The document discusses the structure of atoms and their composition. It explains that atoms are composed of a tiny, dense nucleus containing protons and neutrons, surrounded by electrons in defined orbits. Niels Bohr revised earlier atomic models by suggesting that electrons occupy specific orbits and can jump between these orbits by absorbing or emitting energy. The arrangement of electrons in an atom is called its electronic configuration. Atoms of the same element can have different numbers of neutrons, forming isotopes with the same chemical properties but different physical properties. Isotopes have applications including use in nuclear power and medical treatments.
2. ILLUSTRATIONS OF THE COMPOSITION OF THE
HUMAN BODY
HUMAN
BODY
SYSTEMS
ORGANS
TISSUES
CELLS
3. HISTORICAL BACKGROUND OF atom’S
STRUCTURE
In approximately 430B.C., Democritus coined the
term atomos, which means “uncuttable” or “the
smallest indivisible particle of matter”.
In 1803 A.D John Dalton put forth his celebrated
atomic theory.According to him,
• Matter is made of small particles called atoms.
• Atoms cannot be created, divided into smaller
particles, nor destroyed in the chemical processes.
4. THOMSON’S THEORY OF ATOM
John Thomson in 1867 developed the idea that
atoms are made of negative electrons embedded in
a gel of positive charge (A Plum Pudding).
5. RUTHERFORD’S SCATTERING
EXPERIMENT
Rutherford overtuned Thomson’s atom model in
1911 with his well-known gold foil experiment in
which he demonstrated that the atom has a tiny,
massive nucleus.
Rutherford designed an experiment to use alpha
particles emitted by a radioactive element as
probes to the unseen world ofatomic structure.
6. Using these observations he formulated the theory
of an atom.According to this theory:
• An atom has a tiny, dense, positively charged core
called a nucleus, in which nearly all the mass is
concentrated.
• The light, negative constituents, called electrons,
revolve around the nucleus in paths called
orbits.This is much like a solar system where planets
revolve around the sun.
7. Rutherford’s model had certain
limitations.It could not eplain how the
moving electron would remain in its
orbit, especially when it is was a charged
particle and therefore it would accelerate
due to its movement, finally moving
closer to the nucleus and drop into it.The
atom would not be stable which in turn
would mean that matter would not be
composed of unstable atoms.
8. NIELS BOHR’S ATOMIC MODEL
Nobel Prize winner, Niels Bohr
in 1922, revised Rutherford’s
model by suggesting that:
• The electrons were confined into
clearly defined orbits.
• They could jump between these orbits, but could
not freely spiral inward or outward in intermediate
states.
• An electron must absorb or emit specific amounts
of energy for transition between these fixed orbits.
9. DISCOVERY OF NEUTRON
James Chadwick in 1932 discovered another
particle inside the atom.It had no charge and its
mass was almost equal to the mass of the proton.It
was present in the nucleus along with the
proton.He named it as the ‘Neutron’.
10. DISTRIBUTION OF ELECTRONS IN ORBITS
Let us now see how these electrons are
organised.According to Bohr, electrons move in the
stationary orbits (also called shells) which are
associated with a certain amount of energy.
11. The orbit closest to the nucleus is given the number
‘1’, the second orbit the number ‘2’ and so on.They
are also named as K, L, M, N,…….. Corresponding to
the shell numbers, like n=1, 2, 3, 4,……The number
of maximum electrons in each orbit is given by a
formula 2n2 .The following table gives the maximum
number of electrons in each orbit.
12. ELECTRONIC CONFIGURATION OF
ELEMENTS
The arrangement of electrons of each element in
their orbits or shells is known as its electronic
configuration.
You have seen
that K, L, M, N,……
Shells occupy 2, 8,
18, 32,… electrons
respectively.
13. Every electron has a fixed amount of
energy depending on which orbit it is
present in.Electrons in the first orbit
i.e. K have minimum energy.
Electrons in the
subsequent orbit
posses higher
energy.
Electrons in an atom
occupy shells in increasing
order of energies.
15. ATOMIC NUMBER
Atomic number (Z) is the number of protons
present in an atom.
An atom of an element is electrically neutral.The
number of protons and electrons are equal.
16. ATOMIC MASS NUMBER
Atomic masses are measured in a unit called
Dalton (u).A Dalton is approximately equal to
1 atomic mass unit (amu) which is equal to 1.6
x 10-27 Kg.
The mass of a proton, neutron and electron is
approximately 1 u, 1 u and 0.0055 u
respectively.So you can realize how less the
mass of electron is a s compared to the
masses of proton and neutron.
17. Hence, while calculating mass of an atom of an
element, we only add up the mass of proton and
neutron.The atomic Mass of an element is equal to
the sum of the number of protons and neutrons
present in the nucleus of the element.It is called the
mass number and denoted by symbol A.
The atomic number and mass number of an
element can be represented symbolically as:
For e.g:11Na.
Z
A
Symbol of
the element
23
18. ISOTOPES AND THEIR APPLICATION
In nature it is found that many elements have
atoms with the same atomic number but different
atomic mass number.
Such atoms of the same elements are called
isotopes.Many elements have isotopes and a
mixture of them is present in the element.
e.g: Carbon exists in two isotopic form 12C and 14C.
The number of neutrons in
isotopes is different.The
chemical properties of
isotopes are the same
though some physical
properties may different.
19. USES OF ISOTOPES
Isotopes are widely used for different purposes:-
• Uranium -235 (235U) is used
for nuclear fission.
• Isotopes of some elements are
used in the medical treatment
of deadly diseases like cancer.
• Iodine (131I) is used in the
treatment of goitre, a disease
of the thyroid glands.
Goitre
20. ISOBARSThere are a number of different elements whose
atomic mass numbers are same.Such atoms are
called Isobars.
For Example:
