LIGHT and ELECTRONS Unit 6 Chemistry Langley. LIGHT and its PROPERTIES Pre-1900 Issac Newton explained light and its behavior by assuming light moved

LIGHT and ELECTRONS

Unit 6

Chemistry

Langley

LIGHT and its PROPERTIES

Pre-1900 Issac Newton explained light and its behavior

by assuming light moved in waves

1900 and beyond Experimental evidence began to convince

scientists that that light consists of particles (after the 1902 experiment of Max Planck)

1905-Einstein Dual Wave Particle Theory


Wavelength: distance between two points on two adjacent waves, symbol is (Greek symbol for lamda)

Frequency: number of waves that pass a given point in a given amount of time, symbol is (Greek symbol nu). Units for frequency are cycles per second which SI speaking is a Hertz, Hz (Hz is also a reciprocal seconds-1).


The frequency and wavelength of light are inversely proportional to each other. As the wavelength of light increases, the frequency

decreases As the wavelength of light decreases, the frequency

increases Amplitude: Wave’s height from zero to crest or wave’s

height from zero to trough (can be positive or negative) A complete wave cycle starts at zero goes through its

highest point, back through zero, reaches its lowest point, and back to zero again. One wave cycle starts at zero and has one crest

and one trough


According to the Wave Model, light consists of electromagnetic waves Electromagnetic radiation: light moving in

waves through space Radio waves, microwaves, infrared waves, visible

light, ultraviolet waves, X-rays, and gamma raysElectromagnetic spectrum

Speed of light: depending on the wavlength and frequency, speed of light changes

C = Speed of light in a vacuum = 3.0 x 108 m/s

SPEED of LIGHT PROBLEMS

EXAMPLE 1: Determine the speed of light if the wavelength

is 3.5 x 10-9 m/s and the frequency is 3.5 Hz.


EXAMPLE 2: If light has a speed of 5.6 x 103 m/s and a

frequency of 2.3 Hz, what is the wavelength.


EXAMPLE 3: What is the wavelength of radiation with a

frequency of 1.5 x 1013 Hz? Does this radiation have a longer or shorter wavelength than red light?


EXAMPLE 4: What frequency is radiation with a wavelength

of 5.00 x 10-8 m? In what region of the electromagnetic spectrum is this radiation?

PHOTOELECTRIC EFFECT(supporting work for Atomic Spectra)

The photoelectric effect is a quantum electronic phenomenon in which electrons are emitted from matter after the absorption of energy from electromagnetic radiation such as x-rays or visible light. The emitted electrons can be referred to as photoelectrons in this context. {Wikipedia.org}

PHOTOELECTRIC EFFECT (supporting work for Atomic Spectra)

Expected: Since all light is energy moving in waves, all colors of light should knock electrons off a metal Shine different color lights on a metal Measure the number of electrons knocked off

the metal Found that no electrons were knocked off when

light was below a certain frequency

MAX PLANCK(his work used in Atomic Spectra)

German Physicists, founder of quantum theory Studied the way light came off hot objects

(diffusion of hydrogen through heated platinum)

Concluded that light comes off in small burst of particles, NOT WAVES

Quantum-minimum amount of energy that can be lost or gained by an atom

To calculate quantum/energy: E = h E = energy of the photon h = Planck’s constant = frequency of the incident photon

ATOMIC SPECTRA

As atoms absorb energy, electrons move into higher energy levels. When the atoms release energy (lose the energy), the electron return to the lower energy levels.

The frequencies of light emitted by an element separate to give the atomic emission spectrum of the element No two elements have the same emission

spectrum

ATOMIC SPECTRA

Atomic line spectra and its existence was known before Bohr’s atomic model of hydrogen was produced. What Bohr did was explain why hydrogen had the specific frequencies it had, why it “produced/broke down” into the colors it did; it predicted the values that agreed with the experiements.

ATOMIC SPECTRA

Hydrogen Atom Line Emission Spectrum

ACTUAL:

Current passed through tube with Hydrogen gas.

Pink light is given off.

Light passed through spectrum.

Found only specific frequencies of light given off.

EXPECTED:

Continuous spectrum of light to be given off. (Since e- are moving around nucleus randomly and using different levels of energy.)

