Chapter 5

Scientific Models Models are things used to represent real

phenomena. simplify and explain complex realities. can take many forms scale models, e.g. a globe mathematical models, e.g. P/V = k computer models, etc., e.g. weather

predictions

It explained much about the structure Nucleus: positive, very dense, most of atom’s

mass Electrons: outside the nucleus Empty space: most of the volume of the atom

It could not explain chemical behavior of elements, such as…. Why did elements give off light when heated? Why did one element react with another to

form a new compound?

Rutherford’s model could not explain why matter gave off light when heated

Neils BohrDanish Physicist1913: Proposed new model of the atom

Bohr proposed that an electron is found only in specific circular paths, or orbits, around the nucleus.

Each possible electron orbit in Bohr’s model has a fixed energy.

The fixed energies an electron can have are called

energy levels. Higher energy levels are farther away from the

nucleus A quantum of energy is the amount of energy

required to move an electron from one energy level to another energy level.

Energy levels are like rungs on a ladder

Higher energy levels are closer together

Takes less energy to change between higher levels

When electrons absorb exactly the right quanta of energy….They jump to higher energy levelWhen it jumps back down…It gives off (emits) the same energy as light.

ERWIN SCHRÖDINGER Bohr’s planetary model only worked for hydrogen

But it could not explain motion of electrons

Schrödinger and others developed a new mathematic model of the atom….

Called the quantum mechanical model

Like Bohr’s model, electrons are restricted to certain energy levels

Unlike Bohr’s model, the exact pathway of the electron is uncertain

Locations of electrons are uncertain, and described terms of probability….

i.e. the likelihood of finding the electron at a given point in time

Electrons are found within an “electron cloud” outside the nucleus

The electron cloud is more dense where the probability of finding the electron is high.

A spinning fan blade Forms a ‘fuzzy’ image You know the fan

blade is within the fuzzy region, but at any point in time you don’t know exactly where it is

Electrons are located in regions of probability called “orbitals”

Quantum Number

Defines Describes Values

1st Principal Energy Level ----- n = 1 to 7

2nd Angular Momentum

Energy Sub-level

Shape s, p, d, f

3rd Magnetic Orbital3-D

orientationx, y, z, etc.

4th Spin ----- Magnetic spin +1/2 or -1/2

AN “s” ORBITAL Orbitals are represented by “electron density maps”

Probability is represented by the density of color

The more probable location of the electron is in the darker blue region

Regions of space in which there is a high probability of finding an electron Various types of orbitals exist, depending upon the sublevel

S sublevels have one orbital P sublevels have 3 orbitals each

d sublevels have 5 orbitals each

f sublevels have 7 orbitals each

Energy Level

# Sublevels

# OrbitalsElectron capacity

n n n2 2n2

1 1 1 2

2 2 4 8

3 3 9 18

4 4 16 32

5 5 25 50

6 6 36 72

7 7 49 98

Each orbital can contain up to 2 electrons!

Sublevel # Orbitals per sublevel

Electron capacity per sublevel

s 1 2

p 3 6

d 5 10

f 7 14

In most natural phenomena, change trends toward lower energy

Systems are more stable when they have less energy.

Electrons also tend to arrange themselves in their lowest energy states.

The arrangement of electrons within an

atom is called an electron configuration.

Three rules are used to determine electron configurations

Aufbau Principle Pauli Exclusion Principle Hund’s Rule

Electrons occupy the lowest energy level first

This diagram is known as an electron orbital diagram

4th quantum number is the “spin” number Electrons “spin”, either clockwise &

counter-clockwise Spin is symbolized ↑ or ↓ PEP says….

Two electrons in the same orbital must have opposite spins. Therefore ….

No two electrons in an atom can have the same identical set of 4 quantum numbers.

Electrons fill orbitals within a sublevel such that they have maximum number of unpaired spins

This is because they have the lowest energy this way

Determine the number of electrons in the diagram. How?

Begin filling orbitals at the lowest energy level (Aufbau principle)

Continue filling, applying Hund’s rule All “up” spins Follow by “down” spins

Stop when you have assigned all the electrons to orbitals

A shorthand way for writing orbital diagrams Write the energy level, sublevel, and number

of electrons in the sublevel Li 1s2 2s1

C 1s2 2s2 2p2

N 1s2 2s2 2p3

O 1s2 2s2 2p4

F 1s2 2s2 2p5

Ne 1s2 2s2 2p6 Na 1s2 2s2 2p6 3s1

Periods (rows) in the PT correspond to energy level Certain groups (columns) correspond to the

sublevels (s, p, d, f) (see page 166)

Also called “noble gas notation” An element’s electron configuration

contains the e-config of a noble gas (group VIIIA, 18)

Begin with the preceding noble gas Then complete the e-config

Transition elements (groups 3-12) tend to prefer half-filled or completely filled d-orbitals at the expense of the s-orbital.

For chromium, you would expect ….4s2 3d4,

but in fact one of the 4s electrons is promoted to 3d, resulting in ….4s1 3d5

Try copper….

Much of what is known about the atom is due to the study of light

Light has properties of waves Waves have amplitude, wavelength, and

frequency

Inversely proportional

c = speed of light = 3.00 x 108 m/s (constant) lambda = wavelength (meters) nu = frequency (Hertz, Hz, s-1)

c

Visible light is a small portion of the electro-magnetic spectrum

All EM radiation travels at the same speed

c = 3.00 x 108 m/s EM radiation varies in wavelength and

frequency Longer wavelength → Lower frequency Shorter wavelength → Higher frequency

Light separates into different colors (wavelengths) when it passes through a prism

It is a continuous spectrum

Electrons of an element can absorb energy and emit the energy as EM radiation

These emission spectra are not continuous

Each element has a unique emission spectra

Like a bar code for an element

Electron at ground state absorbs a quantum of energy

Excited electron returns to ground state, emitting the quantum as light

Frequency of the light is directly proportional to the energy change of the electron

Lyman Series is in the UV range

Balmer series is visible

Paschen series is in IR range

Einstein determined that light behaved like a particle

“Particle” of light is the photon Photon is a quantum of light So light can behave as a wave and a

particle, which is it? Both

If light (a wave) can behave as a particle, can a particle behave as a wave?

Yes So electrons can be thought of as waves.

Uncertainty principle It is not possible to know the location and

momentum (speed) of an electron at the same time

Schrodinger equation Mathematically described sublevels and

orbitals