Transcript Atomic Structure

Chapter 2
Atoms and Elements
Electron Arrangement and
Periodic Law
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What had we learned so far?
• Atomic Structure –
– Nucleus
– Electrons
• Essential question: how are those
electrons surrounding nucleus
arranged?
Characteristics of Electrons
 Extremely small mass
 Located outside the nucleus
 Moving at extremely high speeds in a
sphere
 Have specific energy levels
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Energy of Electrons
 When atoms are heated, bright lines appear
called line spectra
 Electrons in atoms arranged in discrete
levels.
 An electron absorbs energy to “jump” to a
higher energy level.
 When an electron falls to a lower energy
level, energy is emitted.
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Experimental Evidence
Discharging Tubes
The Flame Test
The light coming out
of the excited atomic
entities is very specific
to particular element!
Results are quite
reproducible.
Experimental Evidence
There has been no
radioactive decay going
on. Hence the nucleus
does not change when
the atomic entity gets
excited either by
electricity or heat.
So the colored
light must have
come from those
electrons.
Light emitted from excited atomic entities is the
tool used to probe how electrons are arranged.
Loss and Gain of Energy
L
o
s
s
G
a
I
n
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What is light?
Lights, both visible and invisible to human
eyes, are electromagnetic waves.
Time Out!
Before we go
any further, what
is a Wave?
What is a wave?
A wave is a means to transfer energy from point A to
point B.
Waves in
water
Sound
Typical mechanicalwaves
waves such as
those in water and sound waves
DO need medium in which they
propagate. Water and air are the
prerequisites for waves to travel.
Waves – in more the abstract
form
Wavelength - distance from crest to crest
abbreviated Greek letter, l, pronounced “lambda”.
Also can be defined as how far the wave travels in a
cycle.
Note:
great link to an online simulation of waves.
http://phet.colorado.edu/sims/wave-on-a-string/wave-
Waves – in more the abstract form
Frequency – the number
of complete waves
passing any given point
per second.
• SI unit for frequency is
Hertz (Hz), or
cycles/sec.
• Abbreviated Greek
letter, n, pronounced
The graph shows that the top “nu”.
wave
passes any given point 4 complete
wave forms every second; the
middle one 2 complete wave
forms; and the bottom one 1
complete wave form.
High
Energy
Low
Energy
Radio Micro Infrared
waves waves .
Ultra- XGamma
violet Rays Rays
Low
Frequency
Long
Wavelength
Visible Light
High
Frequency
Short
Wavelength
---------------- > decreasing energy ---------------------
----------------> decreasing frequency ---------------->
---------------> increasing wavelength ---------------->
Types of Spectra
• Continuous – all wavelengths within a
given range are included.
• Electromagnetic – all electromagnetic
radiation arranged according to
increasing or decreasing wavelength.
a.unit for wavelength ranges from meters to
nanometers
b.unit for frequency is hertz (Hz) (# waves
per second)
Types of Spectra
• Visible spectrum - light you can see
(ROY-G-BIV)
a. Red has the longest wavelength and the
smallest frequency.
b. Violet has the shortest wavelength and the
greatest frequency.
• Bright Line spectrum (emission
spectrum)
Bands of colored light emitted by excited
electrons when they return to the ground
state.
Passing Light Through a
Prism
• White light is
made up of all
the colors of the
visible spectrum.
• Passing it
through a prism
separates the
colors in white
light.
If the light is not white,
• By heating a gas
with electricity we
can get it to give
off colors.
• Passing this light
through a prism
does something
different.
If the light is not white,
• Each element
gives off its own
characteristic
colors.
• Can be used to
identify the atom.
• This is how we
know what stars
are made of.
Spectroscopy
1. Emission spectra of a substance is
studied to determine its identity.
2. Spectroscope – instrument that
separates light into a spectrum.
3. Spectral lines – represent wavelength of
light emitted when excited electrons fall
back to the ground state.
How Does a Spectroscope Work?
Emission Spectrum (Line Spectrum)
Emission Spectrum
Do you get it?
Answer with:
1) Energy absorbed
2) Energy emitted
3) No change in energy
A. What energy change takes place when an
electron in a hydrogen atom moves from the
first (n=1) to the second shell (n=2)?
B. What energy change takes place when the
electron moves from the third shell to the
second shell?
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Answers
A.
1) Energy absorbed
B.
2) Energy emitted
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Bohr Model
• First model of the electron structure
• Gives levels where an electron is most likely
to be found
• Incorrect today, but a key in understanding
the atom
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