OnRamps Chemistry College Exam 1

Rank Electromagnetic radiation by increasing E and V

radio waves -> Microwaves -> Infrared -> Viisible -> UV -> X rays -> Gamma-rays

Visible Wavelength value

300 to 500 nm

UV wavelength value

300 to 10 nm

Infrared region

800 nm to I um

x rays

.01 to 10 nm

E

hv

v

c/λ

h

6.6x10^8 m/s

Metric Conversions

You can convert between the smaller and larger metric units by simply moving the decimal point.

Scientific notation rules

Decimal must be after first non-zero integer

Positive exponent means move decimal right

Negative exponent means move decimal left

Multiplication/Division: add/subtract exponents

Addition/Subtraction: exponents must match before calculating

Energy (of a photon)

hc/λ

Radio waves

1 nm to 100s m

Spin nucleus excited used in MRI (non-invasive)

Responsible for AM, FM radio

Micro waves

100 um 100 cm

Excited spin of electrons

excites rotation of H20, fats molecules around 12 cm to cook food

Infrared

800 to 1000 nm (1 um)

Causes vibrations in molecules

used in night vision goggles

excited wavelength of -OH in ethanol and used to measure intoxication

Visible

300-800 nm

causes excited value of e-'s to higher energy level

solution for H atom

This is rays of detection for human eye

UV

300 to 10 nm

excites valence electron

Lyman series for H emission, n=1 excited

causes sun burn

X-rays

10 to 0.01 nm

Excites core e-'s in atoms

used in medicine for invasive imaging

cause cell mutations to limit exposure

Gamma and Cosmic Rays

<0.01 nm

associated with decay of atom nucleus

are emitted from stars hence comic ray name

When given molecule absorbs a photon (IR) radiation

it begins to vibrate

Radiofrency radiation has what kind of impact on matter?

It makes the nuclei spin

Photo-electric effect

The emission of electrons from a metal when light shines on the metal

In the photo-electric effect, metals with low ionization energy like alkine metals hit the metal

and do not emit electrons

Electron threshold

the minimum kinetic energy a pair of traveling particles must have when they collide

In the photo-electric effect, high energy (hv) hits the metal and

electrons are emitted

According to classical mechanics,

the intensity of the light should make any

hv energy eject electrons

But this does not happen. Classical mechanics fails.

According to Plank and Einstein, thinking of light as a particle

E = hv ← a photon

of enough energy will eject electrons.

E = hv = E threshold +KE photon

Photons above the threshold cause electrons

to be ejected with higher velocities

Which of the following statement(s) is/are true about the photoelectric effect?

I) The sum of the work function and kinetic energy of an ejected electron is proportional to the frequency of incident light.

II) Given light of high enough intensity, electrons can be ejected from any surface.

III) Einstein employed the concept that photons have quantized amounts of energy to explain the effect.

I, III

Classical mechanisms predicted that light of any wavelength could be

able to eject electron from a metal surface it was sufficiently intense, which was

inconsistent with the observed threshold. This threshold effect, the ejection energy,

required that light energy was quantized. Conservation of energy lets us conclude

that KE = hv − )0(

Light is a wave and a particle

-Light used to be thought of as wave-like, in keeping with classical

mechanics. Wave phenomena includes: diffraction, reflection,

polarization, etc.

-Downfall of classical mechanics: Blackbody radiators, H emission

spectra and photoelectric effect can't be explained by classical

mechanics.

-Light as a particle E ≡ hv ≡ photon is postulated.

Matter is also a wave and a particle

-Matter is a particle. It behaves according to Newtonian physics.

• But as matter gets smaller, it does not behave like particle. Can't assign

trajectories to electrons.

• deBroglie postulated that matter also acts as a wave.

• This is shown in Foreman experiment where electrons have a

diffraction pattern.

The famous electron diffraction pattern is proof of what concept?

Particles can exhibit wave-like properties

"light and matter both have

dual nature:

wave-like and particle-like."

Electron diffraction

The spreading of electrons as they pass through a gap similar to the magnitude of their de Broglie wavelength. It is evidence of the wave-like properties of particles.

de Broglie wave equation

λ = h/mv

λ

wavelength of matter

m

mass of particle

v

velocity of particle

h

Planck's constant

mv

momentum

Note inverse relationship between wavelength and wave

The larger the wave, the smaller the wavelength

Because h = 6.6×10^-34 Js is so small, this says that large particles that you can see have undetectably small wavelengths

Simple estimate is 100 kg person

walking 1 m/s ≡ 10 ^-36 m wavelength.

But electrons and protons are small enough that their wavelength can be measured!!

An e- has λ of nanometers, about the length of an atom. Protons are 10^3 layer so they have a λ of picometers depending on the velocity

According to de Broglie, which of the following objects has the smallest wavelength?

molecules

This is why quantum mechanics Is needed

It explains wave behavior of

electrons ≡ where they are

Rydberg equation

It ties experiment and theory to explain the behavior of

electrons in an H atom. Bohr's atom, that you learned in elementary school, is the bridge

between experiment and theory

Bohr's atom

Electrons move in circular orbits, around a nucleus-- electrons remain in orbits unless disturbed-- when an electron jumps from one orbit to another, energy is released or absorbed

Note that as energy spectra get

closer they become 1/n^3 function

1/2^2, 1/2^2, 1/3^2 , or 1, 0.25, 0.11

UV falls to n=1

lyman series