Atomic Theory

Democritus – 460 – 370 BC

• Matter consisted of tiny particles “atomos”

• Ideas were wrong

John Dalton (1803)

Dalton’s Theory – Explains the Law of Conservation of Matter

Atoms are separated, combined or rearranged in a reaction, they are not created, destroyed or divided.

What is an atom?

• The smallest particle of an element that retains its original properties

Discovering the Existence of the Electron – JJ Thompson

Cathode Ray Tube – Led to the TV

Particles had a negative charge.

Rays consisted of particles.

All types of gases and cathodes produced a beam of particles.

JJ Thomson – 1890s

• Found that the mass of the particle in the cathode ray tube was smaller than the mass of the Hydrogen atom.

Which part of Dalton’s theory did he prove wrong?

X

Robert Millikan – 1909 – Oil Drop Experiment

Determined the mass and charge of an electron.

• Oil droplets of different masses

• Dropped between charged plates

• Became negatively charged

• Drops fell due to gravity

• Negatively charged lower plate repelled them and they became stationary

• The magnitude of the charge could be calculated from the known voltage and mass.

• If electrons are part of all matter and they possess a negative charge, why is matter neutral?

• If the mass of an electron is so small, what accounts for the rest of the mass in an atom?

More Questions

JJ Thomson’s Answer – Plum Pudding Model

Ernest Rutherford’s Experiment - 1911

Evidence Contradicts the Plum Pudding Model

Rutherford Concluded:

1. Atoms consist mainly of empty space in which electrons move freely.

2. A tiny dense space in the center of the atom contains the majority of the mass and the positive charges.

3. The positive charge of the nucleus holds the negative electrons within the atom.

• If electrons are part of all matter and they possess a negative charge, why is matter neutral?

1st Question Answered

Protons and Neutrons

• Rutherford (1920)– Protons are positively charged and found in

the nucleus

• James Chadwick (1932)– Neutrons are found in the nucleus, has no

charge and has a mass equal to the proton

Space Between Atoms - Video

Second Question Answered:

If the mass of an electron is so small, what accounts for the rest of

the mass in an atom?

What do atoms look like?

Silicon Atoms

How can we take pictures of atoms?- Video

Atomic Number

• Henry Moseley

• Each element has a unique positive charge

• Equal to # of Protons and Electrons

Mass Number and Atomic Number

• Mass number = protons + neutrons

1 2 3

1H 1H 1H

Protium Deuterium Tritium

Mass number Atomic number

1 proton 1 proton

1 neutron

1 proton

2 neutrons

Isotopes – atoms with the same number of protons, but a different number of neutrons

Atomic Mass Unit

1 AMU = 1/12 the mass of

the Carbon Atom

1 AMU ≠ mass of a proton or a neutron

Average Atomic Mass

• Chlorine-35 (34.969 amu x 75.770%)• Chlorine-37 + (36.966 amu x 24.230%)

35.453 amu

• Chlorine’s average atomic mass is 35.452 amu. Chlorine-35 has an atomic mass of 34.969 and Chlorine-37 has an atomic mass of 36.966. What are the percentages of each isotope?

Electrons in Atoms

Problems with Rutherford’s Model

Chlorine # 17

Reactive

Potassium # 19

Very reactive

Argon # 18

Not reactive

The Quest for a Better Model• Electromagnetic radiation behaves like a

wave. (video)

Characteristics of a Wave

Wavelength = λ

Frequency = v (number of waves that pass a point per second)

1 Hertz (Hz) = 1 wave per second (SI Unit for frequency)

Speed and Frequency of Light

c = λv

c = speed of light (3.0 x 108 m/s)

↑ wavelength ↓ frequency

↓ wave length ↑frequency

What is the relationship between energy and frequency?

Microwave Oven - Simulation

Problems

Light: Particle or Wave?

Wave model doesn’t address:

Why heated objects emit only certain frequencies of light at a given temperature?

Why some metals emit electrons when a colored light of a specific frequency shines on them?

Iron

• Dark gray = room temp• Red = hot temp• Blue = extremely hotter temp

• ↑ temp, ↑ kinetic energy, emit different colors of light

• Wave model could not explain this

Max Planck - 1900Matter gains or loses energy only in small, specific amounts called quanta

quantum is the minimum amount of energy that can be gained or lost by an atom

Equantum = hv

h – Planck’s constant – 6.626 x 10-34 J·s

J = joule, SI Unit for energy

Photoelectric Effect – The problem with wave theory. - Simulation

Only certain frequencies of light could emit an electron from a plate of Ag.

Accumulation of low frequencies couldn’t

Einstein and the Dual Nature of EMR (1900)

• EMR acts as a wave of individual particles (photon)

Ephoton = hv

Calculating the energy in a Photon

Ephoton = hv

E = (6.626 x 10-34 J·s) x (7.23 x 1014 s-1)

E = 4.79 x 10-19 J

Atomic Emission Spectra

The frequencies of the EMR emitted by atoms of the element.

