a ppt on different models of atom

2. • In the early 1900’s many scientist had turned their
attention to investigate the structure of atom .
Many models were proposed and a handful have
been adopted as ways to describe the atom.
Neither of them are perfect but they have bought us
a long way towards understanding these building
blocks. These are –
• The Rutherford Model
• The Bohr Model
• The Dalton’s Model
• The Thomson’s Model
• The Quantum Mechanical Model

3. The Rutherford ModelThe Rutherford Model
 In 1909 Ernest RutherfordIn 1909 Ernest Rutherford
conduct what is now a famousconduct what is now a famous
experiment where he bombardedexperiment where he bombarded
gold foil with alpha particlesgold foil with alpha particles
(Helium nuclei) . A source which(Helium nuclei) . A source which
undergoes alpha decay is placedundergoes alpha decay is placed
in a lead box With a small hole inin a lead box With a small hole in
it .Ant of the alpha particlesit .Ant of the alpha particles
which hit the inside of the boxwhich hit the inside of the box
are simply stopped by the boxare simply stopped by the box
.Only those which pass through.Only those which pass through
the opening are allowed to escape,the opening are allowed to escape,
and they follow a straight line toand they follow a straight line to
the gold foil .the gold foil .

4.  Observations –Observations –
 Most of the alpha particlesMost of the alpha particles
pass straight through thepass straight through the
gold foil.gold foil.
 Some of the alpha particlesSome of the alpha particles
get deflected by very smallget deflected by very small
amount.amount.
 A very few get deflectedA very few get deflected
greatly.greatly.
 Even fewer get bounced ofEven fewer get bounced of
the foil and back to the left.the foil and back to the left.

5.  Conclusions –Conclusions –
 The atom is 99.99% emptyThe atom is 99.99% empty
space.space.
 The nucleus contains aThe nucleus contains a
positive charge and most ofpositive charge and most of
the mass of atom .the mass of atom .
 The nucleus isThe nucleus is
approximately 100,000approximately 100,000
times smaller then thetimes smaller then the
atom .atom .

6.  The deflection of alpha particles through large angles and evenThe deflection of alpha particles through large angles and even
bouncing off the gold foil is best described by the animationbouncing off the gold foil is best described by the animation
above. Keep in mind that the gold nuclei have a charge of +79above. Keep in mind that the gold nuclei have a charge of +79
and the alpha particle has a charge of +2. these two positiveand the alpha particle has a charge of +2. these two positive
charges repel each other. The closer they get, the greater thecharges repel each other. The closer they get, the greater the
force. The greater the force the greater the amount offorce. The greater the force the greater the amount of
deflection.deflection.

7. The Bohr Model
 While The Rutherford Model focused on describing the nucleus , Niels Bohr
turned his attention to describing the electron. Prior to the Bohr Model, the
accepted model was one which depicted the electron as an orbiting planet.
 The flaw with the planet like model is that an electron particle moving in the
circular path would be accelerating . An accelerating electron creates a
changing magnetic . The changing magnetic field would carry energy away
from the electron , eventually slowing it down and allowing it to be captured
by nucleus

8.  Bohr built upon spectroscopic
observations of atoms.
Spectroscopists noticed that an
atom can only absorb certain
energies (colors) of light (the
absorption spectrum) and once
excited can only release certain
energies (the emission spectrum)
and these energies happen to be
the same. Bhor uses these
observations to argue the energy
of a bound electron is “quantized”.
Quantized is a fancy word mean
only certain quantities of energy
are allow. This explanation
addresses the true origin of light.
Science only certain energy levels
are allow it is actually possible to
diagram the atom in the terms of
its energy level.

