Models of the AtomModels of the Atom

Ernest Rutherford proposed an atom with a nucleus and negative particles moving around the nucleus.

The next major step forward on the model of the atom came after some important breakthroughs in our understanding of the electromagnetic spectrum

light behaves like a wave, but also like a stream of extremely tiny, fast-moving particles (wave-particle duality)

light is a form of electromagnetic radiation

Electromagnetic radiation can be described in terms of…Amplitude: height of wave from origin to crest

Electromagnetic radiation can be described in terms of…Wavelength (): distance between successive crests

(lambda)

Electromagnetic radiation can be described in terms of… (nu) Frequency: ( ) how fast the wave oscillates. The number of waves per second.* High frequency high energy

Speed of light (c): 3.00 x 108 meters per second

c=v

If is low, then v must

be small since c is constant; the reverse is also true.

Long light is low energyShort is high energy

The visible spectrum of light is just part of the electromagnetic spectrum - the part detectable by the human eyeVisible light has a wavelength ranging from around 400-800 nm, with different colors each falling somewhere in that range.

The energy absorbed and given off by objects was thought to be continuous, however…

MAX PLANKMAX PLANK…in 1900, proposed that energy is only available in discrete packets, or quanta (singular is quantum); the size of the quantum is related to the frequency of the radiation by a simple equation.

…The energy E of each quantum, or each photon, equals Planck’s constant h times the radiation frequency symbolized by the Greek letter nu, ν, or simply: E = hν

Now Back to ElectronsNow Back to Electrons• Danish scientist Neils Bohr Danish scientist Neils Bohr

(1885-1962) applied Planck’s (1885-1962) applied Planck’s idea of quantized energy to the idea of quantized energy to the model of the atom model of the atom

• and helped explain why and helped explain why different elements emit different elements emit different spectra by going back different spectra by going back to the electron.to the electron.

Bohr’s modelBohr’s model• Bohr developed a Bohr developed a

model to reflect model to reflect how electrons how electrons move around the move around the nucleus.nucleus.

• The electron is The electron is restricted to restricted to specific orbits or specific orbits or energy levels.energy levels.

• The energy levels The energy levels are said to be are said to be quantized.quantized.

Bohr’s modelBohr’s model

• An atom has several An atom has several orbits, each orbits, each representing a specific representing a specific energy level.energy level.

• The energy levels are The energy levels are like the steps of a like the steps of a ladder…or even stairs.ladder…or even stairs.

• Only specific energy Only specific energy values (steps) can values (steps) can exist – there are none exist – there are none in betweenin between

Bohr’s modelBohr’s model

This looks a lot like This looks a lot like planets held in orbit planets held in orbit around the sun, so this around the sun, so this model of the atom model of the atom became known as became known as Bohr’s Bohr’s planetary modelplanetary model. .

Bohr’s Model of the hydrogen atomBohr’s Model of the hydrogen atomBohr studied the spectrum of Bohr studied the spectrum of excited hydrogen atoms and excited hydrogen atoms and proposed the following…proposed the following…-When an atom is not excited When an atom is not excited ((ground state),ground state),- its electrons are in orbits close to its electrons are in orbits close to the nucleus of the atom the nucleus of the atom -If an atom gains energy (If an atom gains energy (excited excited statestate), an electron is displaced ), an electron is displaced farther away form the nucleus to farther away form the nucleus to one of the higher energy levels.one of the higher energy levels.

- Absorption of energy increases the PE of the electron as it Absorption of energy increases the PE of the electron as it moves further away from nucleusmoves further away from nucleus

- An electron of excited atom absorbs energy in the form of heat An electron of excited atom absorbs energy in the form of heat or electricity to jumpor electricity to jump

Bohr’s Model of the hydrogen atomBohr’s Model of the hydrogen atom- An atom in the An atom in the excited stateexcited state emits energy when the emits energy when the

excited electron falls from a higher energy level to a lower excited electron falls from a higher energy level to a lower energy level in one sudden drop or transition. The energy energy level in one sudden drop or transition. The energy released is electromagnetic energy…light.released is electromagnetic energy…light.

Bohr’s Model of the hydrogen atomBohr’s Model of the hydrogen atom- The frequency of the radiation emitted depends on the The frequency of the radiation emitted depends on the

difference between the higher and lower energy levels in difference between the higher and lower energy levels in the transition. the transition.

