Structure Of The Atom Revision Notes

Class 9 Notes

Please refer to the Structure Of The Atom Revision Notes given below. These revision notes have been designed as per the latest NCERT, CBSE and KVS books issued for the current academic year. Students will be able to understand the entire chapter in your class 9th Science book. We have provided chapter wise Notes for Class 9 Science as per the latest examination pattern.

Revision Notes Chapter 4 Structure Of The Atom

Students of Class 9 Science will be able to revise the entire chapter and also learn all important concepts based on the topic wise notes given below. Our best teachers for Grade 9 have prepared these to help you get better marks in upcoming examinations. These revision notes cover all important topics given in this chapter.

Structure Of The Atom Class 9 Science Notes

• More than 2400 years ago, he named the smallest piece of matter “ATOMOS ,” meaning “not to be cut.” To Democritus,
• Atoms were small, hard particles that were all made of the same material but were different shapes and sizes.
• Atoms were infinite in number, always moving and capable of joining together

Dalton’s Atomic theory:

Structure Of The Atom Class 9 Science Notes

 First recorded evidence that atoms existed.
 Using his theory, Dalton rationalized the various laws of chemical combination
Dalton’s theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights.

 Limitations
• The indivisibility of an atom was proved wrong , for, an atom can be further subdivided into
protons, neutrons and electrons.
• The atoms of same element are similar in all respects , but isotopes of same element have different mass.
• Dalton’s theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights.

Structure Of The Atom Class 9 Science Notes

Discovered electrons in 1897.
• Showed us that the atom can be split into even smaller parts.
His discovery was the first step towards a detailed model of the atom .
• An atom is a uniform sphere of positive charges (due to presence of protons) as well as negative
charges (due to presence of electrons).
• An electron is a negatively charged component of an atom which exists outside the nucleus.
Each electron carries one unit of negative charge and has a very small mass as compared with that of a neutron or proton.

Structure Of The Atom Class 9 Science Notes

JJ Thomson used cathode ray tubes to demonstrate that the cathode ray responds to both magnetic and electric fields.
Since the ray was attracted to a positive electric plate placed over the cathode ray tube (beam deflected toward the positive plate) he determined that the ray must be composed of negatively charged particles. He called these negative particles “electrons.”

Limitation: Model failed to explain how protons and electrons were arranged in atom so close to each other.
Eugene Goldstein:

Structure Of The Atom Class 9 Science Notes

“canal rays” which had electrical and magnetic properties opposite of an electron
Protons:
The canal rays have positively charged sub-atomic, particles known as protons (p).

Rutherford’s Scattering Experiments:

Structure Of The Atom Class 9 Science Notes

Experiment: Rutherford took a thin gold foil and made alpha particles , [ He2+ ] positively charged Helium fall on it.

Structure Of The Atom Class 9 Science Notes


Structure Of The Atom Class 9 Science Notes

• Limitation: In Rutherford’s atomic model , Nucleus & electrons are held together by electrostatic
force of attraction which would lead to the fusion between them. This does not happen in the atom.
Atomic radius ~ 100 pm = 1 x 10-10 m
Nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

Structure Of The Atom Class 9 Science Notes

• In 1932, James Chadwick proved that the atomic nucleus contained a neutral particle which had been proposed more than a decade earlier by Ernest Rutherford officially discovered the neutron in 1932, Chadwick received the Nobel Prize in 1935.
A neutron is a subatomic particle contained in the atomic nucleus. It has no net electric charge, unlike the proton’s positive electric charge.

Sub Atomic Particles:

Structure Of The Atom Class 9 Science Notes

Protons & Neutrons collectively are known as NUCLEONS.

Atomic Number & Mass Number:
“Atomic number of an element is defined as the number of unit positive charges on the nucleus (nuclear charge) of the atom of that element or as the number of protons present in the nucleus.”

present in the nucleus.”
Atomic number, Z = Number of unit positive charge on the nucleus = Total number of unit
positive charges carried by all protons present in thenucleus. = Number of protons in the nucleus
(p) = Number of electrons revolving in the orbits (e)
Eg :- Hydrogen – Atomic number= 1 (1 proton)
Helium – Atomic number = 2 (2 protons)

Mass number[ A] : It is defined as the sum of the number of protons & neutrons present in the
nucleus of an atom.
Mass Number = Mass of protons + Mass of neutrons
Eg :- Carbon – Mass number = 12 (6 protons + 6 neutrons) Mass = 12u Aluminium –
Mass number = 27 (13 protons + 14 neutrons) Mass = 27

Structure Of The Atom Class 9 Science Notes

Niel Bohr Atomic Model:

Structure Of The Atom Class 9 Science Notes

Bohr and Bury Scheme – Important Rules

Structure Of The Atom Class 9 Science Notes

The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has a capacity to accommodate more electrons. This is a very important rule and is also called the OCTET RULE. The presence45 of 8 electrons in the outermost shell makes the atom very stable.

Isotopes & Isobars:

Structure Of The Atom Class 9 Science Notes
Structure Of The Atom Revision Notes