Atomic Structure
Grade -XI
Mind Map
CA Standards
Students know how to relate the position of
an element in the periodic table to its
atomic number and atomic mass.
Students know the nucleus of the atom is
much smaller than the atom yet contains
most of its mass.
Modern Atomic Theory
❖ All matter is composed of atoms
❖ Atoms cannot be subdivided, created, or
destroyed in ordinary chemical reactions.
However, these changes CAN occur in
nuclear reactions!
❖Atoms of an element have a characteristic
average mass which is unique to that
element.
❖Atoms of any one element differ in
properties from atoms of another element
Discovery of the Electron
In 1897, J.J. Thomson used a cathode ray tube
to deduce the presence of a negatively charged
particle.
Cathode ray tubes pass electricity through a gas
that is contained at a very low pressure.
An electron was discovered by cathode ray discharge tubes
experiment.
A cathode ray tube is made of glass containing two thin pieces of
metal called electrodes, sealed in it. The electrical discharge through
the gases could be observed only at very low pressures and at very
high voltages . The pressure of different gases could be adjusted by
evacuation. When sufficiently high voltage is applied across the
electrodes , the current starts to flow through a stream of particles
moving in the tube from the negative electrode to the positive one .
These rays were called the cathode rays or cathode ray particles
.
The flow of current from cathode to anode was further checked by
making a hole in the anode and coasting the tube behind anode with
phosphorescent material called zinc sulphide coating , a bright spot
on the coating is developed .
Results of the Experiment
• The cathode ray starts from cathode and moves towards
•
•
•
•
•
the anode.
Their behavior can be observed with the help of
fluorescent or phosphorescent (which glow when hit by
the).
In the absence of electric or magnetic field these rays
travel in straight line.
In the presence of electric or magnetic field the behavior of
cathode rays are similar to negatively charged particles.
So cathode ray consist negative charged particle, called
electron.
The characteristics of cathode rays do not depend on
material of electrode or nature of gas present in tube.
Thus we can conclude that electrons are basic constituent
of all the atoms.
CHARGE TO MASS RATIO OF ELECTRON
•
8
Conclusions from the
Study of the Electron
❑ Cathode rays have identical properties
regardless of the element used to produce
them. All elements must contain identically
charged electrons.
❑Atoms are neutral, so there must be
positive particles in the atom to balance
the negative charge of the electrons
❑ Electrons have so little mass that atoms
must contain other particles that account
for most of the mass
Discovery of protons
• Electrical discharge carried out in the modified cathode rays
tube led to the discovery of particles carrying positive charge
also known as canal rays.
• The characteristics of these rays are:
⇒ Unlike cathode rays, the positively charges particles depend
upon the nature of gas present in the cathode ray tube.These
gases are simple positively charged ions.
⇒ The charge to mass ratio of the particles is found to depend on
the gas from which they originate.
⇒ Some of the +vely charged particles carry a multiple of the
fundamental unit of electrical charge.
⇒ The behaviour of these particles in the magnetic or electrical
field is opposite to that observed for electron or cathode rays.
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• The smallest and lightest positive ion was obtained from
hydrogen and was called the proton.
• Discovery of neutrons: Chadwick felt that by
bombarding a thin sheet of beryllium by α-particles. When
electrically neutral particles having a mass slightly greater
than that of the protons was emitted. He named these
neutral particles as neutrons. Thus this discovery Was a
very important discovery in the history of chemistry.
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Thomson’s Atomic Model
Thomson believed that the electrons were like
plums embedded in a positively charged
“pudding,” thus it was called the “plum pudding”
model.
An important failure of this model is mass of
the atom is uniformly distributed.
Rutherford’s Gold Foil Experiment
❑ Alpha (α) particles are helium nuclei
❑ Particles were fired at a thin sheet of gold foil
❑ Particle hits on the detecting screen (film) are
recorded
Rutherford’s Findings
❑ Most of the particles passed right through
❑ A few particles were deflected
❑ VERY FEW were greatly deflected
“Like howitzer shells bouncing off of
tissue paper!”
Conclusions
:
❑ The nucleus is small
❑ The nucleus is dense
❑ The nucleus is positively charged
❏ The nucleus is surrounded by electrons that
move around the nucleus with very high
speed in circular paths called orbits like solar
system.
❏ Electrons and nucleus are held together by
electrostatic force.
Drawbacks of Rutherford model
Atomic Particles
Particle
Charge Mass/ u
Location
Electron
-1
0
Electron
cloud
Proton
+1
1
Nucleus
Neutron
0
1
Nucleus
Atomic Number
Atomic number (Z) of an element
is the number of protons in the
nucleus of each atom of that
element.
Element
# of protons
Atomic # (Z)
6
6
Phosphorus
15
15
Gold
79
79
Carbon
Mass Number
Mass number is the number of protons and
neutrons in the nucleus of an isotope.
+ ++ n
0
0
Mass
#
=
p
Nuclide
p
n
Oxygen -
1
Arsenic -87
Phosphorus 5
-
8
33
31
15
10
42
1
6
e-
8
3
3
1
5
Mass #
1
8
7
5
3
1
Isotopes
Isotopes are atoms of the same element having
different masses due to varying numbers of
neutrons.
Isotope
Protons
Electrons
Neutrons
Hydrogen–1
1
1
0
1
1
1
1
1
2
(protium)
Hydrogen-2
(deuterium)
Hydrogen-3
(tritium)
Nucleus
Atomic Masses
Atomic mass is the average of all the
naturally occurring isotopes of that element.
Isotope
Carbon-12
Symbol
Composition of
the nucleus
% in nature
12C
6 protons
98.89%
6 neutrons
Carbon-13
13C
6 protons
1.11%
7 neutrons
Carbon-14
14C
6 protons
<0.01%
8 neutrons
Carbon = 12.011
The Electromagnetic Spectrum
• In 1870 Maxwell proposed that light
and other form of radiant energy
propagate through space in the form
of waves.
• Visible light is a small portion of the
electromagnetic radiation spectrum
detected by our eyes.
• Electromagnetic radiation includes
radio waves, microwaves and X-rays.
• Described as a wave traveling through
•
space.
There are two components to
electromagnetic radiation, an electric field
and magnetic field.
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The Wave Nature of Light
• Wavelength, , is the distance
between two corresponding
points on a wave.
• Amplitude is the size or “height”
of a wave.
• Frequency, , is the number of
cycles of the wave passing a given
point per second, usually
expressed in Hz.
The Wave Nature of Light
•
The fourth variable of light is velocity.
• All light has the same speed in a vacuum.
• c = 2.99792458 x 108 m/s
• The product of the frequency and wavelength is the speed of
light.
c  
• Frequency is inversely proportional to wavelength.
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The Wave Nature of Light
• The arrangement of different types of electromagnetic
radiations in order of increasing wavelengths or decreasing
frequencies is called Electromagnetic spectrum.
• Electromagnetic radiation can be categorized in terms of
wavelength or frequency.
• Visible light is a small portion of the entire electromagnetic spectrum.
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Planck’s Quantum Theory
• Higher T = shorter λ (higher E) maximum.
• Couldn’t explain with classical physics
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Planck’s Quantum Theory
•
•
In 1900 Max Planck studied black body radiation and realized
that to explain the energy spectrum he had to assume that:
1. energy is quantized
2. light has particle character
Planck’s equation is
E  h  or E 
hc

