4.1 Development of a New Atomic Model
Summary
TLDRThis lecture discusses the development of a new atomic model, addressing the limitations of Rutherford's model in explaining the behavior of the nucleus and electrons. It introduces the dual nature of light, behaving as both a wave and a particle, and explores the photoelectric effect, where light interacts with metal to emit electrons. Key concepts include Max Planck’s quantum theory, the relationship between energy and frequency, and Einstein’s photon theory. The lecture also introduces Bohr's model of the atom, focusing on electron orbits, energy states, and the hydrogen atom’s line emission spectrum.
Takeaways
- 🔬 Rutherford's atomic model was incomplete as it didn't explain why the nucleus didn't fly apart despite having positive charges.
- ⚛️ The model also couldn't explain why negatively charged electrons didn't fall into the positively charged nucleus.
- 💡 The development of a new atomic model began with experiments involving light, revealing its wave-particle duality.
- 🌈 Light is a form of electromagnetic radiation, and only a small portion of the electromagnetic spectrum is visible to the human eye.
- 🌊 Two key properties of light waves are wavelength (Lambda) and frequency, which are inversely related to maintain the constant speed of light (C = 3 x 10^8 m/s).
- 📡 The photoelectric effect demonstrated that light must reach a certain frequency before electrons are emitted from a metal surface.
- 📏 Max Planck proposed that energy is emitted in discrete packets called quanta, relating energy to frequency with the formula E = hf.
- ✨ Einstein further developed this by describing light as photons, showing that atoms absorb whole photons, not fractions.
- 🔬 The hydrogen atom's emission spectrum showed that excited atoms release specific wavelengths of light when returning to their ground state.
- 🔭 Niels Bohr proposed a model where electrons move in stable orbits, and only certain energy levels are allowed, explaining why hydrogen emitted discrete spectral lines.
Q & A
Why did scientists need to develop a new atomic model?
-Scientists needed a new atomic model because Rutherford's model was incomplete. It didn't explain why the positively charged nucleus didn't fly apart due to the repulsion of like charges, nor did it explain why negatively charged electrons didn't fall into the nucleus due to their attraction to the positive charge.
What is the significance of white light in the development of the atomic model?
-White light is significant because it led to the understanding that light behaves as both a particle and a wave. This dual nature helped scientists develop new ideas about atomic structure and the behavior of electrons.
What is the electromagnetic spectrum and how does it relate to light?
-The electromagnetic spectrum is a range of all possible frequencies of electromagnetic radiation, including radio waves, visible light, X-rays, and gamma rays. The light we see is a small part of this spectrum, and understanding its properties helped scientists understand atomic behavior.
What are the two basic properties of waves mentioned in the script?
-The two basic properties of waves mentioned are wavelength (Lambda), which is the distance between two crests or troughs in the wave, and frequency, which is how many crests pass by every second.
How is the speed of light related to wavelength and frequency?
-The speed of light (C) is related to wavelength (Lambda) and frequency (F) by the equation C = Lambda * F. This equation shows that wavelength and frequency are inversely related, meaning if one increases, the other must decrease to maintain the constant speed of light.
What is the photoelectric effect and how does it challenge the continuous wave theory of light?
-The photoelectric effect is the emission of electrons from a material when light shines upon it. It challenges the continuous wave theory because electrons are only emitted at a certain minimum frequency, and not at lower frequencies regardless of light intensity, suggesting that light must be quantized rather than continuous.
Who proposed the idea that light is emitted in packets called quanta, and how did this relate to the photoelectric effect?
-Max Planck proposed the idea that light is emitted in packets called quanta. This concept related to the photoelectric effect by explaining that only certain frequencies (and thus certain quanta of energy) could cause electrons to be emitted from a material.
What is Planck's constant and how does it relate to the energy of quanta?
-Planck's constant (H) is a fundamental physical constant that relates the energy (E) of a quantum to its frequency (F) by the equation E = H * F. It is approximately 6.626 x 10^-34 joule seconds and is crucial for understanding the energy carried by photons.
How did Einstein's explanation of the photoelectric effect build upon Planck's idea of quanta?
-Einstein explained that light consists of particles called photons and that atoms and molecules can only absorb whole numbers of photons. This meant that there was a minimum energy required to eject an electron, which explained the observed threshold in the photoelectric effect and built upon Planck's quantized concept of energy.
