The discovery of subatomic particles of an Atom. Its Nuceus and the model of an Atom

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DISCOVERY OF
SUBATOMIIC PARTICLES
&
ATOMIC MODEL

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INTRODUCTION
 The Atom is a basic unit of matter that consists of a
dense, central nucleus surrounded by a cloud of
negatively charged electrons.
 The Indians were the first to make an attempt towards the
discovery of atom in the early 6th century BCE. The
Bhagvad Gita mentions ‘anu’ or “anor” which is sanskrit
for ‘atom’. But the main philosophy of most of the
philosophers such as Dharmakriti was that matter is
made up of 4 elements- fire, earth, water and air.
 In the 5th Century BC, Leucippus and Democritus
proposed that all matter was composed of small invisible
particles called “atoms”.
 For a long time, it was believed that atoms are the
ultimate particles that matter is made up of and that these
atoms cannot be divided further. The experiments
conducted during the latter half of the nineteenth century
and early years of the twentieth century revealed that the
atom is not the ultimate particle. The continued efforts of
the scientists led to the discovery of subatomic
particles. Democritus Leucippus
Democritus Leucippus

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Discovery of Subatomic Particles
 The advances in atomic structure and quantum mechanics have
led to the discovery of other fundamental particles. The discovery
of subatomic particles has been the base for many other
discoveries and inventions.
 The atomic structure of an element refers to the constitution of its
nucleus and the arrangement of the electrons around it. Primarily,
the atomic structure of matter is made up of protons, electrons and
neutrons.
 The protons and neutrons make up the nucleus of the atom,
which is surrounded by the electrons belonging to the atom.
The atomic number of an element describes the total number of
protons in its nucleus.
 Neutral atoms have equal numbers of protons and electrons.
However, atoms may gain or lose electrons in order to increase
their stability and the resulting charged entity is called an ion.
 Atoms of different elements have different atomic structures
because they contain different numbers of protons and electrons.
This is the reason for the unique characteristics of different
elements.

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Atomic Model
 In the 18th and 19th centuries, many scientists attempted to
explain the structure of the atom with the help of atomic models.
Each of these models had their own merits and demerits and were
pivotal to the development of the modern atomic model. The
most notable contributions to the field were by the scientists John
Dalton, J.J. Thomson, Ernest Rutherford, Niels Bohr & James
Chadwick.
Dalton’s Atomic Theory:
 The idea of atomic theory was revived by John Dalton (1766-1844)
2,000 years after Democritus first proposed about it.
 In 1800, John Dalton performed a series of experiments showing
the matter consists of lumpy particles called “atoms”.
 His Findings led to the formulation of what is commonly known as
“Dalton’s Atomic Theory”.
 Dalton formulated his theory based on Antoine Lavoiser’s (1743-
1794) law of conservation of mass and Joseph Proust’s (1754-
1826) law of definite proportion.
 According to the postulates proposed by Dalton, the atomic
structure comprised atoms, the smallest particle responsible for
the chemical reactions to occur.
John Dalton

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Dalton’s Atomic Model & Postulates
 The English chemist John Dalton suggested that all matter is
made up of atoms, which were indivisible and indestructible. He
also stated that all the atoms of an element were exactly the
same, but the atoms of different elements differ in size and mass.
The following are the postulates of his theory:
 Every matter is made up of atoms.
 Atoms are indivisible.
 Specific elements have only one type of atoms in them.
 Each atom has its own constant mass that varies from element to
element.
 Atoms undergo rearrangement during a chemical reaction.
 Atoms can neither be created nor be destroyed but can be
transformed from one form to another.
 Dalton’s atomic theory successfully explained the Laws of
chemical reactions, namely, the Law of conservation of mass, Law
of constant properties, Law of multiple proportions and Law of
reciprocal proportions.
 However, during his time, (1800s), some scientists still did not
believe that there was an indivisible atom found in all Matter.

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Thomson Atomic Model
 The English chemist Sir Joseph John Thomson put forth his
model describing the atomic structure in the early 1900s.
 He was later awarded the Nobel prize for the discovery of
“electrons”. His work is based on an experiment called cathode
ray experiment. The construction of working of the experiment is
as follows:
Conclusions:
Based on conclusions from his cathode ray experiment, Thomson
described the atomic structure as a positively charged sphere into
which negatively charged electrons were embedded.
 It is commonly referred to as the “plum pudding model” because
it can be visualized as a plum pudding dish where the pudding
describes the positively charged atom and the plum pieces
describe the electrons.
 Thomson’s atomic structure described atoms as electrically
neutral, i.e. the positive and the negative charges were of equal
magnitude.
Limitations of Thomson’s Atomic Structure: Thomson’s atomic
model does not clearly explain the stability of an atom.
 With all this evidence, Thompson concluded that cathode rays are
made of negatively charged particles called “electrons”.
J.J.THOMSON

