Chem Exam 2 (concepts)

Bohr frequency rule

ΔE = |E_f - E_i| = hv

equations for light

c = λv, E=hv, 𝒗=λv

List the types of electromagnetic waves from high energy to low energy

gamma, x-ray, UV (ultraviolet), visible, IR (infrared), micro waves, radio waves

what is an electromagnetic wave?

self-propagating waves that travel at the speed of light and are made of electric and magnetic fields that transport energy through space

what does v stand for in the light equations?

frequency, measured in s^-1 (AKA Hz)

What does Planck’s equation, E=hv, calculate?

The energy of a single photon

What is the photoelectric effect?

The phenomenon where if you shine light on certain metals, electrons may be ejected from them

List the unusual characteristics of the photoelectric effect

1. electrons are only ejected if the frequency of the light shone (v) is above a certain threshold (v₀), which depends on the metal

   1. doesn’t matter how intense (how many photons) the light is, if not above v₀, no e- ejected

2. If the frequency of the light shone is above v₀, then the number of e- ejected is proportional to the intensity of the light

   1. all ejected electrons have the same kinetic energy (speed) no matter how intense light is

3. as the frequency of the light shone is increased further (v>v₀) the emitted electrons have more and more KE (they speed up)

   1. if v>v₀, the KE is proportional to the frequency

How did Einstein use Planck’s concept of quantized energy to explain the unusual characteristics of the photoelectric effect?

E=hv means the greater the freq of light (v), the greater the energy of the photon, so a minimum frequency corresponds to a minimum energy needed to remove the electron from the metal

• if energy of photon is below energy (i.e. freq / v_o) needed to eject electron, it won’t be ejected

Explain stationary state orbits

Electrons can only travel in specific circular orbits around the nucleus called stationary states

How does photon emission and absorption work?

when an electron moves from a higher orbit to a lower one, it emits a photon whose energy corresponds to the energy difference between the two orbits

when an electron jumps to a higher energy orbit, it must absorb a photon whose energy corresponds to the energy difference between the two orbits

What is Bohr’s frequency rule?

The frequency of photons emitted or absorbed during an electron’s transition between two energy levels/shells/stationary orbits corresponds to the energy difference between those two energy levels

what is another way to write E = hv?

E = h(c/λ) ; help to derive balmer rydberg

what do the variables in the de Broglie wavelength equation represent?

λ = h/(mv)

h = Planck’s constant

m = mass

v = velocity (speed)

what do the variables in Heisenberg’s uncertainty principle represent?

ΔxΔp_x ≥ h/4π

Δx = uncertainty in position

Δp_x = uncertainty in momentum

h = Planck’s constant

What are the four quantum numbers?

N, l, m_L, m_s

What quantum number does N represent?

• Principal Quantum Number

• Related to size and energy of an orbital

• integer value of 1, 2, 3…etc up to infinity

What quantum number does L represent?

• Angular Momentum Quantum Number

• Related to shape of the orbital

• 0=s, 1=p, 2=d, 3=f, 4=g, 5=h, 6=i

• L = 0,1, 2…up to n-1

What quantum number does m_L represent?

• Magnetic Quantum Number

• Related to orientation of orbital in space

• m_L = -L, … +L

What quantum number does m_s represent?

• Electron Spin Quantum Number

• Related to spin of electron

• up = +1/2

• down = -1/2

What is the difference between shell and subshell?

The shell is the main energy level around an atom’s nucleus (n) while the subshell is a specific group or shape of orbitals located inside the shell

• e.g. for 3s, 3 = shell, s = subshell

What is the Born interpretation of the wave function?

The square of the absolute value of a wave function (wave function = mathematical description that holds every piece of measurable info about quantum system or particle) represents the probability density of finding a particle (electron) at a specific position/state

What is a node in the Born interpretation of the wave function?

Where there is 0 probability of finding an electron

How does Z_eff / shielding affect the energies of s, p, d, and f orbitals within the same shell for a multi-electron atom?

s can shield p even if on the same n level (why the Be/B exception exists) because electrons in s can get closer to nucleus than electrons in p

Pauli exclusion principle

No two electrons in the same atom can have the same four quantum numbers

• dictates the shape of the periodic table

Hund’s Rule

The most stable arrangement of electrons in a subshell is the one with the greatest number of parallel spins

What are the metalloids?