ATOMIC SPECTRA

Lowest possible energy of the electron is referred to as its ground state Normal location of an electron

Electrons circle the nucleus in specific orbits

If an electron absorbs energy, moves up an energy level (absorption)

If an electron gives off energy, moves down an energy level (emission)

QUANTUM MECHANICS

EINSTEIN, AGAIN!!!!!!!!!!!!!!!! Debate between whether light is waves or

particles Einstein creates dual waves particle theory

(1905) Light is small particles (photons) that move in

wave shapes Thought electrons moved around the nucleus

in wave shapes (since electrson are small particles like photons)

QUANTUM MECHANICS

Louis de Broglie: Given that light behaves as waves and particles, can particles of matter behave as waves? Referred to the wavelike behavior of particles as matter

waves Came up with an equation that predicts all moving objects

have wavelike behavior: mv/ = h Thanks to experiments conducted by 2 scientists, his theory

was proven correctNobel Prize Waves Waves have specific frequencies and electrons have

specific orbits/energy levels Waves and electrons can both be bent (diffraction) Waves and electrons can both overlap and interfere with

each other (interference) Creator of Wave Mechanics

QUANTUM MECHANICS

DeBroglie’s equation combines Einstein and Planck’s equations

mv/ = h (Anything with mass and velocity has a

wavelength, so electrons have wavelengths) DeBroglie Problems:

What is the wavelength of an electron that has a mass of 1.5 X 10-30 kg and a velocity of 2.5 X 104

m/s?

QUANTUM MECHANICS

DeBroglie Problems: What is the velocity of an electron with a mass of

8.3 X 10-29 kg and a wavelength of 400 nm? (Hint: convert nm to m)

What is the mass of an electron with a velocity of 4.6 X 103 m/s and a wavelength of 5.6 X 10-2

meters? What is the wavelength of an electron that has a

mass of 2.8 X 10-31 kg and a velocity of 3.0 X 108 m/s?

QUANTUM MECHANICS

Heisenberg 2 Goals in Life:

find the location of an electron find the velocity of an electron

Problem: Electrons cannot be seen under a microscope Only way to find an electron is to shoot a photon

(particle of light) at the electron Problem: when the photon hits the electron, it knocks

the electron off course So with this photon method, you can only know the

position of an electron for a split second, but you still don’t know the velocity

QUANTUM MECHANICS

Heisenberg DeBroglie: Tries to help Heisenberg and offers

his equation = (mv)/h If you know mass and wavelength of an

electron, equation could help you find velocity Problem: Equation does not show location! Equation method will only tell you velocity NOT

location

QUANTUM MECHANICS

Heisenberg Heisenberg Uncertainty Principle: It is

impossible to know both the position and velocity of an electron at the same time.

QUANTUM MECHANICS

Schrodinger Working with Hydrogen atom that only has 1 electron Wants to find general location/area of the one electron

in Hydrogen Creates quantum theory Quantum theory – uses math to describe the wave

properties of an electron (frequency, wavelength, etc) Once he plugged his data into the quantum theory, he

found that electrons do not travel in nice, neat orbits (Bohr model)

Instead, found that electrons travel in 3D regions around the nucleus

QUANTUM MECHANICS

Schrodinger Schrodinger’s equation is used to find the

greatest probable location/area of the Hydrogen atom electron (in the ground state)

QUANTUM MECHANICS

Quantum Theory Ground State-normal location of an electron Excited State-one ring up from the normal location When excited electron falls back to the ground state, a

photon is given off Energy of the photn is equal to the difference in energy

between the excited state and ground state Hydrogen gives off specific colors because its electrons

move from ring 2 to ring 1; Neon gives off a different color because its electrons move from ring 3 to ring 2

LIGHT AND ELECTRONS REVIEW

Light was first thought to be wavelike Equation for the speed of light is c = Photoelectric effect challenges this because only certain

frequencies of light could knock off electrons Max Planck’s experiment proved that light could be a

particle Einstein’s dual wave particle theory says that light is

ACTUALLY small particles (photons) that move in wave like patterns

Equation for energy of a photon is E = h Bohr found that electrons orbit the nucleus in specific

orbitals/energy levels

LIGHT AND ELECTRONS REVIEW

Electrons as Waves: 1924 – Louis de Broglie asked “Could electrons have a

dual wave particle nature like light?” Similarities between waves and electrons Waves have specific frequencies and electrons have

specific orbits/energy levels Waves and electrons can both be bent (diffraction) Waves and electrons can both overlap and interfere with

each other (interference) DeBroglie’s equation combines Einstein and Planck’s

equations mv/ = h (Anything with mass and velocity has a wavelength, so

electrons have wavelengths)

QUANTUM NUMBERS and ATOMIC ORBITALS

REVIEW Energy levelsSpecific energies electrons can have Quantum of energyamount of energy required to

move an electron from one energy level to another energy level

The amount of energy an electron gains or loses in an atom is not always the same

Energy levels in an atom are not equally spaced Higher energy levels are closer together Modern description of the electrons in atoms, quantum

mechanical model, comes from the mathematical solutions to the Schrodinger equation

