THE DEVELOPMENT OF ATOMIC THEORY

Chemistry Rules!

A time when logic ruled the land…

The Philosophical Era (Circa 500~300BCE)

Philosophical Era (Ancient Greece)

o Two ancient Greeks stand out in the advancement of chemistry.

o Their ideas were purely based on logic, without experimental support (as was common in that time)

Democritus (460-370 BCE)

o The most well-known proponent of the idea that matter was made of small, indivisible particles

o Called the small particles “atomos” meaning “that which cannot be divided”

o Believed properties of matter came from the properties of

the “atomos”

Philosophical Era

Aristotle (384-322 BCE)

o Famous philosopher of the ancient Greeks

o Believed matter was comprised of four elementso Earth, Air, Fire, Water

o These elements had a total of four propertieso Dry, Moist, Hot, Cold

o People liked him – so this idea stayed

Philosophical Era

Philosophical Era

The “Dark Ages” of Chemistry where early chemists had to work in secret and encode their findings for fear of persecution

Alchemical Era (300 BCE ~ 1400CE)

Alchemy

o the closest thing to the study of chemistry for nearly two thousand years

o based on the Aristotelian idea of the four elements of mattero If you change the properties, then you

could change elements themselves – lead to gold and immortality

o Very mystical study and experimentation with the elements and what was perceived as magic

o Study was persecuted, findings hidden in code

Alchemical Era

Procedures of Alchemy

o Alchemy brought about many lab procedures

o We use some of the same methods and the names developed in these dark ages of chemistry

Alchemical Era

Elements in Alchemy

o Alchemists studied many different materials, and their properties, in order to find a way to turn lead into gold and achieve immortality

Alchemical Era

Alchemy had to be discussed in secret so that its students could avoid persecution

Alchemical symbols for various materials

Alc

he

mic

al

Era

Alchemists’ Persecution

o Alchemy was tied to witchcraft and druidso it was perceived as heresy by the catholic churcho Practitioners had to hide their trade or hobby

o Information was passed in codeo Coded messages were sent between friendso Symbols were used to avoid readable words

o The growth of Chemistry was stunted by the oppression endured during this era

(No such problems in the Far East –Hence gunpowder)

Alchemical Era

The printing press heralds the widespread transfer and acquisition of knowledge

The Classical Era (1400CE – 1887CE)

Foundations

o Robert Boyle departs from Aristotle (1661)o Suggested in A Skeptical Chymist a substance was

not an element if it was made of more than one component

o Antoine Lavoisier (1743-1794)o Accepted Boyle’s idea of elementso Developed the concept of compoundso Determined Law of Conservation of Mass

o Law: There is no change in mass due to chemical reactions

o Discovered Oxygeno Recognized Hydrogen as an element

Classical Era

Foundations (continued)

o Joseph Proust (1790s)o Determined the Law of Definite Proportions

o Elements combine in definite mass ratios to form compounds

Classical Era

Robert BoyleIrish

Antoine Lavoisier

(and wife) French

Joseph ProustFrench

John Dalton [really famous] (1766-1844)

o Dalton returns to Democritus’ ideas in 1803 with four postulatesI. All matter is made up of tiny particles called

atomsII. All atoms of a given element are identical to

one another and different from atoms of other elements

III. Atoms of two or more different elements combine to form compounds. A particular compound is always made up of the same kinds of atoms and the same number of each kind of atom.

IV. A chemical reaction involves the rearrangements, separation, or combination of atoms. Atoms are never created or destroyed during a chemical reaction.

Classical Era

John DaltonEnglish

(Originally poor and self-

educated)

Defense of Atoms (After Dalton)

o Joseph Gay-Lussac (1778-1850)o 2L hydrogen (g) + 1L Oxygen (g) 2L Water

Vapor (g)o Experimental findings disagreed with some of

Dalton’s beliefs

o Amadeo Avogadro (1776-1856)o Suggested Hydrogen and Oxygen are diatomic

moleculeso This solved the riddle over Gay-Lussac’s

experimental results

Classical Era

Joseph Gay-LussacFrench

Amadeo AvogadroItalian lawyer

Dalton’s Disbelief

o Dalton refused Avogadro's Diatomic moleculeso Dalton wrongly believed that similar types

of atoms would repel, like poles of a magnet – hence no diatoms

o Due to Dalton’s reputation in chemistry, his ideas were believed over Avogadro’s

o Sustaining Dalton’s (wrong) theory, that mass corresponded to amount of atoms, led to confusiono Avogadro’s ideas lived on in Italy (south of

the Alps)

Classical Era

Avogadro’s Number

o In 1860 a council of chemists met to solve the problems they had standardizing atomic masseso This was only a problem because they kept Dalton’s

idea instead of Avogadro’so An Italian chemistry teacher, Cannizzaro,

presentedo His teaching pamphlet used simple math based on a

corollary of Avogadro’s theory– Avogadro's Numbero Avogadro's Number grouped atoms into moles:

6.022×1023 parts = 1mole (6.022×1023parts/mole)

