Chem 103 Midterm 2

What is electromagnetic radiation

light which is photons

Amplitude

height of peaks (intensity)

Wavelength

distance between two identical points

Frequency

#of wave fronts/oscillations per sec

How are wavelength and frequency related by the speed of light?

They’re inversely related by the speed of light so if the wavelength increases the frequency decreases and vise versa

Energy of light relation to frequency and wavelength

Energy of light increases as frequency increases and wavelength decreases

Short wavelengths have

high frequency and high energy

Long wavelengths have

low energy and low frequency

What is the evidence that light is a wave

Diffraction and interference patterns

Define diffraction

(double slit experiment) waves bend around obstacles or through openings causing them to spread out creating a new wavefront as they pass through. Light exhibits this wave like property where as particles don’t.

Define interference

When two sets of light waves overlap

Define what it means for waves to be in-phase and have constructive interference

The waves are completely synchronized. They overlap in such a way that the amplitude doubles.

Define what it means for waves to be out-of phase and have destructive interference

One wave is at it’s max amplitude while the other is at it’s min. These waves interact in such a way that they completely cancel each other out.

What is the evidence that light is a particle

photoelectric effect

Define the photoelectric effect

Electrons are ejected from the surface of metal when light of a certain energy strikes the surface. The light is transferring energy to the electrons at the metal surface where it’s transformed into kinetic energy that give electrons enough energy to “leave” the atoms in the metal.

Relationship between intensity of light and the # of electrons (photoelectric effect)

The higher the intensity the more electrons that are ejected

Relationship between frequency of light (energy of photons) and the kinetic energy or speed of the electrons (photoelectric effect)

The higher the frequency the higher the kinetic energy

Threshold frequency VO

Below VO no electrons are emitted, Vlight>VO the speed of electrons increases linearly with the frequency of light.

What does the energy of a photon depend on

Frequency not intensity

Describe and identify experimental evidence for why electromagnetic radiation is a wave. :

The concept of diffraction, where waves bend around obstacles or through openings, causing them to spread out thus creating a new wavefront as they pass through. And the property of interference,e, which is when two waves overlap each other. In constructive interference, the amplitudes come together and double, but in destructive interference they cancel each other out completely. 

Claim for why we can consider electromagnetic radiation as a wave. 

We consider electromagnetic radiation as a wave because it exhibits wave-like properties: 

Evidence for why we can consider electromagnetic radiation as a wave. 

Diffraction, and interference

Reasoning for why why we can consider electromagnetic radiation as a wave. 

The wave-like nature of electromagnetic radiation is evident through its ability to diffract and interfere. Diffraction occurs when electromagnetic waves encounter an obstacle or pass through an opening, causing them to spread out and form a new wavefront—something only waves can do. Additionally, interference demonstrates the wave properties of electromagnetic radiation, as overlapping waves can either reinforce each other (constructive interference) or cancel out (destructive interference). Since these phenomena are defining characteristics of waves, the fact that electromagnetic radiation exhibits them supports the conclusion that it behaves as a wave.

Describe and identify experimental evidence for why electromagnetic radiation is a particle

Evidence: Photoelectric effect: electrons are ejected when light of a certain energy strikes the surface of metal. This is due to the light transferring energy to the electrons, where its transformed to kinetic energy that gives the electrons enough energy to “leave” the atoms in the metal

Claim for why we can consider electromagnetic radiation as a particle. 

The existence of photons explains the photoelectric effect because light is made up of photons.

Evidence for why we can consider electromagnetic radiation as a particle. 

Photoelectric effect

Reasoning for why we can consider electromagnetic radiation as a particle. 

The photoelectric effect shows that light behaves as a particle because electrons are only ejected when light is above a certain threshold frequency. This means light is made up of photons, rather than a continuous wave. Since only photons with enough energy can knock electrons free, this confirms that electromagnetic radiation has particle-like properties.

Explain how (and why) different atoms emit different wavelengths of light.

each element has different energy levels an the differences between those levels result in different energies of the photons being released, and different energy photons have different colors

Compare and contrast atomic emission and absorption spectra and how they are generated

 Atomic emission spectra are generated when electrons jump from a higher energy level where they are in a more excited state to a lower energy level or to the ground state, and when this happens, photons are emitted. Causing dark lines. 

Atomic absorption spectra are generated when electrons jump from the ground state to a higher energy level, where it’s in a more excited state, causing photons to be absorbed. Causing bright lines. 

Claim for  for the existence of quantized energy levels in an atom.  as it relates to spectra

Spectra are evidence for the existence of quantized energy levels in an atom 

Evidence for:  for the existence of quantized energy levels in an atom (as it relates to spectra)

If the electrons didn't exist in quantized energy levels, there would be a continuous spectrum rather than the distinct lines formed. And these lines correspond to specific wavelengths of light that are emitted or absorbed by electrons transitioning between energy levels

Reasoning for:  for the existence of quantized energy levels in an atom. as it relates to spectra

Only specific wavelengths appear in emission and absorption spectra, which means that electrons can only occupy certain energy levels within an atom. When electrons move between these levels, they must absorb or emit a certain amount of energy, proving that atomic energy levels are quantized.

Compare and contrast absorption, emission, and the photoelectric effect.  Describe the role that photons and electrons play in each phenomenon. 

In absorption: electrons move from a lower energy level to a higher energy level, causing photons to be absorbed. In emission: an electron moves from a higher energy level to a lower energy level, causing photons to be emitted. In the photoelectric effect, photons strike the metal surface, causing electrons to be ejected as long as the frequency of the light is above the threshold VO. When the light strikes the surface, the energy of the electrons increases, increasing the kinetic energy to allow the electrons to have enough energy to leave the atoms/surface. 