9.  In the animation below You will see a hydrogen atom .In the animation you will notice
that if the energy of the photon of light is just right , it will cause the electron to jump
to a higher level.
 When the electrons jumps back down, a photon is created for each jump down. A
photon without the right amount of energy (the pink one) passes through the atom
with no effect. Photons with too much energy will cause the electron to be ejected
with ionizes the atom. A ionized electron is said to be the n = infinity energy level .
Keep in mind that these rings are not actually orbits, but are levels that represent the
electron wave .The number n corresponds to the number of complete waves of
electron.

10. The Dalton’s ModelThe Dalton’s Model
Daltons theory was based on the premise that the atoms of the different elements could be distinguished byDaltons theory was based on the premise that the atoms of the different elements could be distinguished by
difference in their weight . He stated his theory in a lecture to the royal institution in 1803 . The theorydifference in their weight . He stated his theory in a lecture to the royal institution in 1803 . The theory
proposed a number of basic ideas :proposed a number of basic ideas :
 All matter is composed of atoms.All matter is composed of atoms.
 Atoms cannot be made or can destroyed.Atoms cannot be made or can destroyed.
 All atoms of the same element are identical.All atoms of the same element are identical.
 Different elements have different types of atom.Different elements have different types of atom.
Chemical reactions are occur When atoms are rearranged.Chemical reactions are occur When atoms are rearranged.
Compounds are formed from atoms of the constituent elements.Compounds are formed from atoms of the constituent elements.

11.  Using his theory, Dalton rationalized the various lawsUsing his theory, Dalton rationalized the various laws
of chemical combination Which were in existence atof chemical combination Which were in existence at
that time .However, he made a mistake in assumingthat time .However, he made a mistake in assuming
that the simplest compound of the two elements mustthat the simplest compound of the two elements must
binary, formed from atoms of each element in a 1:1binary, formed from atoms of each element in a 1:1
ratio, and his system of atomic weights was not veryratio, and his system of atomic weights was not very
accurate , he gave oxygen an atomic weight of sevenaccurate , he gave oxygen an atomic weight of seven
instead of eight. Despite these errors , Dalton's theoryinstead of eight. Despite these errors , Dalton's theory
proved a logical explanation of concepts.proved a logical explanation of concepts.

12. The Thomson's ModelThe Thomson's Model
 An atom is electrically neutral .ItAn atom is electrically neutral .It
contains positive charges (due tocontains positive charges (due to
protons) and negative chargesprotons) and negative charges
(due to electrons). Hence J.J.(due to electrons). Hence J.J.
Thomson assumed that an atom isThomson assumed that an atom is
a uniform sphere of positivea uniform sphere of positive
charges with electrons embeddedcharges with electrons embedded
into it. Thus, an atom may beinto it. Thus, an atom may be
visualized as a pudding or cake ofvisualized as a pudding or cake of
positive charges with raisinspositive charges with raisins
(electrons) embedded into it .(electrons) embedded into it .
Hence this is also known asHence this is also known as
“Raisin-Pudding Model”“Raisin-Pudding Model”
 This model explains the electricalThis model explains the electrical
neutrality of atom ,but it failed toneutrality of atom ,but it failed to
stand for a long time as it was notstand for a long time as it was not
consistent with the experimentalconsistent with the experimental
results obtained by Rutherford.results obtained by Rutherford.

13. The Quantum Mechanical ModelThe Quantum Mechanical Model
• As we learn that the Bhor model establishes the concept of definite
electron energy levels within atoms . But Bhor’s model was rather
simplistic and as scientist made more discoveries about more
complex atoms , Bhor’s model was modified and eventually was
replaced by more sophisticated models .
• The Quantum Mechanical Model of the atom presents a more
accurate model of the atom. It us a more sophisticated model based
on complex mathematical calculations and interpretations .

14. • The quantum model introduces the concept of -
• Sublevels : According to the calculations in
quantum mechanics each energy level of an
atom consists of one or more sublevels The
principle energy level ,n, determines the number
of sublevels in the principle energy level.
• n = The number of sublevels in the nth principal
energy level
• Atomic orbits: Each sublevel has one or more
atomic orbit with a specific 3-diminsional shape.