Chem Exam 4

Rutherford

Created the Nuclear Atom Model (Planetary model)

Atoms have a dense positive charge middle (Sun) with smaller particles that are negatively attracted to it and orbiting(planets)

Reality of how electrons surround the nucleus

Electrons behave a lot less like orbiting particles and a lot more like standing waves (jump rope)

If the planetary model were true - electrons could have any range of energies (exist on an energetic “ramp” and their energies would fill a visible spectrum)

When do atoms absorb or emit light?

If the wavelength of light corresponds to the energy of a change they can undergo (conservation of energy)

Hydrogen lamp atoms absorb energy from a high voltage source and that energy is emitted as light

Each band of color corresponds to a specific amount of…

Energy

Specific colors are seen because the electron can only emit specific amounts of energy

Emission Spectra…

Show that electrons can only have specific energies (energies are quantized)

Heisenberg Uncertainty Principle

Insignificant for macroscopic objects, but it severely limits what we can know about small particles w/ significant wavelengths (like electrons)

• If a particle has a significant wavelength it’s hard to pinpoint it’s position and momentum at the same time

Electrons cannot be described using classical …

Mechanics

Classical mechanics would allow the electron to have any energy because it could be in any orbit

Electron energies

Electrons are limited to specific energies (standing wave)

They exist in certain allowed energy levels (Orbitals)

Orbitals

The possible states of existence of an electron

The probability of finding an electron at a given location

Electron density

The probability of finding an electron at a given location

Each orbital is described by…

Three quantum numbers: n, l, ml

n

Principle quantum number that corresponds to the shape of the orbital

any number greater than 0

A higher n = bigger orbital and higher energy

l

angular momentum quantum number that corresponds to the shape of the orbital

any number from 0 to n-1

l is given letter: 0 is s, 1 is p, 2 is d, 3 is f, 4 is g

Ex: For n-2, l can be 0 or 1

Ml

Magnetic quantum number that corresponds to an orbital oriented differently in space

Can be any number from -l to +l

Ex: For l = 1, Ml can be -1, 0, or +1

Node

A location in space where the mathematical sign of the wave function changes and electron density equals zero

(probability of finding an electron is zero)

Orbitals have what kinds of nodes?

Radial nodes: n-l-1 (distance from the nucleus)

Angular nodes: l (an angle about the cortesine axis)

Radial Nodes

Distance from the nucleus

n-l-1

Angular Nodes

An angle about the cortesine axis

l

Radial vs. Angular nodes

Radial are spherical shells representing a radial distance from the nucleus

Angular are flat planes/cones that pass through the nucleus that divide lobes

Bigger n = …

Bigger orbitals and higher energy

Why are models circles?

Because the edge doesn’t exist!

Ms

Electron spin quantum number

Can be +1/2 or -1/2

Pauli Exclusion Principle

In a given atom, no two electrons can have the same y quantum numbers

Meaning each orbital (unique n, l, ml) can “hold” 2 electrons, one w/ +1/2 and one w/ -1/2

Orbitals are the solution to the …. equation

Schrodinger

The equation cannot be solved precisely for atoms w/ more than one electron

Orbital in poly electronic atoms

assumed to be like those of single-electron atoms, but they are shielded from the nuclear charge by the other electrons, so they experience a weaker effective nuclear charge

(Orbitals in multi-electron atoms are treated like those in single-electron atoms, but other electrons shield the nucleus, so each electron feels a reduced (effective) nuclear charge.)

Shielding

Process where inner-shell electrons act as a barrier, reducing the positive pull of the nucleus on outer (valence) electrons

At a given n level, … electrons shield the most

s

P electrons shield more than … electrons

d

d electrons shield more than …

f electrons

What letter gets shielded the least?

s

BUT it penetrates the best

What letter penetrates the best?

s

and it shields the least

Higher net attraction to the nucleus = ….

lower energy

In a single electron atom, all orbitals at a given n-level are …

degenerate (they have the same energy)

In polyelectronic atoms, for a given n-level, energy ….

increases

s<p<d<f

Electron configuration

Lists of occupied orbits and the number of electrons they contain

Afbau Principle

Electron configuration can be “built up” by successively adding electrons to the lowest-energy available orbitals

Hund’s Rule

The lowest energy electron configuration in a set of degenerate orbitals has the maximum number of electrons w/ the same spin (same Ms)

Pairing Energy

Electron repel each other, so two electrons are a little higher in energy when they occupy the same orbital

Exceptions to orbital configuration

Cr is [Ar] 4s¹3d⁵ and Cu is [Ar] 4s¹3d¹⁰

Three high energy and close in energy orbitals

4s and 3d

5s and 4d

6s, 5d, and 4f

Valence Electrons

highest energy electrons in an atom

Groups 1-2: Group # = valence electrons

Groups 13-18: Subtract 10 from group #

Helium only has 2 valence electrons

Core Electrons

On average, usually closer to the nucleus than valence electrons

They shield valence electrons very well

Shielding of core vs. valence electrons

Core shield valence very well (nearly cancels the attraction of one proton)

Valence do not shield other electrons well (decreases the effective nuclear charge by only a small, usually negligible amount

Ionization

The removal of an electron from an atom

Energy increases as you go top top right of table

First ionization energy

The energy required to remove highest-energy electron from a neutral gaseous atom

Ex: ∆E for X_(g) → X⁺_(g) + e^-

Separating opposite charges …

Always requires energy, so ionization energies are positive

The harder it is to remove an electron, the greater the …..

ionization energy is

As you go to up and to the right of the table ….

Numbers of protons increase

Numbers of valence electrons increase

Ionization energy increases

BUT core electrons stay constant

Why does ionization energy increase across a row?

Because each additional proton results in additional attraction while each additional (valence) electrons results in minimal additional shielding

The electron being removed experiences a net greater effective charge, so it’s ahrder to remove

As you go top to bottom down a group….

Number of protons increase

Number of valence electrons is constant

Number of core electrons increases

n-level of highest energy orbital increases

Ionization energy decreases

Why does ionization energy decrease down a group?

Because changes in the #’s of protons and core electrons nearly equally oppose one another, but the greater distance from the nucleus (bc of the greater n-level) results in a weaker force of attraction

Second ionization energy

The energy required to remove the highest energy electron from a gaseous +1 ion

Energy is always greater than 1st ionization energy

The second ionization energy experiences less …

shielding and a greater effective nuclear charge, because one electron has already been removed

Or

The second electron is coming off a positive ion, so i experiences a greater attraction than the first one did

Electron Affinity

The energy change associated with the addition of a electron to a neutral gaseous atom

Typically negative because energy is being released when a negative electron associates with a positive nucleus (plus like minus)

A more negative electron affinity is often described as being more …

exothermic or more favorable (they stick together more)

This means that the electron is easier to add (and avoids ambiguous greater/less phasings)

Moving left to right across a row, does it get easier or harder to add an electron and why?

Up until the noble gases, it gets easier to add an electron across a row. Electron affinity becomes more negative, more exothermic, and more favorable. Nuclear charge and # of valence electrons both increase, but valence electrons don’t shield well, so and added electron experiences a greater effective nuclear charge. There is not a consistent trend in electron affinity going down most groups.