Unique to each element

Flame Test Demo

Niels Bohr - 1913• Worked in Rutherford’s lab

• Proposed a quantum model of the atom

• Explain why emission spectra were discontinuous

• Predicted frequencies of light in Hydrogen’s atomic emission spectra

Bohr’s Explanation• Ground state – lowest energy state of an atom• Excited state – when an atom gains energy• Electrons move in circular orbits

– Smaller orbit – lower energy state, “energy level”– Larger orbit – higher energy state, “energy level”

An explanation for the Emission Spectra

Ground state

Excited States

Atoms absorb energy and are excited. As the electron returns to the

ground state they give off energy “photon” equal to the difference in energy

levels.

e-

photon

Electron in ground state

Electron absorbs energy

e-

Electron in Excited State

Energy released “photon”

Returns to Ground state

Atomic Spectra - Simulation

Problem: Bohr’s Model Only explains Hydrogen

• Louis de Broglie (1924) – proposed that the energy levels are based on the wave like nature of electrons

Heisenberg Uncertainty Principle

• It is impossible to know the velocity and position of a particle at any given time

(Video)

Photon and electron are about the

same mass.

Erwin Schrodinger - 1926

• Developed the quantum mechanical model of the atom– Assigns electrons to energy levels like Bohr– Does not predict the path of the electron – It predicts the probability of finding an electron

• An electron’s “atomic orbital”

(Video – Quantum Atom)

Each dot is a picture of an electron during

a given amount of time.

Where does the electron spend most

of the time?

Boundary represents the location of an

electron 90% of the time.

Principle Energy Levels

• 7 energy levels

• Lowest energy is 1 – greatest energy 7

• Each level consists of sublevels

The second energy level is larger and the electrons are farther from the nucleus.

Types of Sublevels

Same energy

Putting it together:

Principal quantum number “energy

level”

Sublevels (Types of orbitals and total number)

1 S – 1

2 S – 1, P – 3

3 S – 1, P – 3, D – 5

4 S – 1, P – 3, D – 5, F - 7

Ground State Electron Configurations

• Most stable – lowest energy

• 3 principals to follow

Aufbau Principle – each electron must occupy the lowest energy state

1.Orbitals in an energy sublevel have equal energy

2.The energy sublevels in a principle energy level have different energies.

3.The sublevels increase in energy from s,p,d,f

4.Principal energy levels can overlap

Aufbau Diagram

Equal energies – 2 p

Sublevels have different energy levels

Energy levels overlap

• Each electron spins

• Electrons must spin in opposite directions

• 2 electrons per orbital

Pauli Exclusion Principle

Written as

Hund’s Rule

Electrons must occupy each orbital before additional electrons can be added.

Pauli Exclusion Principle

Representing Electron Configurations

Electron Configurations

Sub level diagram – indicates the order that orbitals are filled

What are the orbital diagrams and electron configuration notation for

Al and Cl?

Electron Configuration Shorthand

• Substitute noble gases from preceding energy levels in the notation

Li – [He] 2s1

C – [He] 2s2 2p2

Valence Electrons

• Electrons in the outer most energy levels

S [Ne] 3s2 3p4

Sulfur has 6 valence electrons

How many valence electrons

do Al, Ne, and Cl have?

Electron Dot Structures

Valence electrons are

used in reactions and are

represented by an electron dot

structure.

Writing Electron Dot Structures

• Fill the valence electrons 1 at a time in any particular order.

Ca C

O

**

***

*

******

What are the electron dot diagrams for K, Ar and F?

Modern Atomic Theory

• Any electron in an atom can be described by 4 quantum numbers

• Principal Quantum Number

• Azimuthal Quantum Number

• Magnetic Quantum Number

• Spin Quantum Number

Principal Quantum Number (n)

Related to the size and energy of principal energy level.

The farther away from the nucleus the more energy the electron has

1 < 2 < 3 < 4 < 5 < 6 etc….

Azimuthal Quantum Number (Angular Momentum) = l

• Refers to the subshells in each principal energy level (n)

• S = 0• P = 1• D = 2• F = 3

n l

1 0

2 0

1

3 0

1

2

4 0

1

2

3

Magnetic Quantum Number (ml)

• Specifies the orbital within a energy level where an electron is likely to be found

n l Orbital designation

ml

1 0 1s 0

2 0 2s 0

1 2p -1,0,+1

3 0 3s 0

1 3p -1,0,+1

2 3d -2,-1,0-,1,2

4 0 4s 0

1 4p -1,0,+1

2 4d -2,-1,0-,1,2

3 4f -3,-2,-1,0,1,2,3

Spin Quantum Number (ms)

• + ½ or – ½

• Electrons in the same orbitals must have opposite spins (Pauli Exclusion Principle)

n l Orbital designation

ml ms

1 0 1s 0 + ½, - ½

2 0 2s 0 + ½, - ½

1 2p -1,0,+1 + ½, - ½

3 0 3s 0 + ½, - ½

1 3p -1,0,+1 + ½, - ½

2 3d -2,-1,0-,1,2 + ½, - ½

4 0 4s 0 + ½, - ½

1 4p -1,0,+1 + ½, - ½

2 4d -2,-1,0-,1,2 + ½, - ½

3 4f -3,-2,-1,0,1,2,3 + ½, - ½

n l ml ms

2 1 -1 + ½ or

What are the quantum numbers for A? B?

A B