What are energy states in the context of atomic physics?
-Energy states are levels of energy that an atom can have, which include both kinetic and potential energy. The ground state is the lowest energy state, and higher states are called excited states.
How did the study of hydrogen's line emission spectrum contribute to atomic theory?
-The study of hydrogen's line emission spectrum showed that light was emitted at specific wavelengths when hydrogen atoms returned to their ground state from an excited state. This indicated that atoms have quantized energy levels and that electrons can only occupy certain stable orbits, which was a key part of the development of quantum theory.
Outlines
🔬 Understanding Rutherford's Model and the Need for a New Atomic Model
The first paragraph discusses the limitations of Rutherford's atomic model. It explains that Rutherford's model couldn't explain why the positively charged nucleus didn't fly apart and why negatively charged electrons didn’t fall into the nucleus due to electrostatic attraction. To address these issues, scientists developed a new model. Experiments with light revealed that light behaves both as a particle and a wave. The light we see is a form of electromagnetic radiation, which includes radio waves, UV rays, X-rays, and gamma rays. The paragraph introduces key concepts such as the electromagnetic spectrum, wavelengths, and frequency, leading to the constant speed of light (C), which ties the wavelength and frequency together.
💡 Max Planck and the Introduction of Quanta
The second paragraph introduces Max Planck's work on quantizing electromagnetic radiation. It explains how Planck proposed that energy is emitted in discrete packets called 'quanta,' rather than continuously. Planck's equation (E = hf) relates energy to frequency, with Planck's constant (h) as a key factor. The importance of units in Planck's constant is emphasized, as it ties frequency to energy in Joules. This concept of quanta paved the way for understanding how atoms absorb and emit energy in fixed amounts, which is essential in explaining phenomena like the transparency of glass.
🌟 Einstein and the Photon Theory of Light
This paragraph expands on Planck’s quantum theory with Einstein’s idea of photons. Einstein proposed that light consists of discrete particles called photons, which atoms can absorb or emit in whole numbers, not fractions. This concept explains the photoelectric effect, where light above a certain frequency causes metal to emit electrons. The idea of light as a continuous wave was refuted, showing that specific frequencies and intensities of light are necessary to knock electrons loose. This theory further clarified the threshold behavior observed in the photoelectric effect.
🚀 Exciting Atoms and the Hydrogen Emission Spectrum
The fourth paragraph discusses how scientists explored the hydrogen atom’s behavior when excited by electricity. When hydrogen gas is excited, it emits a pink light, which, when split, reveals four specific colors or wavelengths. These wavelengths represent the light emitted as excited electrons return to their lower energy or ground states. The paragraph introduces the Bohr model, which suggests that electrons move in stable orbits around the nucleus. Electrons move to higher orbits when they absorb photons and return to lower orbits by emitting photons, corresponding to specific energy levels.
🔭 Bohr's Atomic Model and Quantum Energy States
The final paragraph elaborates on Bohr's atomic model and the concept of energy states in atoms. It explains that electrons in an atom can exist in either a ground state (the lowest energy state) or excited states (higher energy levels). Electrons move between these states by absorbing or emitting photons. The Bohr model proposes that electrons orbit the nucleus in fixed, stable paths, much like rungs on a ladder. This model helps explain why hydrogen emits specific wavelengths of light when electrons transition between these energy levels. However, this model works primarily for hydrogen and has limitations for more complex atoms.
Mindmap
Keywords
💡Rutherford's Model
💡Electromagnetic Spectrum
💡Wavelength (λ)
💡Frequency (f)
💡Photoelectric Effect
💡Quanta
💡Planck’s Constant
💡Photon
💡Bohr Model
💡Ground State and Excited State
Highlights
Rutherford's atomic model couldn't explain why the nucleus, composed of positive charges, doesn't fly apart, and why electrons don't fall into the nucleus despite their attraction.
The study of light led to the development of a new atomic model, as light behaves both as a particle and a wave.
Electromagnetic radiation includes various types, such as radio waves, UV rays, X-rays, and gamma rays, with visible light being just a small part of the spectrum.
Wavelength (lambda) is the distance between crests or troughs, and frequency is the number of crests passing per second; they are inversely related, maintaining a constant speed of light (C).
The photoelectric effect occurs when light shines on a metal, emitting electrons only at a certain minimum frequency, with more intensity producing more electrons.