Classical Era

Mendeleev’s Table (1869)

o Once a standard for atomic masses was made, people started to see trendso These trends showed that properties

gradually changed with atomic mass, but seemed to cycle periodically

o Dmitri Mendeleev was a Russian teachero He arranged the elements in a table so that

his students could learn more easilyo Listed atoms by atomic masseso New columns whenever the properties cycledo Empty spots left – He predicted undiscovered

elements

Classical Era

Dmitri MendeleevRussian teacher

Here is a black and white copy of the manuscript, and an English textbook version

Mendeleev’s table quickly became famous

Cla

ssic

al

Era

**Don’t Forget Newton!!! (1643-1727)

o Isaac Newton was very important to scienceo He is most remembered for his contributions to physics,

including gravity and much work in optics (light)o He was the first person to divide white light into its partso Splitting light into parts lead to many interesting

discoveries

Classical Era

The relatively quick discovery of things smaller than the once “indivisible” atom

The Subatomic Era (1897CE – 1932CE)

It’s Electric!

o Electricity was studied throughout the classical era

o Ben Franklin’s kite in a thunderstorm (1752)o Electricity could flow through gasses

(atmosphere)

Subatomic Era

Cathode Ray Tubes

o Glass chambers used to study electricity in gasseso Crooke observed glowing rays

emitted from the cathodeo Glowing rays were observed in all

gasses, and even gasless set-ups

Subatomic Era

J.J. Thompson English (1897)

o Subjected cathode rays to magnetic fieldso Using three different arrangements of CRTs he was able to

determine that the Cathode rays…o Were streams of negatively charged particleso Those particles had very low mass-to-charge ratios

o The observed mass-to-charge ratio was over one thousand times smaller than that of hydrogen ionso The CRT particles had to be much lighter than hydrogen

and/or very highly charged

Subatomic Era

Robert Millikan American (1909)

o Thompson needed to know either the mass or the charge of his negative particles to describe them

o Millikan’s oil drop let him find that the charge on objects is always some multiple of 1.60×10-19Co He proposed this as the basic increment of chargeo Applying this charge to Thompson’s particles, he

found the mass to be much less than any atom

Subatomic Era

Plumb Pudding Model (1904)

o With the combined work of Thompson and Millikan the first subatomic particle was established!o Electrons – one part of an atom with one

negative fundamental increment of electrical charge

o Since whole atoms were known to be electrically neutral, Thompson developed the plumb pudding model of the atom

Subatomic Era

Negatively (-) Charged electrons

Positively (+)

charged majority

Ernest Rutherford New Zealander (1910)

o Rutherford worked with radiation and had heard of Thompson’s plumb pudding modelo He wanted to use radiation to prove

Thompson’s model o He set-up an alpha particle gun (with

help from Marie Curie) to shoot at an ultra-thin piece of gold foil, with a Geiger counter on the other side

Subatomic Era

Ernest RutherfordNew Zealand

Marie CuriePolish/ French

Rutherford’s Results

o Rutherford’s results were not what he expectedo Expected to have all alpha particles go

straight through all of the atomso Saw that occasionally an alpha particle

would ricochet o Determined the positive charge of an

atom must be held in a massive, centrally located, “nucleus”

Subatomic Era

The Second Subatomic

o After more realizations and experiments the second subatomic particle was formally named (1911)o Through more Nuclear physics Rutherford determined

all atomic nuclei were made up of hydrogen nuclei o Hydrogen nuclei are deemed Protonso Antonius van den Broek suggested elements on the

periodic table are in order by their increasing number of protons, not Mendeleev’s atomic masses

o Proton: The massive subatomic particle, within the nucleus of an atom, with a single positive charge

Subatomic Era

The Planetary Model (1911)

o Earnest Rutherford took his idea of a nucleus, and the known electrons, to construct a new atomic modelo There is a compact nucleus

o The nucleus, made of nucleons, is the location of positive charge in the atom

o The charge of the nucleus might be proportional to its mass

o The orbit of the electrons kept them from falling directly into the nucleus, just like planetary motion

Subatomic Era

The Rutherford Model

orThe Planetary

Model

The Third Subatomic (1932)

o Electrons and Protons were identified as particles, but these alone could not fully describe atomso The charge-to-mass ratio of atoms

was off without another additiono James Chadwick studied an

unnamed form of radiation– he found it to be electrically neutral and about the mass of a protono Including these particles in the

nucleus of the atom solved all discrepancies that were previously observed

Subatomic Era

James ChadwickEnglish

Subatomic Review

o Electronso Orbit the nucleuso Very small mass: 9.10938215×10−31 kgo Negatively charged: −1.602176487×10−19 C

o Nucleons: all particles that make up the nucleus

o Protonso Reside in the nucleuso Relatively large mass: 1.672621637×10−27 kgo Positively Charged: 1.602176487×10-19 C

o Neutronso Reside in the Nucleuso Relatively large mass: 1.67492729×10−27 kgo No electric charge

Subatomic Era

Atomic Variance

An atom’s element is defined by the number of…Protons

Any atom with a non-neutral charge is called an…Ion

Ions exist because the atom has either more or fewer than

There are several different forms of elements called that vary in amounts of

Subatomic Era

Electrons

Protons

Isotopes Neutrons

The Quark Era starts in 1964, but that advance can be regarded as outside the realm of chemistry – instead a part of nuclear physics

The Modern Era (1900CE – Present)

Chapter 5 in your book!

