chem exam 2

Amplitude:

height of peak

Wavelength (λ)

distance between two identical points (units: m)

Frequency (ν)

number of wavefronts per second (units: Hz = s -1 )

Claim: Light is a Wave

-diffraction: Diffraction is the bending and spreading out of waves when they encounter an obstacle or pass through a narrow slit. Because light travels as a wave, it can bend around the edges of objects rather than just casting a perfectly sharp, dark shadow

-Interference: If waves are in-phase the wave reinforces/the amplitude gets bigger, out of phase crests/troughs cancel

why is light not just a wave?

-we know the amplitude of a wave is related to the energy of

that wave

-The energy of electromagnetic radiation does NOT simply depend on the

amplitude of the wave

Light is a Particle

photoelectric effect: Metals can emit electrons when certain electromagnetic radiation (light)

shines on the surface

-Electromagnetic radiation transfer energy to the electrons in the metal, where

the electrons gain enough energy to leave the metal.

photoelectric effect observations

-When we increase the intensity of the light, more electrons are

emitted

-When we decrease the wavelength (increase the frequency) of the

light, the speed (kinetic energy) of the electrons increases.

is light simply a wave?

NO!!!! increasing its intensity should

increase its energy, meaning:

– with bright enough light of any frequency, electrons should be emitted.

– increasing the intensity of light should increase the speed of the

emitted electrons.

• This is not what is observed!

– Light does not just behave like a wave. It also behaves like a particle!

• photons:packets of energy

energy of a photon depends on

frequency (and wavelength), not on the intensity of the light

ntensity (brightness) of light relates to

the number of photons

If the photon does not have enough energy,

no electron is ejected

regardless of how many photons (intensity of light) hit the metal

why is ruthford wrong? (Electrons circling the nucleus like planets around the sun)

-Charged particles circling in an electric field would decay

energetically. leading to a continuous emission spectrum

-Also would have led to the atom imploding

The Bohr Model

Electrons move in orbits around the nucleus.

• These orbits have definite energies and are at definite

distances from the nucleus.

• The energies of electrons in atoms are quantized

the photoelectric effect,

-an atom absorbs a photon resulting in the ejection of an electron

-electron completely leaves the atom (the equivalent of moving to energy level n=∞) (ionization)

Limitations of the Bohr Model… only works on

hydrogen and other one electron systems

𝜆

wavelength

h

-numerator

-planck’s constant

mv

-denominator

-mass, velocity

Particles with more mass have

larger momentum (𝑚v) and shorter

wavelengths (𝜆𝜆)

-definate position

Particles with less mass have

-smaller momentum (𝑚v) and longer

wavelength (𝜆𝜆):

-uncertain position

Electron Density Distribution

-illustrate where that electron is likely to be found

-electron density distribution for the atom; the electron density (probability) is highest near the nucleus for a ground state H atom.

-90% of atom

Atomic orbital and three quantum numbers

wave function (ψ) describing an electron in an atom

(|ψ|2 : its probability distribution). N, L, M

Atomic orbital n

relates to energy/size

principle quantum number

atomic orbital l

shape

azimuthal quantum number

mℓ

-orientation

-magnetic quantum number

s orbitals

are spheres (ℓ = 0)

• have 1 orientation (mℓ = 0)

-can have 2 electrons

• have 0 angular node

• have n-1 spherical nodes

• Orbital size increases as n increases

• Electron spends more time further away from nucleus

p orbitals

• are dumbbells shaped (ℓ = 1)

• have 3 orientations (mℓ = −1, 0, +1)

• There are three p orbitals in a shell (e.g.

three 2p orbitals: 2px, 2py, 2pz).

-can have up to 6 electrons

• have 1 angular node

• have n-2 spherical nodes

• Orbital size increases as n increases

• Size of 2p < 3p

What is the probability of finding a 2p electron within a

nodal plane?

0%

d orbitals

have ℓ = 2

• shape: two dumbbells or a torus with a dumbbell

• have 5 orientations (mℓ = −2, −1, 0, +1, +2)

-can have up to 10 electrons

• There are five d orbitals in a shell

• have 2 angular nodes

• have n-3 spherical nodes

Fourth Quantum Number Ms

• Spin magnetic quantum number, ms

• It describes the electron rather than the atomic orbital

• Spin is a property of the electron

• ms can take on either of two values: +½ or -½

• Referred to as “spin up” or “spin down”

• Electrons are not spinning – this is

outdated terminology.

Pauli exclusion principle

no two electrons in an atom can have the same

set of four quantum numbers

Core electrons

in inner shell

Valence electrons

-are in the highest-energy occupied orbitals

-These electrons participate in forming chemical bonds!

Atomic radius

• A measure of the size of the atom

• Half the distance between the two nuclei

• Depends on how it is determined (e.g. via covalent interaction)

• Experimentally measured or calculated

What determines the size of the atom?

The atomic radius represents the state

where the forces of attraction between the

electrons and protons are equal to the

forces of repulsion between the electrons.

Effective nuclear charge increases across a row, What happens to the attractive force across a row?

the force INCREASES

As you move left-to-right across a period (row), what

happens to the atomic radius?

-it decreases

-stronger attractive force makes radius smaller

As you move down across a group (column), what happens

to the atomic radius?

-atomic radius increases

Monoatomic Ions

electrons have been added to or removed from a

neutral atom

Cations are

-pos charged

-outermost electron removed

-smaller than neutral

anions

-negatively charged

-e- added to next available orbital

-bigger than neutral

Which has a larger radius?

A. Li

B. Li+

Li because Li+ is positive because it lost an electron, making it smaller

Which has a larger radius?

A. F

B. F ̶

F- because it has another electron

Isoelectronic

Exact same electron configuration/same number of e-

ionization energies generally

-increase across a period

• decrease down a group