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CBSE | Class 9 | science
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Summary Note
Key concept recap
Atoms are the building blocks of matter, but further research revealed that they are not indivisible as Dalton proposed.
Atoms consist of sub-atomic particles: electrons, protons, and neutrons.
Understanding how these particles are arranged inside the atom became a major focus for scientists in the 20th century.
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Key concept recap
J.J. Thomson discovered the electron through cathode ray experiments and concluded it carried a negative charge.
E. Goldstein discovered positively charged canal rays, leading to the identification of the proton.
Protons are positively charged particles nearly 2000 times heavier than electrons, and are located in the nucleus.
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Key concept recap
Thomson proposed that atoms are positively charged spheres with negatively charged electrons embedded in them.
This model was popularly known as the 'plum pudding' model.
The model could explain electrical neutrality but failed to account for other experimental results.
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Key concept recap
Rutherford performed the gold foil experiment using alpha particles and found that most particles passed through the foil, but a few deflected.
He concluded that atoms have a dense positively charged center called the nucleus where most mass resides.
Electrons revolve around the nucleus, and the nucleus is much smaller compared to the atom's overall size.
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According to classical physics, electrons moving in circular paths should emit energy and spiral into the nucleus.
This would make atoms unstable, which contradicts the observed stability of matter.
Hence, Rutherford's model could not fully explain atomic stability.
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Niels Bohr proposed that electrons revolve in fixed orbits called energy levels or shells.
Electrons do not radiate energy while in these orbits.
Bohr’s model successfully explained atomic stability and energy levels.
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Key concept recap
James Chadwick discovered neutrons in 1932; they are neutral particles with mass nearly equal to protons.
Neutrons are present in the nucleus along with protons, and contribute to the atom’s mass.
All atoms, except hydrogen, contain neutrons.
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Key concept recap
Bohr and Bury proposed rules for electron distribution: maximum electrons in a shell = 2n², where n is the shell number.
Electrons fill lower shells before occupying higher ones.
Maximum 8 electrons can be present in the outermost shell.
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Key concept recap
Valency is the combining capacity of an atom, determined by the number of electrons in the outermost shell.
If the outermost shell has fewer than 4 electrons, valency equals the number of valence electrons.
If more than 4, valency is 8 minus the number of valence electrons.
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Key concept recap
Atomic number (Z) is the number of protons in the atom.
Mass number (A) is the sum of protons and neutrons.
Atoms are represented by the notation: A/Z Symbol (e.g., ¹⁴₇N).
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Key concept recap
Isotopes are atoms of the same element with same atomic number but different mass numbers (e.g., ¹H, ²H, ³H).
Isobars are atoms of different elements with the same mass number but different atomic numbers (e.g., ⁴⁰Ca and ⁴⁰Ar).
Isotopes have similar chemical properties but different physical properties; some are used in medicine and energy.
Summary Note
Key concept recap
Electron – Negatively charged sub-atomic particle.
Proton – Positively charged sub-atomic particle.
Neutron – Neutral sub-atomic particle.
Nucleus – Dense central core of an atom containing protons and neutrons.
Valency – Combining capacity of an atom.
Isotopes – Atoms of same element with different mass numbers.
Isobars – Atoms of different elements with same mass number.
Atomic Number – Number of protons (Z).
Mass Number – Sum of protons and neutrons (A).
Bohr Model – Model of atom with electrons in fixed energy levels.