Read pages 133-148

It all begins… (1900)

o Scientists believed that we had answered all major questions- only leaving a few items to finisho Max Plank was commissioned

to build a better light bulbo He wanted to answer questions

about “black body radiation”o He reluctantly used statistics to

solve questions (he was very conservative)

o December 14, 1900

Modern Era

Max PlankGerman, Physicist

Statistics in Science

o Most science uses regular math (ex: F=ma)

o This era starts to deviate from tradition…o The second law of thermodynamics

(Boltzmann)o All systems move toward a less organized

stateo Plank knew about Boltzmann’s ideas –but

disproved of deviation from traditiono Plank reluctantly adopted statistics to

best explain experimental findings, although he didn’t want to be progressive

o Einstein interpreted Plank’s use of statistics to start Quantum theory

Modern Era

Quantum Theory

Energy can only be transferred in small packets

Plank saw the emission of light could not be explained by classical physics of the day Energy transferred in whole-number

multiples of hν ΔE = energy transferred n = integer multiple ν = frequency of light h = Plank constant (4.134×10-15eV·s )

Modern Era

ΔE = nhν

Photon – light packets

o Light partially behaves like particles that Einstein called Photons

De Broglie said - all matter can be described by similar wave packets This blurred the line between particles and

waves λ=h/p

Modern Era

λ=h/p …or(λ=h/mv)

o Wavelength = Plank’s constant / momentum Wavelength – wave property Plank’s constant – a fundamental constant

6.626068 × 10-34 m2 kg / s Momentum – a mechanical property

Momentum = mass × velocity (p=mv)

o Find the wavelength of lots of things!

Modern Era

Explaining Data

o The quantum theory suddenly meant energy could only be transferred in discrete amounts

o We had observed emission spectra and knew the Rutherford model, but neither was fully explained

Emission Spectra of Iron (Fe)

Emission Spectra of Hydrogen (H)

Modern Era

Bohr’s Planetary Model of the Atomo integrated all known

information into a new, mathematically based, model of the atomo He kept electrons in orbits

around the nucleus o Only allowed certain specific

electron orbits for each atomo Electron transitions between energy

levels (orbits) could only be jumps – nothing could be in between these energy levels (like steps on stairs)

Modern Era

Niels BohrDanish Physicist

Discrete Electron Energy Levels DeBroglie said that electrons always act

like waves This supported the idea of discrete energy

levels Only certain wavelengths will “fit” around

the atom

Modern Era

Bohr Energy levels

Electrons can only travel in specific energy levels

n=1

n=2

n=3

E = The actual energy of the given energy level

Z = the nuclear charge (number of protons)

This linked the properties of atoms with the observations of emission spectrum

E=-13.6eV

Z2

n2

Modern Era

Bohr Energy Levels

Atoms typically found in “Ground State” Electrons want to exist in

the lowest energy levels available

Atoms can be raised to an “Excited State” Electrons can be put into

higher energy levels than usual, but energy has to be added to do so

Modern Era

Energy Level Transitions

Electron jump: Quantum leap! Electrons can jump

from any lower energy level to a higher energy level and vice versa

Total energy of atom changes

Light is absorbed to get to higher energy states

Light is emitted when electrons jump to lower energy states

Modern Era

Electron Transitions

Only Specific wavelengths of light are absorbed and emitted by atoms – you have seen these before Light emitted by atoms is the emission

spectra ΔE = Efinal –Einitial

E = hν h=Plank’s Constant

4.134×10-15eV·s 6.63×10-34 m2kg/s

Modern Era

Some Practice!

Colors of light are identified by their frequency and/or wavelength Find the

frequency of light for transitions 1-3

Find the wavelength of light for transition 3

What does 4 mean?

Modern Era

1

2

3

4

The Fall of Bohr…

Bohr had easily come up with the best model for the atom so far, and his impact is still felt today but…

Werner Heisenberg, a student of Bohr’s, stated: It is impossible to know the

absolutely exact position and momentum of anything at the same time

Δx Δp ≥ h4π

Werner HeisenbergGermany

Modern Era

The New Quantum Model

o In 1926 Erwin Schrödinger developed an equation that took care of all inconsistencies of Bohr’s modelo Completely treated electrons as waves (Ψ)o Accounted for uncertainty principle

o This took the electron from existing in defined orbits to living in a “probability cloud”o Concentric probability clouds expand out from the

nucleuso Probability cloud – the area where an electron is

likely to be found

Modern Era

The Modern (current) Atom

We don’t know any electron’s exact location or momentum Heisenberg uncertainty principle We know electrons act like

waves Electrons are likely to exist in

some areas around a nucleus, and not in other areas We can find probabilities where

electrons can be foundErwin Schrödinger

Austria

Modern Era

What does it look like?

Likely electron locations are now represented by probability clouds – a way to graph probability in three dimensions

Electron Clouds

Electron

Bubbles

Modern Era

Electron OrbitalsModern Era