Comparisons between emission and absorption:

 both involve the transition of electrons from different levels of energy within an atom.

Comparisons between abs and ems spectra and photo electric effect:

both involve photons and electrons

Contrasts between abs and ems spectra and photo electric effect

Spectra involve a change in energy levels for the electrons within an atom, whereas photoelectric effect is the complete ejection of electrons from a surface as long as the photons have a high enough frequency to do so. 

Describe how the model of the atom changed from Dalton through Thompson,  Rutherford, and Bohr to Schrodinger.  Explain why each model changed and point out the problems with the previous model. 

Dalton: Claimed atoms were indivisible

Thomson: Proved Dalton wrong with the existence of electrons and the idea of the plum pudding model where atoms are composed of a mainly positive structure with electrons surrounding it.

Rutherford: Disproved Thomson with the discovery of the nucleus which then changed the model to mostly space with a positively charged dense nucleus in the middle of the atom, and the electron cloud surrounding it.

Bohr's Model: Proposed that electrons moved in fixed orbits around the nucleus which combated the instability of Rutherford's model, but this only applied to hydrogen and not more complex elements. 

Schrodinger: Introduced the quantum mechanical model in which he stated that orbitals represented a 90% probability of the electron being located within a particular volume of space 

Describe experimental evidence for the wave nature of electrons.

Evidence: The Davisson and Germer experiment, which showed the wave-like behavior of electrons, because it showed that electrons could diffract.

Describe an atomic orbital and what it represents. :

Atomic orbitals represent the probability of an electron of a certain energy being located within a volume of space

Explain the concept of effective nuclear charge and how it affects atomic radius

An element with a higher effective nuclear charge will have a smaller atomic radius. This is because when an element has a higher ENC, the charge pulling the valence electrons to the positively charged nucleus is much stronger, holding the electrons tighter together, decreasing the atomic radius. 

Apply Coulomb’s law to explain the relative sizes of neutral atoms and ions. 

Neutral Atoms: The atomic radius is determined by the balance between nuclear attraction and electron-electron repulsion, decreasing across a period as nuclear charge increases, and increasing down a period.

Cations: Losing electrons reduces the electron-electron repulsion as their proton to electron ratio is greater than 1, decreasing the atomic radius.

Anions: Gaining electrons increases electron-electron repulsion and results in a proton to electron ratio less than one, making anions larger than their neutral atoms, increasing the atomic radius.

Apply Coulomb’s law to explain periodic trends in atomic radii

As you move from left to right on the periodic table, the atomic radius will decrease. This is because according to Coulomb's law, the greater the ENC that the electron experiences, the greater the force of attraction between the valence electrons and the positively charged nucleus. And the ENC inc bc the #of protons increases but the core electrons remain the same.

As you move down a period on the periodic table, the atomic radius will increase. This is because there is an increase in energy levels, which means that there is a greater amount of distance between the electrons and the positively charged nucleus, as the energy levels increase. and results in a weaker attraction between electrons resulting in a larger atomic radius.  (ENC remains the same)

Apply Coulomb’s law to explain periodic trends in ionization energies. 

Ionization energies increase across a period because the ENC increases, as the #of protons inc but the core electrons remain the same, so the attraction between the valence electrons and the nucleus increases (electrostatic attraction increases while e- to e- repulsion dec), thus requiring more energy to overcome the interaction and remove an electron. 

Ionization energies decrease down a period because the valence electrons become further and further from the nucleus as the energy levels (quantum # n ) increase, weakening the force of attraction requiring less energy to overcome the interaction making it easier to remove an electron. 

Describe how ionization energies support the idea of quantized energy levels in atoms.

If electrons could occupy any level of energy any amount of energy could possibly be used to remove an electron but since Ionization energies show that electrons are removed in specific replicable energy amounts, it shows that a certain amount of energy is needed to remove the electron

ENC definition:

Net postive charge between the nucleus and valence electrons

Why do we take into account the wavelengths of electrons:

The wavelength of an electron is similar in size to the atom and affects it properties whereas the wavelength of the macroscopic object is much smaller than the object and does not effect its properties

Ionization energy definition:

 The energy required to remove an electron from an atom in the gas phase. 

Isoelectronic series definition:

Attraction between protons and electrons increases as the charge of the nucleus increases but the electron-electron repulsion remains the same

What does the quantam number n represent

The energy level and size of the orbital (# of sublevels)

What does the quantam number L represent

The shape of the orbital

What does the quantam number mL represent

the orientation

What does the quantam number MS represent

the spin of the electron up arrow ½ down arrow -1/2

Define what it means for energies of electrons to be quantizied

It means that electrons can only occupy specific energy levels within an atom.

Explain why ENC inc across a period

It inc bc the # of core electrons remain the same but the # of protons increase meaning that there is a stronger pull between the nucleus and valence electrons

Paulis exclusion principle

no two electrons in an atom can have the same set of 4 quantum numbers (cant have same spin direction)

Hunds rule

a principle in atomic and molecular physics that describes how electrons fill orbitals in atoms and molecules

Relationship between frequency and intensity

Once the frequency threshold is met inc the intensity results in a greater number of electrons ejected. Intensity is the #of photons in a given area so if you inc the intensity you inc the #of photons hitting the area.

Why does hydrogen have the same emission and absorption spectra compared to other elements?

The photons that are absorbed and emitted have the same energy and therefore same wavelength (same color) This is bc the difference between energy levels is the same wether your transitioning up or down. But with other elements that have more than 1 electrons the transitions can become more complex resulting in more complex absorption spectra.

Reasoning for why electrons behave as waves

quantum mechanical model of the atom supports wave particle duality