms of the same elements are exactly identical (except isotopes and isobars). Atoms of different elements are always different in size shape and

Charged Particles In Matter

• An atom is divisible and consists of charged particles.
• Two objects when rubbed together, the objects become electrically charged. This proves that there is also the presence of sub-atomic particles.
• Ionization of gases in the discharge tube proved that atoms have sub-atomic particles.

Discovery Of Electrons

• Discovery of Electrons by Cathode Rays Experiment – J.J. Thomson in 1897.
• CONCLUSION:
• Cathode rays are nothing but a stream of negatively charged particles called electrons.
• These negatively charged particles are an integral part of all atoms and have equal mass.
• Electron carries negative charge of magnitude -1.602×10^-19 Coulombs.

Discovery Of Protons

• Discovery of Protons by Anode Rays Experiment – E. Goldstein – 1909
• CONCLUSION:
• Anode rays are nothing but a stream of positively charged particles called protons.
• These rays were known as positive rays or anode rays or canal rays.
• Protons carries positive charge of magnitude +1.602×10^-19 Coulombs.

Characteristics Of Subatomic Particles

Characteristics	Electron	Proton	Neutron
Symbol	e	p	n
Relative charge	-1	+1	0
Nature	Negatively charged	Positively charged	Neutral
Discovered by	J.J. Thomson	E. Goldstein	James Chadwick
Mass	1/2000 times mass of hydrogen atom	1 unit	Mass is nearly equal to that of proton

Thomson’s Model Of Atom

• An atom is a uniform sphere of positive charges (due to the presence of protons) as well as negative charges (due to the presence of electrons) which are embedded in it.
• This model is often called the ‘Water Melon Model’.
• An atom, as a whole, is electrically neutral because the negative and positive charges are equal in magnitude.

• Limitations of Thomson’s model of atom

1. The model failed to explain how protons and electrons could be arranged in an atom so close to each other.
2. It does not have any experimental evidence in its support.
3. It didn’t mention the nucleus of the atom.

Rutherford’s Model Of An Atom

• Rutherford predicted that if J.J. Thomson’s model, which proposed a uniform distribution of positive and negative charge, was accurate, alpha particles striking a gold atom would be evenly deflected. However, this was not observed.
• Since alpha particles also have a positive charge, so he did not expect to see large deflections.
• Rutherford bombarded alpha rays on a thin sheet of gold foil in an evacuated chamber.
• Selection of gold metal for Rutherford’s α-particle scattering experiment:
• Gold is easily malleable and can be beaten into very thin sheets.

• Observations:

1. Most of the α-particles passed straight through gold foil without suffering any deflection from their original path.
2. Some of the α-particles were deflected by the foil at small angles.
3. One out of every 12000 particles appeared to rebound.

• CONCLUSIONS:

1. Most of the space inside the atom is empty. Hence, it allows the α-particles to pass straight through it without any deflection.
2. Very few particles were deflected from their path, which suggests that the positive charge of the atom occupies very little space.
3. The total volume occupied by a nucleus is very small compared to the total volume of the atom, as very few α-particles are reflected by 180⁰, and all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

Rutherford’s nuclear model of an atom

1. There is a positively charged centre in an atom called the nucleus and the entire mass of the atom resides in the nucleus.
2. Electrons revolve around the nucleus in closed orbits with high speed.
3. Size of the nucleus is very small as compared to the size of an atom.
4. The diameter of the atom is 1,00,000 times larger the diameter of the nucleus.
5. This model was similar to the solar system.

Defects in Rutherford’s model of atom

1. Rutherford had proposed that electrons move around a positively charged nucleus at a very high speed in circular orbits. Electron would have to be accelerated centripetally (tending to move toward a center) to remain in a circular orbit, but according to electromagnetic theory, if charged body (electron) is accelerated around another charged body (nucleus) then there would be continuous radiation of the moving body (i.e., electron). This loss of energy would slow down the speed of electron and eventually electron would fall into nucleus. But Rutherford’s model could not explain such a collapse.
2. Rutherford had proposed that electrons revolve around the nucleus in fixed orbits. He did not specify the number of electrons in each orbit.

Postulates put forward by Bohr regarding model of atom

1. Electrons revolve around the nucleus in a limited number of orbits called discrete orbits of electrons that are also called as permissible orbits.
2. While revolving in discrete orbits, the electrons do not radiate energy i.e., energy of an electron remains constant so long as it stays in a given orbit. Electrons present in different orbits have different energies.
3. When an electron jumps from lower energy level to higher energy level, some energy is absorbed, while energy is released when electron jumps from higher energy level to lower one.
4. Orbits or shells are represented by the letters K,L,M,N…… or the numbers, n=1, 2, 3, 4……

Bohr-Bury Scheme For Distribution Of Electrons In Different Orbits

1. Maximum number of electrons that can be accommodated in a shell is given by 2n², where n is the shell number i.e., first shell can accommodate two electrons, second shell can accommodate eight electrons, third shell can accommodate 18 electrons and so on.
2. Outermost orbit of an atom can accommodate a maximum number of eight electrons.
3. Electrons are not accommodated in a given shell, unless the inner shells are filled, i.e., the shells are filled in a step-wise manner.

Discovery Of Neutron

1. According to Rutherford the nucleus of an atom consist of only a proton and this proton decides the mass of an atom but we know hydrogen has 1 proton in its nucleus whereas Helium has two protons and if the proton where only one particle in the nucleus then helium nucleus would have two protons and there twice the mass of hydrogen.
2. However , Mass of Helium = Four times the mass of hydrogen
3. This suggests there is some other type of neutral particle present in the nucleus.
4. J. Chadwick discovered another sub-atomic particle which had no charge and a mass nearly equal to that of a proton in 1932.
5. Chadwick bombarded beryllium with alpha particles and found that a new particle was ejected which had almost the same mass as that of a proton but no charge because the electric charge detector remain unaffected.

Isotopes

1. Isotopes are the atoms of same element having same atomic number but different mass number.
2. Isotopes have similar chemical properties because they have same number of valence electrons.
3. Isotopes have different physical properties such as boiling point and melting point because they have different mass numbers.
4. Atomic masses of elements are fractional, due to the fact that all elements have isotopes.

Applications of isotopes:

1. An isotope of uranium is used in nuclear reaction.
2. An isotope of cobalt is used to remove brain tumours and their treatment.
3. Isotope of sodium has been used to diagnose restricted circulation of blood.
4. Example : 3 isotopes of hydrogen-protium, deuterium and tritium.

Isobars

• Isobars are the atoms of different elements with different atomic numbers, but same mass number.
• Example: ₂₀Ca⁴⁰,₁₈Ar⁴⁰.