Bohr’s Model of the hydrogen atomBohr’s Model of the hydrogen atom- Remember that a specific frequency correlates with a Remember that a specific frequency correlates with a

specific wavelength (and color!) because specific wavelength (and color!) because c=v

Bohr model of hydrogenBohr model of hydrogen

Atoms Can Be Identified by the Light Atoms Can Be Identified by the Light They EmitThey EmitWhen atoms When atoms absorb energy absorb energy (from heat or (from heat or electricity) that electricity) that atom is atom is temporarily temporarily energized and will energized and will emit light.emit light.

Atoms Can Be Identified by the Atoms Can Be Identified by the Light They EmitLight They Emit

The atoms of a particular element The atoms of a particular element can emit only certain frequencies can emit only certain frequencies (colors) of light. (colors) of light. As a result, each element As a result, each element produces its own distinct glow produces its own distinct glow when energized.when energized.

Atoms Can Be Identified by the Light Atoms Can Be Identified by the Light They EmitThey Emit

Na atoms emit Na atoms emit bright yellow light. bright yellow light. That is why they are That is why they are often used for often used for street lamps. street lamps.

Ne atoms emit very Ne atoms emit very bright red-orange bright red-orange light.light.

Specific elements Specific elements are used in are used in fireworks to give fireworks to give them their colors.them their colors.

Atoms can be identified by Atoms can be identified by the light emit.the light emit.

A A spectroscopespectroscope helps us see the individual helps us see the individual color components of light. color components of light. When we view light from glowing atoms When we view light from glowing atoms through a spectroscope, we see that the through a spectroscope, we see that the light consists of separate frequencies rather light consists of separate frequencies rather than a continuous spectrum (like we see than a continuous spectrum (like we see with a prism). with a prism).

The pattern of frequencies formed by a The pattern of frequencies formed by a given element is referred to as that given element is referred to as that element’s element’s atomic spectrumatomic spectrum. .

Atoms can be identified by Atoms can be identified by the light emit.the light emit.

The atomic spectrum is like an element’s The atomic spectrum is like an element’s “barcode”. No two elements have exactly “barcode”. No two elements have exactly the same spectrum, so you can identify the same spectrum, so you can identify what an element is by the light it emits. what an element is by the light it emits.

Here are the atomic spectrums for some Here are the atomic spectrums for some common elements:common elements:

Atoms can be identified by Atoms can be identified by the light emit.the light emit.

Here are some other element’s atomic Here are some other element’s atomic spectra:spectra:

The Shell ModelThe Shell ModelAn important aspect of this model is that An important aspect of this model is that each shell can only hold a limited number of each shell can only hold a limited number of electrons.electrons.

The innermost shell can hold 2 electrons.The innermost shell can hold 2 electrons.The second and third can hold 8 each.The second and third can hold 8 each.The fourth and fifth, 18 each. The fourth and fifth, 18 each. The sixth and seventh, 32 each. The sixth and seventh, 32 each.

Problems with Bohr’s Problems with Bohr’s ModelModel

Remember no model is a perfect representation. Remember no model is a perfect representation. We use them to help us physically, or We use them to help us physically, or conceptually, understand something better. conceptually, understand something better. However, all models have limitations.However, all models have limitations.

Problems with Bohr’s Model:Problems with Bohr’s Model:1.1.It only explained the line spectrum for It only explained the line spectrum for hydrogen hydrogen because it only has one electron because it only has one electron 2.2.He couldn’t explain why the electrons, which He couldn’t explain why the electrons, which he pictured as circling the nucleus like tiny he pictured as circling the nucleus like tiny planets, did not fall into the nucleus every time planets, did not fall into the nucleus every time they emitted lightthey emitted light

Bohr model tutorialhttp://www.youtube.com/watch?

v=Ic8OnvRonb0&safety_mode=true&persist_safety_mode=1&safe=active

Bohr model explanation.http://www.youtube.com/watch?

v=CUk3enr-m0w&safety_mode=true&persist_safety_mode=1&safe=active 

very good video on bohr model.http://www.youtube.com/watch?

v=QI50GBUJ48s&safety_mode=true&persist_safety_mode=1&safe=active

The visible spectrum of light is just part of the EM spectrum- the part detectable by the human eye