Light

The Nature of Light Visible light is one type of electromagnetic

radiation (EM). Other types include: x-rays, microwaves, and

radiowaves Light has dual nature

Wave (moves through space as a wave) and particle (has energy)

Wave Nature of Light When thinking of light as a wave we must

consider: Frequency (ν, Greek nu) = how fast the wave

moves up and down per second (s-1, also Hz (hertz))

Wavelength (λ, Greek lambda) = distance between waves; expressed in meters (m)

How fast does a wave move? Depends on frequency (ν) and wavelength (λ) In a vacuum waves travel at 3.0 x 108 m/s

This is called the speed of light (c) c = ν x λ

Particle Nature of Light We need a little help from

Einstein showed that different colors of light have different amounts of energy and it takes a certain amount of energy for e- to be emitted.

How to calculate the energy Well here’s the formula: E(energy) = h(planck’s constant) x ν (frequency)

Rutherford’s Model How do we picture an atom? The nuclear model

Mimicks solar system: “sun” = nucleus “planets” = electrons

Uh-Oh! Rutherford…we have a problem! The e- moves in a circular path, is constantly

accelerating, constantly changing direction, radiates energy, orbit decays, and then SMASH (into the nucleus)!!!

Rutherford’s atom should collapse instantly.

So, how do we make sense of it all? Lets take a look at atoms with light.

When you look through a prism or diffraction grating, you see a rainbow that is continuous (no breaks in color)

When you look through a diffraction grating at a light given off by energized atoms, you see a discontinuous spectrum (breaks in color)

Bright-Line Emission Spectrum Each element has a unique…

emission spectrum; the simplest is hydrogen. Each element and compound have a unique

absorption pattern, too.

Bohr atom Niels Bohr advanced a model for hydrogen

that accounted for hydrogen’s line spectrum and addressed the flaw in Rutherford’s model.

Bohr retained the “solar system” image Dense nucleus and e- circling the nucleus

Bohr added… e- orbits could only have certain sizes and

energies, other orbits were forbidden Bohr quantized the model of the atom

Bohr’s model

A closer look at Bohr’s model… The lowest energy value the e- can have is n

= 1 This is most stable energy state; called the

“ground state”

When an e- absorbs quantum energy it leaps to a higher energy level (“excited state”). The e- returns to ground state, emitting energy.

A closer look at Bohr’s model… The emitted energy comes out as a photon

that corresponds to the energy difference allotted for a given orbit.

n = 5 → n = 2 (violet) n = 4 → n = 2 (blue-green) n = 3 → n = 2 (red) n = 6 → n = 2 (barely visible)

And…another one bites the dust! The electron does return to n = 1, but that

transition is so energetic that the photon is in the ultraviolet region.

Bohr’s model only works for hydrogen.

The wave mechanical model 1924 - Louis deBroglie shows that the e- has

wave properties. 1926 -Erwin Schrödinger applied the

mathematics of waves to electrons. e- circles all around nucleus Found that a whole # of e- fit the orbits described

by Bohr for hydrogen

What about the space between the orbits? Between the "orbits", the wave would not fit

right; when it circled around to its starting point it would not meet up correctly. Such a wave would self-destruct

Schrödinger worked out the math that described the three-dimensional waveforms of the electron at various energies. The waveforms are called orbitals to

distinguish them from orbits (orbits can be calculated and predicted)

A closer look at an orbital Orbitals are waveforms.

Can’t pinpoint location of e-

Probability region ( 3D space in which most likely to find e- )

Most useful to look at e- in terms of energy Relates to wave and particle nature (dual nature) So long to “solar system” model

Particle personality = the electron is just somewhere within the cloud (position and velocity unknown)

Wave personality =the electron is the cloud, a standing wave of electron energy electrons.

Waveforms are identified by three characteristics electrons. Principle energy level (n) = describes the size of

the electron wave (how far it extends from the nucleus). n increases 1 to ∞ (orbital energy increases).

Each energy level contains n sublevels. Level 1 = one sublevel (s) Level 2 = two sublevels (s, p) Level 3 = three sublevels (s, p, d) Level 4 = four sublevels (s, p, d, f)

Each sublevel contains 1 or more orbitals s = 1 orbital p = 3 orbitals *Each orbital holds 2 e-

d = 5 orbitals

s orbital and p orbital

Aufbau Principle and e- configuration Aufbau (“to build up”) Principle - the electron

is most stable in the ground or lowest energy state.

To describe energies of all e- in atom , we need to specify waveforms we can use e- configuration.

Let’s practice e- configuration…

Write the e- configuration of: a)Beb)Bc)C

The Periodic Table and e- configuration

The Periodic Table and e- configuration

The Periodic Table and e- configuration