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The essential terms related to the atom are an atomic number, mass number, electronic configuration, valency, isotopes and isobars. Atomic Number represents the total number of protons present in the nucleus of an atom. The word 'atom' has been derived from the Greek word 'a-tomio' which signifies 'non-divisible'. It is a neutral atomic particle. To describe the arrangement of subatomic particles within an atom, many models of an atom are proposed. Some of them are Thomson’s Model, Rutherford’s Model, Bohr’s Model. Atoms comprise of three fundamental particles: protons, electrons, and neutrons.

Basic Requirement

  • Class 9 Student

Skills Covered

  • Thompson's Structure Of An Atom

  • Rutherford's - Alpha Particules

Expert Review

Science was revolutionized with the discovery of atoms. Despite an atom's minute size, it contains an array of essential concepts. This is explained in detail in the chapter Structure of an Atom in class 9. In this chapter, you will learn about the fundamental components of an atom, atom models, electron distributions, valences, atomic number, and mass number. Let's have a look at the CBSE syllabus for class 9 and begin.

1) Fundamental Constituents of an Atom

A proton, a neutron, and an electron make up an atom. Atoms contain protons and neutrons, where protons have a positive charge and neutrons have a neutral charge. At the outermost regions are electrons.

2) Electron

Cathode rays were discovered by J. J. Thomson in 1897 as negatively charged particles emanating from the cathode towards the anode. This negative charge is generated by electrons.

3) Protons

In 1886, Ernest Goldstein discovered that anode and cathode emit positively charged particles called Canal rays or proton, depending on the situation in the chamber.

4) Neutrons

In the nucleus of all atoms, J. Chadwick discovered a subatomic particle with no charge and the mass of protons. This particle is called a neutron.  

5) Atomic Structure of Carbon

Carbon atoms contain six protons, six electrons, and six neutrons, giving it a mass number of 12. 

6) Atomic Structure of Oxygen

The oxygen atom has eight protons, eight electrons, and eight neutrons, giving it a mass number of 16.  

7) Atomic Structure of Hydrogen

Hydrogen atoms (H) contain only one proton, one electron, and no neutrons.

8) Atomic Structure of Helium

An atom of helium consists of two protons, two electrons, and two neutrons, giving it a mass number of 2.  

Models of the structure of an atom   

There have been a variety of theories formulated since the discovery of atoms. The following are the important theories about atom structure as per the chapter. 

Thomson's Model of an Atom     

Thomson proposed that atoms were like Christmas puddings, with electrons embedded within the sphere like currants. According to Thomson:

1) Atoms are positively charged spheres embedded with electrons

2) Electrons and protons in an atom have equal magnitudes, so an atom is electrically neutral

3) A drawback of Thomson's Model: Thomson's structure was not able to explain in detail the arrangement of protons and electrons inside an atom. 

Rutherford’s Model of an Atom

Rutherford bombarded a gold foil with alpha (*)-particles. In his observation of alpha (*)-particles after passing through an atom, he formulated the following postulates:

1) Since the gold foil does not impede the particles, most of the space in an atom is empty.

2) The positively charged center of an atom is called the nucleus, and all of its mass resides in it. After bombarding the nucleus 1800 particles were deflected.

3) An electron orbits the center in a specific path

4) Nuclei are small in comparison with the total size of an atom

5) Rutherford presented these postulates based on scattering of alpha (*)-particles on gold foil. 

Drawbacks of the Model:

Even though Rutherford proposed an entirely new model of atom structure, he failed to explain many of its drawbacks, including the following:  

1) During the rotation of the electrons, they are exposed to acceleration radiation. The electrons lose energy as they revolve. Soon the electrons would collide with the nucleus. An atom with this tendency would be highly unstable but an atom with this tendency would be highly stable

2) Rutherford's structure of an atom could not explain the atomic number concept, because it explained only the presence of protons in the nucleus.

Bohr’s Model of an Atom 

As a response to Rutherford's model's objections, Bohr devised his own. His postulates were as follows: 

1) Atoms permit electrons to orbit only a discrete amount of orbitals, which makes up their outer structure

2) These orbitals and energy levels do not lose energy as the negatively charged particles revolve

3) A change in magnitude occurs when an electron jumps from one energy shell to another

4) In Bohr's model, the structure of an atom is explained in a comprehensive manner and all other models on the structure of an atom have been overcome.

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Course creator

                                 Rashmi T

Rashmi T

I have More than 35 years of Management and teaching experience in chemistry subject also I have done Ph.D. from IIT Roorkee in chemistry.