• History of atom
• What is an atom?
• Structure of atom
• Nucleus of atom
• Thomson’s model of
• Rutherford’s model of
• Bohr’s model of an
• What this particles
consists of ?
• Atomic structure
HISTORY OF THE
460 BC Democritus develops
the idea of atoms
he pounded up materials in
his pestle and mortar until
he had reduced them to
smaller and smaller
particles which he called
He suggested that all matter was made up of
tiny spheres that were able to bounce around
with perfect elasticity and called them ATOM
Atom, tiny basic building block of matter. All the
material on Earth is composed of various
combinations of atoms. An atom consists of a
cloud of electrons surrounding a small, dense
nucleus of protons and neutrons.
Atoms are the smallest particles of a chemical
element that still exhibit all the chemical
properties unique to that element. A row of 100
million atoms would be only about a centimetre
ATOMS MADE VISIBLE
Individual atoms of the element silicon can be seen in this image
obtained through the use of a scanning transmission electron
microscope. The atoms in each pair are less than a ten-millionth of a
millimeter (less than a hundred-millionth of an inch) apart.
Atoms are made of smaller particles, called
electrons, protons, and neutrons. An atom
consists of a cloud of electrons surrounding a
small, dense nucleus of protons and
neutrons. Electrons and protons have a
property called electric charge, which affects
the way they interact with each other and
with other electrically charged particles
An atom consists of a cloud of electrons
surrounding a small, dense nucleus of
protons and neutrons.
The nucleus contains nearly all of the mass
of the atom, but it occupies only a tiny
fraction of the space inside the atom. The
diameter of a typical nucleus is only about
1 × 10-14 m (4 × 10-13 in), or about
1/100,000 of the diameter of the entire
Electrons (e-) were discovered by sir. J.J.
Thomson.Electrons are tiny, negatively charged
particles around the nucleus of an atom. Each electron
carries a single fundamental unit of negative electric
charge–1.602 x 10-19 coulomb and has a mass of 9.109 x
10-31 kg. The electron is one of the lightest particles
with a known mass. Electrons cannot be split into
anything smaller, also electrons do not
have any real size, but are instead true
points in space-that is, an electron
has a radius of zero.
Proton (p+) was discovered by E.Goldstein.
Proton has 1 unit mass. Proton Protons have
a positive electrical charge of 1.602 x 10-19
coulomb. This charge is equal but opposite
to the negative charge of the electron. A
proton’s mass is about 1,840 times the mass
of an electron. Protons carry a positive
charge of +1, exactly the opposite electric
charge as electrons. The number of protons
in the nucleus determines the total quantity
of positive charge in the atom.
Neutron (n) was discovered by Sir James
Chadwick. The neutron is slightly heavier
than a proton and 1,838 times as heavy as
the electron. Neutron, electrically neutral
elementary particle that is part of the
nucleus of the atom. The neutron is about
10-13 cm in diameter and weighs
1.6749 x 10-27 kg.
According to Sir Joseph model of
an atom, it consists of a positively
charged here and the electrons are
embedded in it. The negative and
the positive charges are equal in
magnitude, as a result the atom is
neutral. Thomson proposed that
the atom of an atom to be similar
to that of a Christmas pudding
or a watermelon
An atom consists of a
positively charged center in
the atom called the nucleus.
The mass of the atom is
contributed mainly by the
nucleus. The size of the
nucleus is very small as
compared to the size of the
atom. The electrons revolve
around the nucleus in well-
Bohr agreed with almost all
points as said by Rutherford
except regarding the revolution
of electrons for which he added
that there are only certain
orbits known as discrete orbits
inside the atom in which
electrons revolve around the
nucleus. While revolving in its
discrete orbits the electrons do
not radiate energy.
Hydrogen atom Hydrogen atom
the number of protons in an
the number of protons and
neutrons in an atom
4 Atomic mass
Number Of Electrons = Number Of Protons
Electrons are arranged in Energy Levels or Shells around
the nucleus of an atom.
• first shell a maximum of 2 electrons
• second shell a maximum of 8 electrons
• third shell a maximum of 8 electrons
Isotope, one of two or more species of atom
having the same atomic number, hence
constituting the same element, but differing in
mass number. The nucleus, and mass number is
the sum total of the protons plus the neutrons
in the nucleus, isotopes of the same element
differ from one another only in the number of
neutrons in their nuclei.
The average mass of naturally occurring copper
atoms is equal to the sum of the atomic mass for
each isotope multiplied by its isotopic abundance.
For copper, it would be
(62.930 amu x 0.692) + (64.928 amu x 0.308)
= 63.545 amu.
The atomic weight of copper is therefore 63.545 g.
The total number of nucleons is the
same in the atoms of this pair of
elements. Atoms of different
elements with different atomic
numbers, which have the same mass
number, are known as isobars.