Transcript ATOMIC STRUCTURE AND BONDING

ATOMIC STRUCTURE
R.SANGEETHA
VSMHS SCHOOL,
CHENNAI
ATOMS AND SUBATOMIC
PARTICLES
An atom is very small. Its mass is between 10-21 and 1023g. A row of 107 atoms (10,000,000 atoms) extends
only 1.0 mm.
The subatomic particles are:
1.proton(+ve charge)
2.electron(neutral)
3.electron(-ve charge)
Properties of the proton, neutron, and electron
PARTICLE
ACTUAL RELATIVE RELATIVE
CHARGE
MASS (g) MASS
(amu)
PROTON
1.6726 X 10-24
1.007
+1
NEUTRON
1.6749 X 10-24
1.008
0
ELECTRON
9.108 X 10-28
5.45 X 10-4
-1
A. Atomic Number Equals Electrons or
Protons
• Atomic number= number of protons per atom=
number of electrons per neutral atom
B. Mass Number Equals Protons plus
Neutrons
Mass number = number of protons + number of neutrons
Thomson’s Model Of Atom
• Thomson in 1904 put forward a model of atom according
to which, an atom consists of a sphere of positive charge
in which the negatively charged electrons are embedded
like the seeds in water melon. the model was rejected as
it could not explain Rutherford’s scattering experiment
Embedded
electrons
Diffused positive
charge
Rutherford's Experiment
• In 1904 Rutherford investigated the radioactive
rays with the help of strong electric and
magnetic fields and classified them into three
kinds Alpha(α),Beta(β) and gamma(γ).
• Alpha particles were identified with helium atoms
that have a mass of 4 and the charge on them
as 2+. Rutherford performed many experiments
in radioactivity. Scattering of alpha particles is
one such experiment led to a further
understanding of atomic structure.
Results of Rutherford’s Experiment
1.There is a lot of hollow space in the atom.
2.The central part of the atom called nucleus, is positively
charged.
3.The deflection of positively charged α particles is due to
repulsion by the positively charged nucleus. The space
occupied by the central part i.e. nucleus of the atom is
much less than the much larger space in which the
electrons revolve around the nucleus in closed orbits.
4.The nucleus is heavy and dense.
5.Since the nucleus is very small only a few α particles get
deflected. these α particles get scattered through large
angles by the nucleus because of its large mass and
positive charge.
Bohr’s atom model
• To overcome the objections raised against
Rutherford’s model of an atom, Neils Bohr in
1913 put forward a new theory of atomic
structure known as Bohr’s theory. The
important postulates are:
1.Atom consists of a small but massive
nucleus situated at the centre with the
electrons revolving around it in closed
circular orbits.
2. The electrons in an atom revolve around the
nucleus in definite circular paths known as
energy levels. the energy of the electron
remains constant as it moves in the same
orbit. This implies that each orbit is
associated with a certain definite amount of
energy.
3.The different energy levels or shells are also
called as stationary states designated by
K,L,M,N etc. starting from the nucleus as
shown in figure or with integers 1,2,3,4 etc.
The energies of different energy shells follow
the sequence 1<2<3<4 etc. or K<L<M<N etc.
4.Energy is emitted or absorbed only when an
electron moves from one energy level to
another. The amount of energy thus emitted
or emitted is equal to the difference of
energies of the two energy levels concerned.
Since the various energy levels have certain
definite or discrete quantities called quanta or
photons.
Rules for filling up of electrons in
the atom
1.Pauli’s exclusion principle
“No two electrons in an atom can have all the
four quantum numbers same.”
2.Hund’s rule of maximum multiplicity
This rule states that “electron pairing will
take place only when all the available sub
shells have at least one electron each.”
3.Aufbau principle
“This principle states that electrons enter in
to orbital in the increasing order of energy.”