The document provides an overview of the structure of an atom, detailing its main constituents: protons, neutrons, and electrons, along with their properties and historical context. It discusses various atomic models by notable scientists, including Thomson's, Rutherford's, and Bohr's models, highlighting their strengths and limitations. Additionally, it explains concepts such as mass number and atomic number, along with examples from different elements.

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Structure of the Atom – NCERT Class 9 Science Notes
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Structure of the Atom Notes
Structure of the Atom Class 9 Notes
Structure of the atom – An atom is a particle of matter that uniquely defines a chemical
element.
Atom Constituents
An atom consists of a nucleus located in the middle that is surrounded by electrons. The
nucleus contains relatively heavy particles known as protons and neutrons.
Therefore, protons, neutrons, and electrons are the three fundamental particles found in an
atom. Protons and neutrons make up the atom’s nucleus. The electron shell is the
outermost area where the electrons are situated.
Electrons
Electrons are negatively charged particles located inside all kinds of atoms. Electrons were
first discovered in an experiment by J. J. Thomson in 1897, who noticed that negatively

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charged particle discharged from the cathode toward the anode.
Protons
In the same chamber, under a different circumstance, Ernest Goldstein found in 1886 that
the anode released positively charged particles known as Canal rays or subsequently
designated as Protons.
Neutrons
A subatomic particle with no charge and a mass comparable to the protons in the nuclei of
all atoms was found by J. Chadwick. Hence, they came to be known as neutrons, meaning
those charged neutrally.
Atomic Structure of different elements
Carbon
The carbon atom has a mass of 12 due to the presence of six protons, six electrons, and
six neutrons.
Oxygen
The oxygen atom has a mass of 16, consisting of eight protons, eight electrons, and eight
neutrons.
Hydrogen
There is just one proton, one electron, and no neutrons in the hydrogen atom (H).
Helium
Two protons two electrons and two neutrons make up the helium atom giving it a mass

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Two protons, two electrons, and two neutrons make up the helium atom, giving it a mass
number of two.
Structure of an Atom: Different Models
Many ideas have been developed since the discovery of atoms by several eminent
scientists.
The key atomic structure hypotheses are listed below.
Thomson’s Model
According to J. J. Thomson, an atom’s structure is comparable to a Christmas pudding
because electrons are entangled in the sphere like currants.
He proposed that:
An atom’s structure is a positively charged sphere with embedded electrons.
Because the magnitudes of the protons and electrons in an atom are equal, it is
electrically neutral.
Drawbacks of Thomson’s Model
Thomson’s model did not adequately explain the configuration of protons and electrons in
an atom’s structure.

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Rutherford’s Model
Rutherford bombarded a gold foil with alpha ()-particles in an experiment.
He prepared certain postulates for the experiment based on his observations of the alpha
()-particles’ track after passing through an atom, including:
Since the particles quickly went through the gold foil, the majority of the space in an
atom is vacant.
All of an atom’s mass is located in the nucleus, which is the positively charged center
of the atom. One thousand eight hundred particles bombarded the nucleus before
deflecting.
The route on which the electrons orbit the center is known.
The nucleus is tiny compared to the size of the entire atom.
Rutherford made these hypotheses via an experiment involving the scattering of alpha ()-
particles on gold foil.
Drawbacks of the Model
Rutherford offered a brand-new model for the atom’s structure, but it had several flaws that
he did not address, including the following:
The electrons accelerate as they spin in an unsteady direction, emitting energy. The
electrons expend energy as they spin. The nucleus would soon contain all of the
electrons. While the atom is relatively stable, this inclination would make it
exceedingly unstable.
Rutherford’s theory of the structure of an atom could only account for the existence of
protons in the nucleus, not the idea of atomic number.

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Bohr’s Model
To address the issues Rutherford’s model presented, Bohr created a model.
So, he stated the following postulates:
Only a limited number of electron orbitals are allowed in an atom, which is how the
atom’s exterior structure is formed.
The negatively charged particles in these orbitals or energy levels do not lose energy
while rotating.
A shift in magnitude occurs as the electron moves from one energy shell to another.
Bohr’s model addresses the problems that all prior atomic structure models had and
provides a detailed description of atomic structure.
Mass Number (A)
The mass number depicts the “Total number of protons and neutrons” in an atom’s nucleus.
The notation (A) depicts the Mass number. The notation (N) depicts the total number of
Neutrons.
Or A = Z+N
Mass Number is also referred to as Nucleon number.
Atomic Number (Z)

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The number of protons in the nucleus of an atom, which determines its atomic number, is
present in every atom of an element. The atom has the same amount of protons and
electrons since it is electrically neutral
A = Z+N
The atomic number is represented by the letter Z. For example, since hydrogen only
contains one proton, it has an atomic number of one.
You will benefit from understanding these key components of the chapter structure of an
atom:
In an atom,
NCERT Solution for Class 9 Science, Chapter 4: Structure of the atom
FAQs (Frequently Asked Questions)
State the limitations of J.J Thomson’s model of the atom.
According to this theory, the positively charged spheres have electrons embedded all over
them. However, research has revealed that electrons are dispersed all around an atom’s
nucleus while protons are only found in the center of an atom.
State the limitations of Rutherford’s model of the atom.
In accordance with this theory, the electrons circle the nucleus in a circular pattern. Any
such particle would experience acceleration and release energy as it swung around the
nucleus. The atom would be extremely unstable since the rotating electron would eventually
lose all of its energy and fall into the nucleus. The atoms are quite stable, despite this.
State the properties of electrons, protons, and neutrons.
In the case of Electrons,
Properties Weight Location Affinity Charge
Electrons Negligible
mass
Outside
Nucleus
Attracts positively
charged
Negative
(-)

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g ( )
In the case of Protons,
Properties Weight Location Affinity Charge
Protons 1 a. m. u Within Nucleus Attracts negatively charged Positive (+)
In the case of Neutrons,
Properties Weight Location Affinity Charge
Neutrons 1 a. m.
u
Inside
Nucleus
Neither positive nor negative
charge
Positive
(+)
Check out Worksheet on Magnetism and Electromagnetism
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