8 th Science, 5.1, Inside the Atom, Notes. 

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• Molecules are formed from atoms.
• What is an atom?

1.  An atom is the smallest particle of an element which retains its chemical identity in all the physical and chemical changes.  Or
2.  The basic unit of a chemical element and the fundamental building block of matter is called an atom. Or
3.  The smallest fundamental particle of matter that retains the properties of an element.  Or
4. The smallest fundamental particle of matter that retains the properties of an element.

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• What is the smallest unit of matter?

Ans: An atom is the smallest unit of matter.

• What is meant by matter?

  Ans: Any substance that has mass and occupies space is called as matter.

• The molecules of a few substances contain only one atom.

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🎸 Indian philosopher Kanad:

6th century B.C.

Kanad stated that there is a limit to divide matter into small particles. 

The indivisible particles that constitute matter were named by Kanad Muni as ‘Paramanu’ (meaning the smallest particles).

He also stated that ‘Paramanu’ is indestructible/ Unbreakable.

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🏈Greek philosopher Democritus:

 5th century B.C.

Democritus stated that matter is made of small particles and these cannot be divide. 

The smallest particle of matter was name by Democritus as ‘Atom’. 

In Greek language ‘Atomos’ means the one which cannot be cut.

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🎯  Dalton’s atomic theory : 

British scientist John Dalton put forth in 1803 A.D. his celebrated ‘Atomic Theory’. 

According to this theory matter is made of atoms and atoms are indivisible/ undividable and indestructible. 

All atom of an element are alike while different element have different atom with different

mass.

• (Indivisible means something that cannot be broken up or divided.
• Indestructible means unable to be destroyed )

1. The atom, as described by Dalton, turns out to be a hard, solid sphere with no internal structure.
2. According to Daltons atomic theory the mass is distributed uniformly in an atom.

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🏇  The scientist J.J. Thomson demonstrated experimentally that the  negatively charged  Electron particles inside an atom have a mass 1800 times less than a hydrogen atom.

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🎺 Thomson’s plum pudding model of atom:

The plum pudding model of atom put forth by Thomson is the first model of atomic structure. According to Thomson's model the positive charge is distributed throughout the atom and the negatively charged electron are embedded in it. 

The distributed positive charge is balanced by the negative charge on the electrons. Therefore the atom becomes electrically neutral.

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🏀 Rutherford’s scatterring experiment

Rutherford took a very thin gold foil (thickness : 10^-4 mm) and bombarded it with

positively charged α - particles emitted by a

radioactive element. 

 He observed the path of α - particles by means of a fluorescent screen around the gold foil.

It was expected that the α- particles would get reflected from the gold foil if the positively

charged mass were evenly distributed inside the atoms. 

Unexpectedly, most of the α- particles went straight through the foil, a small number of α-particles get deflected from the original path through a small angle, a still smaller number of α- particles get deflected throught a larger angle and susprisingly one α - particle out of 20000 bounced back in the direction opposite to the original path.

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🏁 What is conclusion of Rutherford’s scatterring experiment

Ans:

The large number of the α - particles that went straight through the foil indicates

that there was no obstacle in their path. It meant that there must be mainly an empty

space inside the atoms in the solid gold foil.

The small number of α - particles that get deflected through a small or a big angle must

have faced an obstacle in their path. It meant that the positively charged and heavy part causing obstruction would be in the centre of the atom.

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🎻  Describe Rutherford nuclear model.

Ans:

1. There is a positively charged nucleus at centre of an atom. 

2. Almost the entire mass of the atom is concentrated in the nucleus.

3. Negatively charged particles called electrons revolve around the nucleus. 

4. The total negative charged on all the electron is equal to the positive charge on the nucleus.

As the opposite charges are balanced the atom is electrically neutral. 

5. There is an empty space between the revolving electron and the atomic nucleus.

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🎾 The atom described in Rutherford’s model turns out to be unstable. In reality, however all atom, except radioactive atom, are stable. This shortcoming of Rutherford’s atomic model was removed by the atomic model put forth by Niels Bohr in the year 1913.

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In the year 1913 Danish scientist Niels Bohr explained the stability of atom by putting forth stable orbit atomic model.

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Write are the important postulates of Bohr’s atomic model.

1. The electrons revolving around the atomic nucleus lie in the concentric circular orbits at certain distance from the nucleus.
2.  Energy of an electron is constant while it is in a particular orbit.
3.  When an electron jumps from an inner orbit to an outer orbit it absorbs energy equal to the difference of its energy level. and when electron jumps from an outer orbit to an inner orbit it emits energy equal to the difference of its energy level.

 

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