The quantum mechanical model determines the allowed energies an electron can have and how likely it is to find the electron in various locations


QUANTUM NUMBERS Quantum numbers are used to describe the

location and behavior of an electron (zip code for electrons)

First quantum number = Principal = n Second quantum number = Angular Momentum Third quantum number = Magnetic Quantum Fourth quantum number = Spin Quantum


Principal (first) quantum number = n Main quantum number Describes the size of the electron cloud (the smaller the number, the

smaller the cloud) ALSO, shows the distance from the nucleus, the smaller the

number, the closer the cloud is to the nucleus Called energy levels or shells Positive integers (1,2,3,4,…) Symbol is n Each energy level has a maximum number of electrons it can hold

n # Electrons1 22 83 184 32

Example: Energy level 1 2 electrons

close to the nucleus small electron cloud


Second Quantum Number: Each energy level has sublevels The number of sublevels is equal to n Example: Energy level 1 has 1 sublevel Sublevels are called: s,p,d,f


Third Quantum Number Divides sublevels into orbitals Also tells the shape the electron is moving in The number of orbitals for each level is:

S has 1 P has 3 D has 5 F has 7

The number of orbitals for an energy level is equal to n2

Example: 2nd Energy level n2 = 4 1s, 3p

Each orbital can only hold a maximum of 2 electrons Shapes of orbitals:

S is spherical P is peanut shaped D is daisy shaped F is unknown


Fourth Quantum Number: Describes the electron spin Both electrons in an orbital are negative, so

they repel each other and spin in opposite directions

Use arrows to represent electrons


Pauli Exclusion Principle: No two electrons in an atom can have the

same set of 4 quantum numbers because electrons repel each other

Example: 2 electrons may both be: in the first energy level (same first number) sitting in an s sublevel (same second # moving in a sphere shape (same third #) BUT one electron spins clockwise and one spins counter clockwise ( which means they

have different fourth #s)

ELECTRON CONFIGURATIONS

Example 1: Map out the quantum numbers for all the electrons in Hydrogen Find the # of electrons in hydrogen (atomic

number will give you this number) What order do you fill in s, p, d, f in the rings?


Diagonal RulePattern that shows the order the electrons fill in the orbitals: Some People Do Forget

1s2s 2p3s 3p 3d4s 4p 4d 4f5s 5p 5d 5f6s 6p 6d7s 7p

Notice that the electrons do not fill in all of the level 3 first (3s, 3p, 3d) and then move to level 4

Instead, electrons fill in the orbitals in the order that is easiest to them (easier for an electron to fill in a 4s before it fills in a 3d)

Aufbau Principle: Electrons have to fill in the lowest (easiest) energy level or orbital first


Hund’s Rule: Every orbital must get

one electron first, before you double up. “Cookie Rule”

Example 2: Helium


Example 3: Lithium Example 4: Fluroine


Orbital Notation drawing out

configurations with arrows

Electron Configuration Notation/Superscript Notation: writing configurations

with superscripts to represent electrons


Do Orbital Notation and Electron Configuration for the following:

Zn I Cl Mg As

NOBLE GAS CONFIGURATIONS

Noble Gas Configurations: Write out the superscript notations for:

Neon:Sulfur:Sulfur has the same configuration as Neon plus a 3s23p4

So, you could use the noble gas as a shortcut and write Sulfur’s configuration as

[Ne] 3s23p4 OR [Ne] Noble gas configuration: write the noble gas (group 18) that comes

directly before the element in question and then add the rest of the configuration

Practice: Write the noble gas superscript notation for the following elements.

C Np W Sn

DOT DIAGRAMS

Lewis Dot Diagrams: Way to show the number and position of

electrons on the outermost energy level Since the energy levels all overlap and cover

one another, only the outermost energy level is able to bond with other elements

The electrons involved in bonding are called the valence electrons (to get these electrons look at the column number)

DOT DIAGRAMS

Lewis Dot Diagrams: Chemical symbol + Number of valence

electrons The rules for orbitals still apply, so no side can

have more than two dots, and each “p” orbital side gets one dot, before you double up

X

p1

p3

p2

sp orbitals

s orbital

DOT DIAGRAMS

Noble gases have a full valenceThere are no empty spaces so the

element does not need any more electronsStable octet – 8 electrons in the valence

so the element does not want to bond (this means it is stable)

Only the noble gases have a stable octet

DOT DIAGRAMS

Practice: Write the noble gas superscript notation and then draw the dot diagram for the following: V Br Al K

Documents

LIGHT and ELECTRONS Unit 6 Chemistry Langley. LIGHT and its PROPERTIES Pre-1900 Issac Newton explained light and its behavior by assuming light moved