Visible light is a form of electromagnetic radiation

Radiation carries

energy through

space

6.1 The wave nature of light6.1 The wave nature of light

Chapter 6Chapter 6 Electronic structure of atoms Electronic structure of atoms

Electromagnetic radiation can be imagined as a self-propagating

transverse oscillating wave of electric and magnetic fields.

The number of waves passing a given point per unit of time is the frequency

For waves traveling at the same velocity, the longer the wavelength, the

smaller the frequency.

All electromagnetic radiation travels at the same velocity

The wavelength and frequency of light is therefore related in a

straightforward way:

Q. What is the wavelength of UV light with v = 5 x 1015 s-1?

Q. What is the frequency of electromagnetic radiation with a wavelength of 0.5 m?

Thomas Young’s sketch

of two-slit diffraction of

light (1803)

6.2 Quantized Energy and Photons6.2 Quantized Energy and Photons

1. Blackbody radiation

2. The photoelectric effect

3. Emission spectra

Wave nature of light successfully explains a range of different phenomena.

Hot Objects and the Quantization of EnergyHot Objects and the Quantization of Energy

Heated solids emit radiation (blackbody radiation)

In 1900, Max Planck investigated black body radiation,

and he proposed that energy can only be absorbed or

released from atoms in certain amounts

The relationship between energy, E, and frequency is:

The Photoelectric Effect and PhotonsThe Photoelectric Effect and Photons

The photoelectric effect provides evidence for the particle nature of light and

for quantization.

Light shining on the surface of a metal can cause

electrons to be ejected from the metal.

Einstein proposed that light could have particle-like properties, which he called

photons.

Below a threshold frequency

no electrons are ejected

Light has wave-like AND particle-like properties

Q. MRI body scanners operate with 400 MHz radiofrequency energy. How much

energy does this correspond to in kilojoules/mol?

Q. A mole of yellow photons of wavelength 527 nm has __________ kJ of energy.

6.3 Line Spectra and the Bohr Model6.3 Line Spectra and the Bohr Model

Line spectraLine spectra

Radiation composed of only one wavelength

is called monochromatic.

When radiation from a light source, such as a light

bulb, is separated into its different wavelength

components, a spectrum is produced,

White light passed through a prism

provides a continuous spectrum

BohrBohr’’s Models Model

Rutherford assumed that electrons orbited the nucleus

analogous to planets orbiting the sun; however, a charged

particle moving in a circular path should lose energy

Niels Bohr noted the line spectra of certain elements and assumed that

electrons were confined to specific energy states.

1. Only orbits of specific radii are

permitted for electrons in an atom

2. An electron in a permitted

orbit has a specific energy

3. Energy is only emitted or absorbed

by an electron as it moves from one

allowed energy state to another

The Energy States of the Hydrogen AtomThe Energy States of the Hydrogen Atom

Colours from excited gases arise because

electrons move between energy states in the

atom.

Bohr showed mathematically that

where n is the principal quantum number

(i.e., n = 1, 2, 3…) and RH is the Rydberg

constant.

The first orbit in the Bohr model has

n = 1 and is closest to the nucleus.

Electrons in the Bohr model can only move between orbitsby absorbing and emitting energy in quanta (E = h!).

The ground state = the lowest energy state

The amount of energy absorbed or emitted by moving between states is given by

Q. When the electron in a hydrogen atom moves from n = 6 to n = 2, is light

emitted or absorbed?

Q. What is its wavelength in nm?

However, the model introduces two important ideas:

1. the energy of an electron is quantized: electrons exist only in certain energy

levels described by quantum numbers

Limitations of the Bohr ModelLimitations of the Bohr Model

The Bohr Model has several limitations:

Louis de Broglie posited that if light can have material properties, matter

should exhibit wave properties

6.4 The wave behavior of matter6.4 The wave behavior of matter

de Broglie proposed that the characteristic wavelength of the electron or of

any other particle depends on its mass, m, and on its velocity, v

Matter waves is the term used to describe wave characteristics of material

particles.