h  Planck’ s constant  6.626 x 10-34 J  s
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The Particulate Nature of Light
• Photoelectric effect: light striking a metal surface
generates photoelectrons.
• The light’s energy is transferred to electrons in metal.
• With sufficient energy, electrons “break free” of the
metal.
• Electrons given more energy move faster (have higher
kinetic energy) when they leave the metal.
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The Particulate Nature of Light
•
•
The photoelectric effect is not explained using a wave description but is
explained by modeling light as a particle.
Wave-particle duality - depending on the situation, light is best described
as a wave or a particle.
•
Light is best described as a particle when light is imparting energy to
another object.
•
Particles of light are called photons.
•
Neither waves nor particles provide an accurate description of all
the properties of light. Use the model that best describes the
properties being examined.
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The Particulate Nature of Light
• The energy of a photon (E) is proportional to the
frequency ().
• and is inversely proportional to the wavelength ().
• h = Planck’s constant = 6.626 x 10-34 J s
E  h 
hc

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The Particulate Nature of Light
•
Binding Energy - energy holding an electron to a metal.
•
Threshold frequency, o - minimum frequency of light needed to
emit an electron.
•
For frequencies below the threshold frequency, no electrons are
emitted.
•
For frequencies above the threshold frequency, extra energy is
imparted to the electrons as kinetic energy.
•
•
Ephoton = Binding E + Kinetic E
This explains the photoelectric effect.
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Atomic Spectra and the Bohr Atom
• Every element has a unique spectrum.
• Thus we can use spectra to identify elements.
• This can be done in the lab, stars, fireworks, etc.
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The Bohr Atom
•
Bohr model - electrons orbit the
nucleus in stable orbits. Although
not a completely accurate model, it
can be used to explain absorption
and emission.
• Electrons move from low
energy to higher energy orbits
by absorbing energy.
• Electrons move from high
energy to lower energy orbits
by emitting energy.
• Lower energy orbits are closer
to the nucleus due to
electrostatics.
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The Bohr Atom
• Ground state: the lowest state (orbital)where the
electrons are initially present is called the ground
state.
• Excited state: the state where the electrons on
gaining energy I subjected to go to a higher energy
level is called the excited state.
Atoms return to the ground state by emitting energy as light.
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Atomic Spectra and the Bohr Atom
• The Rydberg equation is
an empirical equation that
relates the wavelengths of
the lines in the hydrogen
spectrum.
 1
1 
 R  2  2 

 n1 n 2 
R is the Rydberg constant
1
R  1.097  107 m -1
n1  n 2
n’ s refer to the numbers
of the energy levels in the
emission spectrum of hydrogen
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Atomic Spectra and
the
Bohr
Atom
n  4 and n  2
Example 4-8. What is the
2
wavelength of light emitted 1 
when the hydrogen atom’s 
energy changes from n = 4 1 

to n = 2?
1
 1
1 
R 2  2 
 n1 n 2 
 1 1 
1.097  107 m -1  2  2 
2 4 
1
1 1 
 1.097  107 m -1   

 4 16 
1
 1.097  107 m -1  0.250  0.0625 

1

1

 1.097  107 m -1  0.1875 
 2.057  106 m -1
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