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Rutherford Atomic Model
 Rutherford, a student of J. J. Thomson modified the atomic
structure with the discovery of another subatomic particle called
“Nucleus”. His atomic model is based on the “Alpha ray
scattering experiment.”
 It was Ernest Rutherford (1871-1937) who coined the term
“proton” for the positively charged particle in an atom
Rutherford’s Structure of Atom:
 Based on the experiment and conclusions, Rutherford proposed
his own atomic structure which is as follows.
 The nucleus is at the center of an atom, where most of the charge
and mass are concentrated.
 Atomic structure is spherical.
 Electrons revolve around the nucleus in a circular orbit, similar to
the way planets orbit the sun.
 The electrons move around the nucleus, the protons found inside
the nucleus are positively charged.
 The number of protons is equal to the number of electrons.
Later he postulated the existence of a neutral particle in the
nucleus to make up for, he calculated mass efficiency in the atom
studied.
Rutherford

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Niels Bohr & James Chadwick Model
Niels Bohr’s Planetary Model
 The electrons move around the nucleus in fixed orbit & electron in
a particular orbit has constant energy.
 An electron can absorb energy and move to a higher energy orbit
of larger radius.
 An excited electron can fall back to its original orbit by emitting
energy as radiation.
James Chadwick’s Model
 The “Neutron” was discovered by James Chadwick in the year
1932.
 James atomic model is known as the quantum mechanical
model of the atom. In 1932, James Chadwick bombarded
beryllium atoms with alpha particles. An unknown radiation was
produced. Chadwick interpreted this radiation as being composed
of particles with a neutral electrical charge and the approximate
mass of a proton.
 According to him, the nucleus of an atom contains
“Neutrons”,electrically neutral particles with a mass similar to that
of proton.
The continued efforts of the scientists led to the discovery of
subatomic particles. The three primary subatomic particles that
constitute an atom are Electron, Proton & Neutron.
Niels Bohr
J.Chadwick

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Electron, Proton & Neutron
Electrons
Protons
Proton:
 Protons are positively charged subatomic particles. The charge of
a proton is 1e, which corresponds to approximately 1.602 × 10-19.
 The mass of a proton is approximately 1.672 × 10-24
 Protons are over 1800 times heavier than electrons.
 The total number of protons in the atoms of an element is always
equal to the atomic number of the element
 The discovery of protons is credited to Ernest Rutherford.
Neutron:
 The mass of a neutron is almost the same as that of a proton i.e.
1.674×10-24.
 Neutrons are electrically neutral particles and carry no charge.
 Different isotopes of an element have the same number of protons
but vary in the number of neutrons present in their respective
nuclei.
 The neutron was discovered by James Chadwick .
Electron:
 The charge of an electron is -1e, which approximates to -1.602 ×
10-19.
 The mass of an electron is approximately 9.1 × 10-31.
 Due to the relatively negligible mass of electrons, they are ignored
when calculating the mass of an atom.
 J.J.Thompson is credited with the discovery of electrons
Proton
Electrons

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Nucleus of an Atom
Electrons
Nucleus:
 An atom nucleus is made up of protons and neutrons. Protons
bear a positive charge equal to the electrons orbiting around, and
the neutrons bear no charge but weigh the same as the protons.
 The atom nucleus creates the force required to keep the
atom together and in order.
 A nucleus is identified as in the example below by its atomic
number Z (i.e., the number of protons), the neutron number, N,
and the mass number, A, where A = Z + N. The convention for
designating nuclei is by atomic number, Z, and mass number, A,
as well as its chemical symbol.
Summary:
 The nucleus is a small, dense region at the center of the atom.
 Consisting of positive protons and neutral neutrons, the nucleus
has an overall positive charge.
 The nucleus contains virtually all of the atom's mass.
 Strong nuclear force holds together protons and neutrons in the
nucleus, and overcomes the electric force of repulsion between
protons.

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Thank You
Bibliography :
1) https://www.britannica.com/science/atomic-model
2) https://en.wikipedia.org/wiki/Rutherford_model
3) https://www2.lbl.gov/abc/wallchart/chapters/02/2.html -Nucleus
4) https://www.livescience.com/37206-atom-definition.html
5) https://edu.glogster.com/glog/james-chadwick/23jfieeaktw?=glogpedia-source
6) https://byjus.com/chemistry/rutherfords-model-of-atoms-and-its-limitations/
7) https://byjus.com/jee/atomic-structure/
Acknowledgement:
I wish to express my deep gratitude and sincere thanks to the Principal, Mrs Sudha Menon,
North Point School for her encouragement and facilities that she provided for this project work.
I extend my hearty thanks to Mrs.Aditi Neb Chemistry teacher, who guided me to the
successful completion of this project. I take this opportunity to express my deep sense of
gratitude for her invaluable guidance, constant encouragement, immense motivation, which
has sustained my efforts at all the stages of this Project work…
I can’t forget to offer my sincere thanks to parents and also to my sister who helped me to
carry out this project work successful and for their valuable advice and support, which I
received from them time to time….