Do you ever thought that what if we start dividing matter till its last pace? This chapter deals with the solution of such confusions. In this article Structure of the atom Class 9 notes we will read about discovery of atoms, subatomic particles, discovery of electrons – Cathode ray experiment by JJ Thomson, protons – anode rays, Neutrons by James Chadwick, Atomic models of JJ Thomson, Rutherford, Neil Bohr, Atomic number, Mass Number, Distribution of electrons in different Shells, Isotopes and Isobars.

## Introduction:

• John Dalton considered atom as the indivisible particle i.e. further it can’t be divided.
• After discovery of ‘subatomic particles,’ Dalton’s theory was discarded.
• Sub-atomic particles include Electrons, Protons and Neutrons.
• Further read this article on Structure of the atom class 9 notes as a revision material.

## Dalton’s Atomic Theory:

• All matter was composed of atoms.
• Atoms are indivisible and indestructible
• All atoms of an element were identical, whereas atoms of different elements had differing size and mass.
• All compounds were composed of combinations of these atoms in defined ratios.

### Limitations of Dalton’s Atomic Theory:

• Atom consists of particles like electrons, protons, neutrons etc. thus atom is divisible.
• Isotopes discovery revealed that atoms of the same element possess different weights.

Notes of Structure of the class 9 notes includes all the important topic based on CBSE curriculum.

## Subatomic Particles:

 Synonyms Electrons Protons Neutrons Discovered by J.J Thomson E. Goldstein J. Chadwick Charge -1.6 x 10-19 C + 1.6 x 10-19 C 0 Mass 9.1 x 10-31 Kg 1.673 x 10-24 gm 1.673 x 10-24 gm

## Structure of the atom:

Structure of the atom for class 9 CBSE notes is here with all the atomic models. Proposed models are given below:

## Thomson’s Model of an atom:

• By Cathode ray experiment Thomson explained the presence of electrons in atom.
• This model of atom is also known as plum pudding model.
• According to him, atom is a positively charged sphere and the electrons are embedded in it.
• The magnitude of positive and negative charge is same inside atom, so atom is neutral.

### Limitations of Thomson’s Model of an atom:

• Electrically neutral behavior of atom was not explained well using this model.
• The further experiments deviated from the facts that were considered by J.J Thomson.
• Read and revise notes for Structure of the atom class 9.

## Rutherford’s α – particle scattering experiment:

• In his experiment, he bombarded fast moving α-particle on thin gold foil and noted the observations.
• He selected a gold foil because thin layer was required. This gold foil was about 1000 atoms thick.
• α – particles are doubly-charged helium ions (He2+). They are highly energetic as they have 4 u mass.

Observations:

• Most of the α-particles passed straight through the gold foil.
• Some of the α-particles were deflected by small angles.
• One out of 12000 α particles returned back to the same path.

Conclusion of experiment:

• Most of the space inside an atom is empty.
• Positive charge occupies a little space in an atom.
• Inside an atom positive charge and mass of an atom is concentrated in a small volume (called nucleus).
• Radius of nucleus is 105 times less than the radius of the atom.
• Thus revise the terms in structure of the atom for class 9 regularly.

## Rutherford’s model of an atom:

• There is a positively charged centre in an atom where all the mass of atom lies is called nucleus.
• The electrons of the atom revolve around the nucleus in circular paths.
• The size of the nucleus is very small as compared to the size of the atom.

### Limitations of Rutherford’s Model:

• Accelerating electron releases energy, thus while moving electron will lose energy and thus is negatively charged will fall in the nucleus of positive charge.
• By this possession, the electron must be highly unstable, but in reality, atom is highly stable.
• This lead to the failure of his model. Read more in structure of the atom for class 9 notes.

## Bohr’s Model of an Atom:

• Inside the atom only certain special orbits known as discrete orbits of electrons, are allowed.
• While moving in that discrete orbits electrons do not lose its energy.
• These shells or obits are called energy levels.
• These shells are represented by letters K, L, M, N… or n=1, 2, 3, 4……
• Energy is absorbed or emitted when electrons transfer from one energy level to the other.

In the notes of Structure of the atom for Class 9 there is little explanation of Bohr’s model. Further you will read in higher sections.

## Atomic number:

• The number of protons present in the nucleus of an atom is called atomic number.
• It is denoted by ‘Z’.
• Atomic number = number of protons = number of electrons
• In case of ions the number of electrons varies.

## Mass Number:

• The sum total of number of protons and neutrons present in the nucleus of an atom is said to be mass number.
• It is denoted by ‘A’.
• Mass number = no. of protons + no. of neutrons
• Proton together with neutron are called nucleons.
• The number of neutrons present in the nucleus of an atom is denoted by

## Isotopes:

• When atoms of same elements having same atomic number but different mass number is said to be isotopes.
• Isotopes of carbon are: [ 12C6 , 14C6 ] also the isotopes of hydrogen are: [ 1H1 and 2H1 ].
• They have same chemical properties but different physical properties.

### Application of Isotopes:

• Isotope of uranium is used in the nuclear reactor as fuel.
• Cobalt isotopes are helpful in treatment of cancer.
• One of the Iodine isotope is used in the treatment of goiter.
• The basics is in structure of the atom class 9 notes.

## Isobars:

• When atoms of different element having same mass number are said to be isobars.
• Also we can say that elements having same number of nucleons.
• As, 40Ar18 and 40Ca20 have same mass number but different atomic number, hence they are isobars.

## Distribution of electrons in different orbits or shells:

• The distribution of electrons into different orbits of an atom is said to be electronic configuration.
• The arrangement followed the Bohr-Bury Scheme.

Rules of Electronic distribution:

• An orbit can accommodate a maximum of 2n2 electrons, where ‘n’ is the orbit number.
 Shell Maximum No. of electrons K- Shell or n = 1 2n2 = 2(1)2 = 2 L- Shell or n = 2 2n2 = 2(2)2 =8 M- Shell or n = 3 2n2 = 2(3)2 =18 N- Shell or n = 4 2n2 = 2(4)2 =32
• First inner shells are filled then outer, and so on.
• The outermost occupied shell of an atom can have a maximum of 8 electrons even if has capacity to hold more electrons.

Structure of the atom class 9 notes deals with the subjective evaluation for SA.

## Valency:

• We have already discussed valency in previous chapter atoms and molecules.
• Read here in Structure of the atom for class 9 in details.
• The combining capacity of an atom is known as valency.
• To attain stable electronic configuration the number of electrons the atoms should gain or lose or share is said to be its valency.
• K – shell can accommodate a maximum of 2 electrons.
• The electrons which is present in the outermost orbit of an atom is known as valence electrons.

## Structure of the atom for class 9 important questions:

Name the subatomic particles.

Electron, Proton and Neutrons are subatomic particles.

Explain Canal rays.

Canal rays are the positively charged rays. They are also called anode rays. It was used by E. Goldstein in his discharge tube experiment.

What is the electronic configuration of Cl? Also write the number of valance electrons in it and its valency.

The electronic configuration of Chlorine is [1s2 2s2 2p6 3s2 3p5]. i.e. (2, 8, 1). The number of valance electrons are 1 thus to gain stable electronic configuration it will lose electron. Hence its valency is 1.

Explain neutrality of an atom on the basis of Thomson’s model of an atom.

According to Thomson’s model of an atom the total number of electrons as well as the number of protons remains the same. So the having opposite charges on proton as well as neutron they neutralize each other.

Learn and revise Structure of the atom for class 9 notes to score better in the examinations.