Maharashtra Board Class 8 Science Chapter 5 Inside the Atom PDF Download

Inside The Atom

What is meant by matter? What is an atom? What is the smallest unit of matter?

Indian philosopher Kanad (6th century B.C.) 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.

Greek philosopher Democritus (5th century B.C.) stated that matter is made of small particles and these cannot be divided. The smallest particle of matter was named by Democritus as 'Atom'. (In Greek language 'Atomos' means the one which cannot be cut.)

We have seen that matter is made of molecules. Molecules are formed from atoms. Effectively an atom is the smallest unit of matter. An atom is the smallest particle of an element which retains its chemical identity in all the physical and chemical changes.

The table 5.1 shows names and formulae of some substances. Complete the table by putting tick marks in appropriate box to indicate the information of the smallest particle and the type of matter.

Types Of Substances

We have learnt in the earlier standard that the smallest particles of most of the substances are molecules. The molecules of a few substances contain only one atom. Molecules are formed by chemical combination of atoms. From this we understand that the smallest particle of an element taking part in chemical combination is an atom. The concept of atom is more than 2500 years old. However, it was forgotten in the course of time. In the modern times, scientists on the basis of experiments explained the nature of atom as well as the internal structure of atom. It started with Dalton's atomic theory.

Teacher's Note

An atom is the tiniest part of everything around us. Just like a drop of water is made of tiny water molecules, all things are made of atoms. In India, we use atoms in nuclear power plants to make electricity.

Exam Trick

Remember: Atom means "cannot cut" - it is the smallest piece. Like how you cannot cut an Aadhaar number into smaller pieces, you cannot cut an atom into smaller pieces.

Points to Remember

Atoms are the smallest units of matter.
Molecules are made from atoms joined together.
Elements have only one type of atom.
Compounds have two or more types of atoms.
An atom cannot be divided into smaller parts.

Dalton's Atomic Model

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 and indestructible. All atoms of an element are alike while different elements have different atoms with different mass.

A 'Bundi Laddu' is found to have an internal structure. It is formed by sticking smaller particles, the 'Bundis' to each other. However, the solid ball, broadly speaking, does not have any internal structure. The atom, as described by Dalton, turns out to be a hard, solid sphere with no internal structure. According to Dalton's atomic theory the mass is distributed uniformly in an atom. The scientist J.J. Thomson demonstrated experimentally that the negatively charged particles inside an atom have a mass 1800 times less than a hydrogen atom. Later these particles were named as electron. Common substances are usually electrically neutral. Obviously the molecules of substances and the atoms which combine chemically to form molecules are electrically neutral.

Thomson's Plum Pudding Model Of Atom

How is an atom electrically neutral in spite of having negatively charged electrons in it? Thomson overcame this difficulty by putting forth the plum pudding model of atomic structure.

The plum pudding model of atom put forth by Thomson in the year 1904 is the first model of atomic structure. According to this model the positive charge is distributed throughout the atom and the negatively charged electrons are embedded in it. The distributed positive charge is balanced by the negative charge on the electrons. Therefore the atom becomes electrically neutral.

Plum pudding or plum cake is a sweet dish prepared during Christmas. In old times, this dish was made in Western countries by adding pieces of dried fruit called plum. These days, raisins or dates are used.

Teacher's Note

Thomson's model was like a pudding with dried fruit mixed in it. The pudding is positive charge and the fruit pieces are electrons. This is like how in Indian sweets like laddu, different things are mixed together.

Exam Trick

Remember: Plum pudding = Electrons spread in positive charge. Think of rava laddu with sesame seeds mixed all over - that is how electrons are in the positive part.

Points to Remember

Thomson discovered that atoms have smaller particles called electrons.
Electrons are negative in charge.
Positive charge is spread throughout the atom in Thomson's model.
The atom is neutral because positive and negative charges balance.
This model was the first to show atoms have inner structure.

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