The Uncertainty PrincipleThe Uncertainty Principle

Q. What is the wavelength of a bullet (7.5 g) traveling at 700 ms-1?

Q. At what speed must a 3 mg object be moving in order to have a de Broglie

wavelength of 5.4 ! 10-29 m?

sets a fundamental limit on how precisely we

can know the location and momentum of an object.

Heisenberg imagined a gamma ray microscope

to explain his uncertainty principle.

Heisenberg related the uncertainty of the position, !x, and the uncertainty in

momentum !(mv) to a quantity involving Planck’s constant:

Erwin Schrödinger proposed an equation containing both wave and particle

terms. The solution of the equation is known as a wave function, " (psi).

6.5 Quantum Mechanics and Atomic 6.5 Quantum Mechanics and Atomic OrbitalsOrbitals

OrbitalsOrbitals and quantum numbers and quantum numbers

If we solve the Schrödinger equation we get wave functions and corresponding

energies.

The probability density (or electron density) described by an orbital has a

characteristic energy and shape. The energy and shape of orbitals are described

by three quantum numbers. These arise from the mathematics of solving the

Schrödinger equation.

Q. Tabulate the relationship among values of n, ℓ and mℓ through n = 4.

must be a positive integer n = 1,2,3,4,…

maximum value is (n-1), i.e. ℓ = 0,1,2,3…(n-1)

use letters for ℓ (s, p, d and f for ℓ = 0, 1, 2, and

3).

maximum value depends on ℓ, can take integral

values from – ℓ to + ℓ

Orbitals can be ranked in terms of energy.

6.6 Representations of 6.6 Representations of OrbitalsOrbitals

The The ss orbitalsorbitals

• All s orbitals are spherical

• As n increases, the s orbitals get larger

• As n increases, the number of nodes increases

The The pp orbitalsorbitals

• p orbitals are dumbell-shaped

• 3 values of mℓ

The The dd orbitalsorbitals

d orbitals have two nodes at the nucleus

6.7 Many-Electron Atoms6.7 Many-Electron Atoms

OrbitalsOrbitals and Their Energies and Their Energies

In a many-electron atom, for a given value of

n, the energy of an orbital increases with

increasing value of ℓ

Therefore, the energy-level diagram looks

slightly different for many-electron systems

Electron Spin and the Pauli Exclusion PrincipleElectron Spin and the Pauli Exclusion Principle

Line spectra of many-electron atoms show each line as a closely spaced pair of

lines.

Stern and Gerlach designed an experiment to determine why. A beam of atoms

was passed through a slit and into a magnetic field and the atoms were detected:

Two spots were found,

corresponding to silver

atoms with electron

spinning one way or the

other

Pauli’s exclusion principle states that:

6.8 Electron Configurations6.8 Electron Configurations

Electron configurations tell us how the electrons are distributed among the

various orbitals of an atom.

When writing ground-state electronic configurations:

How do we show spin?

HundHund’’ss Rule Rule

“For degenerate orbitals, the lowest energy is attained when the number of

electrons with the same spin is maximized”

Q. Draw the electron configurations of Li, Be, B, C, N, O, Ne and Na.

Condensed Electron ConfigurationsCondensed Electron Configurations

Electron configurations may be written using a shorthand notation (condensed

electron configuration):

Q. Draw the condensed electron configurations of Li, Na and P.

1. Write the core electrons corresponding to the noble gas in square brackets

2. Write the valence electrons explicitly

Transition MetalsTransition Metals

The block of the periodic table in which the d orbitals are filling represents the

transition metals.

6.9 Electron Configurations and the Periodic Table6.9 Electron Configurations and the Periodic Table

The periodic table can be used as a guide for electron configurations.

Blocks of elements in periodic table relate to which orbital is being filled

Anomalous Electron ConfigurationsAnomalous Electron Configurations

There are elements that appear to violate the electron configuration guidelines:

When atomic number > 40, energy differences are small and other anomalies

often occur. These usually act to reduce electron repulsions.