B, Si, Ge, As, Sb, Te, Po

Transition metals

Groups 3-12

Alkali metals

Group 1

Alkaline earth metals

Group 2

Halogens

Group 17

Noble gases

Group 18

What does each variable stand for in the coulombic potential equation?

V=q₁q₂/(4πε₀r)

q_1/2 = charge of particle ½
ε₀ = constant, 8.854 × 10^-12 C²/(N*m²)

r = distance between particles

What does coulombic potential equation calculate?

The potential energy between charged particles

Explain how/why Z_eff changes for atoms as you go across period

Z_eff (generally) increases as you go across a period because more protons are added to the nucleus (greater positive charge) while the shielding stays the same

Explain why the atomic radius generally decreases as you go across a period and up a group

Across a period:

• More protons are added to the nucleus (greater positive charge) while the shielding stays the same, causing electrons to be pulled in more strongly to nucleus

Up a group:

• Principal quantum number corresponds to size, n=2 orbitals larger than n=1 orbitals, so e- is more likely to be found further away

paramagnetic vs diamagnetic

• paramagnetic = unpaired electron/s in orbital diagram; m_s values don’t cancel; weakly attracted to external magnetic field

• diamagnetic = no unpaired electron/s in orbital diagram; m_s values do cancel; weakly repelled by external magnetic field

Why do cations have a smaller radius than their neutral atom?

Losing electron/s increases effective nuclear charge on the remaining outer electrons and reduce electron-electron repulsion

Why do anions have a larger radius than their neutral atom?

Adding electron/s increases electron-electron repulsion, so the electron cloud gets bigger

What is ionization energy?

The energy required to completely remove the outermost electron from a gas-phase atom in its ground electronic state

What is ionization energy measured in?

kJ/mol, so more accurate to say the energy required to remove one MOLE of electrons from one MOLE of gas phase atoms in their ground states

Why does IE generally increase as you go across a period?

Z_eff also increases as you go across a period, so takes more energy to remove an electron with a greater Z_eff

Explain Be —> B IE exception

2p electron in B is slightly shielded by the 2s electrons (because s orbitals are more likely to be close to the nucleus), but 2s electrons in Be don’t shield each other, so B has lower first IE because it feels less Z_eff

Explain N —> O IE exception

More favorable to remove an electron from O than from N because of electron repulsion in the first 2p orbital, so O has a lower first IE

What is electron affinity?

The energy change that accompanies the addition of an electron to an atom

Why is electron affinity almost always negative?

Free electrons are energetically unstable (an electron has less energy when its attached to anything because then its closer to the positive charge in a nucleus) so energy is given off when an anion forms

Why do elements in group 17 (halogens) have the most negative E_ea of any NEUTRAL atom?

They are only one electron away from achieving a stable electron configuration, so they pull the electron more strongly toward the nucleus and that releases a larger amount of energy

Why are electron affinities for noble gases zero or positive?

The very stable noble gas configuration is disrupted by the addition of an electron (forcing an e- into a higher energy level requires an input rather than release of energy)

Difference between ionic and covalent bonds

Ionic:

• Electrons are transferred, then ions are formed which are then attracted to each other to form a crystal lattice

• Between metal and nonmetal

Covalent:

• Electrons are shared

• Only between nonmetals

What is a formula unit?

The smallest electrically neutral ratio of ions in an ionic compound

How do you name ions?

Just the element name plus ion for most of them, except for group 15, 16, and 17 (add -ide to the name resulting ion, e.g. chloride /sulfide/phosphide ion)

what is the prefix for half a hydrogen when naming hydrates?

hemihydrate

what is the prefix for 9 hydrogen when naming hydrates?

nonahydrate

What does Heisenberg’s uncertainty principle state?

The more you know about the momentum of a particle, the less you know about its position, and vice versa

Equation for the energy of an electron in a specific energy level

E_n = -k/(n²)