An entry in the 'schools for you' project. By Aneesh Bapat, class 8 from Abhinav Vidyalaya English Medium High School, Pune, India.About the various theories by different scientists about the structure of the atom.
The ancient Arabs invented the science of
This was the precursor to modern chemistry.
In alchemy, it was believed that everything was
made up of the five ‘elements’- water, earth,
fire, air and aether.
This theory, which was accepted in Europe for
centuries was in contradiction of the atomic
theory and is now known to be false.
But although it was widely accepted, a very
small group of people had in fact proposed the
3. Ancient Atomic Theories
In ancient Greece, Democritus(400BC) and
Lucretius (50 BC) had suspected that
everything was made of atoms.
Lucretius has backed up his atomic theory in
his book De Rerum Natura.
But his ‘atom’ was closer to the modern
molecule than atom.
He claimed that there was a lot of space
between atoms; that was what allowed fluids to
change shape and metals to be molded into
4. The Ancient Indian Atomic Theory
Maharshi Kanad was once walking with his
food, splitting it into morsels, when it occurred
to him that at some tiniest size, it would be
impossible to split it into further smaller bits.
This laid the foundation of his atomic theory,
where atoms were called Peelavs or smallest of
the small particles.
But both ancient theories were absurd in some
ways, for example stating that atoms of metals
were rigidly joined together by hooks or that
chilli tasted hot because its atoms were spiky.
5. About John Dalton
John Dalton was an
He is regarded as the
father of the Atomic theory
because he put forth the
first complete atomic
This is because his theory
completely explained two
discovered laws: The law of
conservation of mass and
the law of definite
6. Dalton’s Theory
Dalton’s theory Consisted of the following
All matter is made of atoms
Atoms are indivisible and cannot be
created or destroyed.
Atoms of the given element are identical
in all aspects.
Atoms combine in fixed ratios of whole
numbers to form compounds.
7. Prout’s Hypothesis
At the time when Dalton’s atomic model
was prevalent, William Prout made a
hypothesis based on his observations
stating that only hydrogen was the
fundamental particle or protyle and all
other atoms were made out of jointed
This theory, although it was incorrect,
influenced the name of the proton.
8. About JJ Thomson
The Full name of JJ
Thomson was Sir Joseph
He was an English physicist
who got the Nobel Prize for
his discovery of the electron
in the year 1906.
He also discovered the
positively charged and
heavier than electron alpha
particle which we now know
to be a helium nucleus.
9. The Big Experiment
When J.J. Thomson was
working with a cathode ray tube,
he observed blips (blinking white
spots) on the screen.
When he held the tube between
one positively charged and one
negatively charged metal plate,
the blips deflected towards the
This proved the existence of tiny
negatively charged particles we
now call electrons.
10. Thomson’s Model
After discovering the electron,
Thomson revised Dalton’s theory
and put forth his new theory.
In this theory, the atom was a
positively charged sphere with
electrons floating in it.
As this model looks like a pudding
(atom) with plums (electrons)
floating in it, it was called the plum
11. About Rutherford
Ernest Rutherford was born in New
Zealand but lived most of his life in UK.
He did path breaking work in the field
of radioactivity and correctly identified
and named alpha and beta forms of
radiation. He also discovered the
proton or positively charged subatomic
But he is most commonly known for
his atomic theory …
12. The Gold Foil Experiment
In order to test Thomson’s Atomic
theory, Ernest Rutherford performed an
experiment with gold foil which was
used because it can be made into a
super thin layer.
He bombarded the gold foil with the
positive alpha particles.
Thomson had predicted that all would
pass through. Most of the alpha particles
did pass through indicating that an atom
is mostly empty space.
But very few were deflected or bounced
back, indicating that an atom had a
positive nucleus.Thus, Thomson was
13. Rutherford’s Model
Rutherford Based his model on the
deductions of the gold foil
He thought that an atom has a
positively charged nucleus with
negatively charged electrons
revolving around it.
An atom is mostly empty space – If
an atom were the size of a soccer
field, the nucleus would be only as
large as the ball.
Rutherford’s model is also called the
14. Limitations of Rutherford’s Model
One major shortcoming in Rutherford’s
model is that the electron should
describe a spiral before collapsing into
the nucleus. But this does not happen.
Another major drawback is that it does
not say anything about the actual
positions of the electrons.
15. The Third Component of an Atom
The third component of an atom, the
neutron was discovered in 1932 by
James Chadwick was a student of
Ernest Rutherford and he got the nobel
prize for this discovery.
The actual construction of the
experiment is complicated, but it is
similar to the gold foil experiment.
16. Niels Bohr
Niels Bohr was a famous Danish atomic
physicist most famous for his atomic
Now for an unconventional introduction, his
university had failed him for answering the
question ‘how to find out the height of a
building using a barometer’ saying that ‘by
taking a long enough string, going to the
top of the building dangling down the
barometer and adding the length of string
used and the height of the barometer.’
When he protested, his headmaster asked
him to answer a question on which his
passing would depend. He gave four or
five methods of answering the question!
17. Bohr’s Model
Niels Bohr improved the planetary model of the
atom by saying that only orbits of certain radii
were allowed. An electron cannot exist between
two adjacent allowed orbits.
The radius of the orbit was dependant on the
energy level of the electron it contains; closer
orbits have higher energy electrons.
An electron must gain or lose energy as
required to jump from one orbital to the other
and a non-jumping electron does not give out
This model was satisfactory in most ways but
could only explain the hydrogen atom.
18. The Shell Model
After Bohr proposed his model, he revised it
with the help of Erwin Schrodinger , Werner
Heisenburg and Max Born.
This model says that electrons are located in
spherical regions called shells around the
nucleus, similar to Bohr’s orbits.
The four common shells are called K,L,M,N
where K is the innermost and N the outermost.
These shells can hold 2, 8, 18 and 32 electrons
respectively from innermost to outermost.
The formula for finding out the number of
electrons for the nth
shell is 2n2
20. Properties of Components of
The proton, neutron and electron are the
components of the atom.
An electron has unit negative charge and
A proton has unit positive charge and
mass nearly equal to a hydrogen atom
(1proton +1electron) .This known as 1
atomic mass unit (1 amu).
A neutron also has a mass of 1 amu but
lacks any charge. It is electrically neutral
and so, called a neutron.
21. Atomic Number and Mass
The atomic number is the number of protons in
an atom. This is also equivalent to the number
of electrons in that atom. It is represented by
the letter Z.It affects the chemical properties of
The atomic mass number is counted in amu’s
and is equal to the total number of protons and
neutron in an atom. It is represented by letter A.
It affects the physical properties of the element.
An isotope is an atom of the same
element, but with a different atomic mass
It has the same chemical properties and
the same atomic number.
The below diagram shows the isotopes of
Hydrogen.( The black balls are neutrons.)
23. Isobars and Isotones
Isobars are different elements having an equal
atomic mass (protons +neutrons)
Eg: Calcium and argon both have an atomic
mass of 40 atomic mass units.
Isotones are also necessarily different elements
(Can even be isotopes) but having the same
number of neutrons.
E.g.: chlorine 37 and potassium 39
(The number indicates the atomic mass of the
element which is an isotope.)
24. Why an Atom Holds Together
Due to complicated interactions between
quarks of protons and neutrons, (Quarks are
particles that make up protons and neutrons.) a
strong Adhesive force is generated.
This force, called the nuclear force, is stronger
than the electric repulsive force of the protons
and hence the atom is held together.
So we see that it is the neutron which is
responsible for holding the atom together.