SCH3UB
Sept12
Early History and Development of Atomic Theories
John DALTON (1805)
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Atom is a solid sphere
Matter composed of indivisible particles (atoms)
Atoms of any one element are identical to each other, different from any other element’s atoms
JJ THOMSON (1897)
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 Atoms consist of negatively charged particles called electrons embedded into positive material
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Ernest RUTHERFORD (1911) ______________________________________
 Gold foil experiment: fired positively charged alpha particles at a thin gold foil sheet; most
passed straight through the foil, some were deflected at angles as they passed through and
others were reflected straight back or at an angle
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http://www.kentchemistry.com/links/AtomicStructure/RutherfordTutorial.htm
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Most of the mass of an atom is concentrated in a tiny nucleus of positively charged protons
and neutral neutrons both of which have approx the same mass
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SCH3UB
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Sept12
Most of the atom is empty space around the nucleus occupied by very small negative
electrons with negligible mass
Planetary model of the atom:
 Neutral atoms have an equal number of + and – charges
 Atomic number = Z = # of protons
 Mass number = A = # protons + # neutrons
Problems with Rutherford Model: an orbiting electron should continuously emit electromagnetic
radiation (ie light) because orbiting bodies are continually accelerating; this means electrons would be
losing energy and should collapse towards nucleus, but they don’t;
Bohr Atomic Theory
Niels BOHR (1913)
Used evidence from spectroscopic analysis of different elements to deduce that electrons in an atom
exist at various definite energy levels [ this is why excited electrons of a particular element always
produce the same bright-line or emission spectrum which is characteristic of the element and different
elements produce different coloured flames].
www.green-planet-solar-energy.com
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Electrons travel in the atom in circular orbits with “quantized” energy ie, only certain quantities
of energy are possessed by the electrons
There is a maximum # of electrons in each orbit: 2, 8, 18, etc
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SCH3UB
Sept12
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An electron can be excited to a higher
energy level (further from the nucleus)
through the addition of electrical or heat
energy
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When an electron falls from an excited
state back to ground state, light is emitted
of a certain wavelength corresponding to
the energy difference between excited and
ground state
Skullsinthestars.com
Assessment statement
Describe the electromagnetic
spectrum.
Students should be able to identify the ultraviolet, visible and infrared
regions, and to describe the variation in wavelength, frequency and energy
across the spectrum.
Distinguish between a continuous
spectrum and a line spectrum.
Continuous Spectrum:………………………………………………………………………………….
………………………………………………………………………………………………………….
………………………………………………………………………………………………………….
Line Spectrum:…………………………………………………………………………………………
………………………………………………………………………………………………………….
………………………………………………………………………………………………………….
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SCH3UB
Sept12
Emission Line Spectrum
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Each element emits a different characteristic emission spectrum of coloured light that
corresponds to the different quantities of energy possessed by its electrons
These different colours of light / energies correspond to differences between the energy levels
present in different atoms
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classes.mhcc.edu
No two elements have the same atomic emission spectrum; the atomic emission
spectrum of an element is like a fingerprint
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SCH3UB
Sept12
EMISSION SPECTRUM of HYDROGEN
2.3.3 Explain how the lines in the
emission spectrum of hydrogen are
related to electron energy levels.
Students should be able to draw an energy level diagram, show transitions
between different energy levels and recognize that the lines in a line
spectrum are directly related to these differences.
An understanding of convergence is expected. Series should be
considered in the ultraviolet, visible and infrared regions of the spectrum.
Calculations, knowledge of quantum numbers and historical references will
not be assessed.
Aim 7: Interactive simulations modelling the behaviour of electrons in the
hydrogen atom can be used.
When an electric current is passed through a glass tube that contains hydrogen gas at low pressure the
tube gives off blue light. When this light is passed through a prism (as shown in the figure below), four
narrow bands of bright light are observed against a black background.
www.horrorseek.com
uwec.edu
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SCH3UB
Sept12
2.3.4 Deduce the electron arrangement for atoms and ions up to Z = 20
a) Use the handout provided of the first 20 elements on the periodic table to draw the Bohr
diagrams and then write the simple electron arrangement for each neutral element.
b) Write the ion that could be formed and the simple electron arrangement for each ion that
could be formed by the first 20 elements. What patterns do you see?
He
2
Ne
2,8
Na+
2, 8
Ar
2,8,8
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SCH3UB
Sept12
Test Your Understanding:
1.
State the electron arrangements of the following species:
Si ........................................................................................................................................
P
2.
3–
(a)
............................................................................................................................. ...........
List the following types of electromagnetic radiation in order of increasing wavelength (shortest first).
I.
Yellow light
II.
Red light
III. Infrared radiation
IV. Ultraviolet radiation
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(1)
(b)
Distinguish between a continuous spectrum and a line spectrum.
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(1)
(c)
The thinning of the ozone layer increases the amount of UV-B radiation that reaches the Earth’s
surface.
Type of Radiation
Wavelength / nm
UV-A
320–380
UV-B
290–320
Based on the information in the table above explain why UV-B rays are more dangerous than UV-A.
.....................................................................................................................................
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.....................................................................................................................................
4.
Which statement is correct about a line emission spectrum?
A.
Electrons absorb energy as they move from low to high energy levels.
B.
Electrons absorb energy as they move from high to low energy levels.
C.
Electrons release energy as they move from low to high energy levels.
D.
Electrons release energy as they move from high to low energy levels.
5.
In the emission spectrum of hydrogen, which electronic transition would produce a line in the visible region
of the electromagnetic spectrum?
A.
n = 2 →n = 1
C.
n=2→n=3
B.
n=3→n=2
D.
n=∞→n=1
6.
What is the electron arrangement of the Mg
A.
2,2
B.
2,8
2+
ion?
C.
D.
2,8,2
2,8,8
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SCH3UB
Sept12
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