Max Planck proposed that electromagnetic radiation is emitted in packets called quanta, not continuously, and related energy to frequency with the equation E = HF.
Planck's constant, 6.626 x 10^-34, is key in calculating the energy of photons based on frequency.
The quantum theory explains that atoms and molecules can only absorb or lose whole numbers of photons, leading to a minimum energy threshold required to knock electrons out of place.
Einstein further built on quantum theory by explaining that light consists of photon particles, and atoms must absorb entire photons for excitation to occur.
The photoelectric effect revealed that light must have a minimum frequency to eject electrons, debunking the idea that any continuous light wave could build up energy over time.
The hydrogen atom's line emission spectrum showed discrete wavelengths when excited, indicating that energy is released in specific quantities as the atom returns to its ground state.
Bohr's model of the atom introduced the idea of stable orbits where electrons reside, with electrons jumping between orbits by absorbing or emitting photons.
Bohr proposed that electrons can only exist in specific energy levels or orbits, like the rungs of a ladder, and can't occupy in-between states.
Bohr's model successfully explained the hydrogen atom's spectrum but was limited in its application to more complex atoms.
The energy differences between electron orbits correspond to the wavelengths of light emitted or absorbed, providing the basis for the quantum model of the atom.
Transcripts
So today we're discussing chapter 4
section 1 development of a new atomic
model and this comes about because
Rutherford's model which we previously
discussed was incomplete it didn't
explain why the nucleus which we'll
represent by that ball here uh didn't
just fly apart because if you'll
remember the nucleus is made up of a
bunch of positive charges and if you
have like charges that are really close
to each other they should try to fly
apart but they don't they would have sit
there in the middle and it also didn't
explain why the electrons that are in
the area around the nucleus which are
negatively
charged why these electrons which are
points on here don't just fall into the
nucleus due to their attraction to the
positive charge so they had to develop a
new model and after some experiments
with light they came up with some new
ideas so as they looked into the
properties of white light they figured
out that it could be behave as both a
particle and a
wave now what you have to understand is
that the light we see is a form of
what's known as electromagnetic
radiation now so are radio waves UV
rays uh xrays gamma rays there's all
kinds of different electromagnetic
radiation and we actually see a tiny
part of what's known as the
electromagnetic spectrum and I'll give
you an example right here now you can't
see the labels on it but radio waves and
stuff are down here and it goes through
the visual light is about
here and then it goes all the way
through to what are known as gamma rays
on the end and here are some examples of
the size gamma rays have a wavelength
which is the distance between two
troughs represented by the Greek letter
Lambda
about the size of a
nucleus and there's two properties of
waves two basic properties at least
there is the wavelength Lambda which is
the distance between
two crests or troughs in the wave as
well as what's known as the frequency
and that is how many of these crests
pass by every second now if you take
these two numbers Lambda which is the
distance
between crests and the
frequency which is how many pass per
second and you multiply
them what you end up finding is that it
equals a constant C and this C is a very
important number this is the speed of
light which
is which comes out to 3 * 10 8
m/s approximately
now because C always comes out to be
this number it means that Lambda and F
the wavelength and frequency are
inversely related meaning that if one
goes up the other has to go down in
order to maintain this constant over
here so now that we've looked at uh
light as a wave we're going to look at
the basis for light as a particle and
that comes from a effect in nature known
as the photo electric effect and
basically what the photo El electric
effect is is if you shine light onto a
piece of metal like
this then what you'll get is at a
certain minimum
frequency the metal will start
emitting electrons now before this
frequency if it's anything below this
frequency you won't get get any
electrons at all no matter how much
light you're shining on it however once
you get above this
frequency you'll
start getting electrons shooting off and
the more intense the light the more
electrons you'll get however to increase
the speed of the electrons what you need
to do is increase the frequency and
people were confused about this
threshold down here if you will because
if light were a continuous wave then
what it would do is it would bombard the
electrons and get them moving faster and
faster and faster continuously until
eventually they could escape the atom so
a solution to this phenomenon was
initially proposed by a German Scientist
by the name of Max Plank and what plank
said was that hot objects and things
that emit uh electromagnetic waves like
this uh don't emit them continuously but
rather they released as sort of packets
that still have a
wavelength and he called these packets
quanta now plank was able to relate the
energy of these quanta to their
frequency by the equation eal HF where e
is the energy in Jews H is what is known
as Plank's constant now Plank's constant
is 6 * 6. 626 rather time
10 to the
-34 so an absolutely T minuscule
number Jewel seconds and frequency
is seconds to the 1 now it's important
to use the units on Plank's constant
because if you'll notice you need the S
to cancel out with this S1 and then
you're left with the jewels to give give
you the energy and this is how he was
able to relate frequency to energy and
still maintain a particle idea of light
now building off his equation eal
HF he proposed a new idea which is
something called the quantum which is
basically the
minimum energy that a atom or
molecule can absorb
or uh
lose now this interestingly enough is
the reason why glass a pane of glass is
see-through is
because the photons of visual light
which remember have a frequency because
they are electromagnetic
radiation don't have a high enough
frequency though to have the energy
which equals the Quantum of of the glass
basically they don't carry enough energy
to excite an atom so what they end up
doing is they just pass straight
through now Einstein further built upon
this idea by explaining that light is a
wavelike stream of
particles called
photons and he said that atoms and
molecules could only absorb whole
numbers of photons so you can't have
half a photon you have to absorb the
whole packet if you're going to take the
thing you can't divide it in half like
that and so as you can see if you have
to absorb whole numbers of photons what
you end up happening what you end up
doing is limiting the amount of energy
you can take in at one time if you're
not getting a continuous beam of or a
continuous ray of light constantly
bombarding an
electron then what happens is the
electron quickly absorbs and then
re-emits it'll absorb it and then
quickly Reit the
photon but if you were to have a
continuous wave as many people proposed
what this would do is it would
constantly bombard the electron and any
frequency of light would be able to push
it off if you just had enough intensity
and shined it at the electron for long
enough but Einstein
building on Plank's idea of
quanta explained using photons
that you have to absorb whole
numbers therefore there's a minimum
energy required based on this frequency
here to build up enough momentum to
knock an electron out of place and this
in effect explained the previous mystery
behind the photoelectric effect so now
we're going to be studying the hydrogen
atom line emission spectrum which is a
fancy way of saying uh we're
studying the wavelengths of light that
hydrogen emits when it's excited now
we'll get into all the technical
details about what all those terms mean
but first thing you have to know is that
atoms have
something called Energy
States and what these energy states are
is a measure of the
kinetic energy or the temperature
basically how fast an atom is moving as
well as the potential energy of the atom
and the lowest State an atom can be
in is something called the ground state
and for these explanations I'm going to
use the simple bore model of the
atom which we'll discuss
later however you do need to know that
the lowest energy
state of an atom is called the ground
state and then anything that is higher
energy than the ground state is
called the excited
state now how did people found find this
out so what they did was they decided to
excite the atoms basically what they did
was they
passed an electric current through
hydrogen
gas and what they found was that this
hydrogen gas then emitted a pink
light but when they broke up this light
they found out that it was not a
continuous Spectrum rather it was made
of only four colors in the visible
spectrum and these colors correspond to
the different
wavelengths of Light
released by the hydrogen atoms after
being excited by this electric current
going back to their ground
state so now we'll get back over to
these diagrams I have on the left which
are what are known as bore models of
atoms and they're named after a man
named Neil's
bore and
bore proposed that electrons went around
the atom in stable circular or
elliptical orbits like I have drawn
here and he proposed also that the
lowest energy state was when this
electron was close here which is why I
have the ground state with the electron
very close to the nucleus with a larger
distance in the excited
state and basically he proposed that
electrons could move from this ground
state to the higher state by
absorbing a photon of light and what
this would do it is it would impart more
energy energy into the electron and move
it higher up however he also said there
were only a few select stable orbits
corresponding to these various lines
found in the
Spectrum sort of like the rungs on a
ladder so you can
go from the Bottom
Rung up one two or three but you can't
be on rung 1.5
basically you can increase your
gravitational energy by going up at
whole numbers to stable rungs if you
will corresponding to the stable orbits
and the method for moving between these
rungs or orbits was the absorbing and
reemission of
photons given by the energy difference
between the orbits eal HF so you can see
if there's only a few select orbits
you're only going to get a few select
frequencies when you change the energy
which is why they only got four lines
when they measured the spectrum of
excited hydrogen however further
Advanced
applications came to realize that this
model only works for
hydrogen
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