Chem 1050 Chapters 2 & 3

Atomic Structure

What is matter made of? (History of the Atom)

• Democritus – Greek philosopher – said that the universe made up of an infinite number of indestructible atoms moving in empty space

• Pretty profound for someone who could not see an atom!

But as politics go:

• Aristotle said Matter was continuous, infinitely indivisible and made up of 4 elements…

• Earth, Air, Fire, Water

History of the Atom• And chemistry as an earnest

and respectable science is often said to date from 1661 when Robert Boyle of Oxford published The Sceptical Chymist

Dark Ages• What happened with science

during this time?

Late 1700’s• Antoine Lavoisier – came up

with law of conservation of mass

• Called the father of modern chemistry – quantified it

Industrial Revolution• What made science leap ahead

during this time?

• Proust and Dalton – law of definite proportions

• Henry Cavendish – broke down H2O2 into H2O

John Dalton• 1803 English Quaker

• Atomic Theory

• 1. All matter is composed of extremely small particles called atoms.

John Dalton• 2. All atoms of a given element

are alike but atoms of one element are different from atoms of another.

• 3. Compounds are formed when atoms of different elements combine in fixed proportions.

John Dalton• 4. A chemical reaction involves

rearranging of atoms. No atoms are created or destroyed in a chemical reaction.

John Dalton

• English schoolmaster, and a Quaker, born 1766

• Published in 1808 1st carefully reasoned arguments to show that atoms must exist

John Dalton• There in a short chapter (5

pages out of 900) in a book entitled A New System of Chemical Philosophy, Dalton established his reputation.

John Dalton• Dalton’s simple insight was that

at the root of all matter are exceedingly tiny, irreducible particles. Dalton’s contribution was to consider the relative sizes and characters of these atoms and how they fit together.

John Dalton

• Most of his theory is still acceptable

• But atoms are not all the same, more on that later.

• The great Caltech Physicist Richard Feynman once observed that if you had to reduce scientific history to one important statement, it would be “All things are made of atoms.” They are everywhere and constitute everything. Look around you. It is all atoms!

• And not just the solid things like walls and furniture, but the air in between. And they are there in numbers that you really cannot conceive! At sea level, 00C, one cm3 of air (a space the size of a sugar cube) will contain 45 billion billion molecules. Every cm3 has them!

• Atoms, in short, are very very abundant!

• And they are very durable

• You are likely part of Shakespeare and Genghis Khan, but not Elvis yet!

• Why?

• No one actually knows how long atoms can survive, but it is probably in the order of 1035 years!

• (a number I am happy to put in scientific notation!)

Definition: Atom• Neutral system of negatively

charged electrons moving around a dense, positively charged nucleus

• That definition is 20th century stuff!

Atom• Well, for the next century after

Dalton made his proposal, the idea of the atom remained entirely hypothetical and many scientists doubted the existence of atoms

Mid 1800’s• 1869 to be exact a Russian

chemist by name of Dmitri Ivanovich Mendeleev invented the periodic table.

• It was organized in rows called periods and in columns called groups and organized by mass

• Why?

Mid 1800’s• He left gaps in the table on

purpose. He knew there were missing elements.

• Example

• Sc ……Ge

Late in the 1800’s:• Experimenters used discharge

tubes (glass tubes with gases and metal electrodes) – studied conduction of electricity in gases

• J.J. Thomson, of Cambridge, England also used discharge tubes in his studies

J.J. Thomson• He put meaning into the nature

of the beams

• He called them corpuscles…

• And later called them electrons

J.J. Thomson• Thomson found out the

properties of the e- beams were… independent of the materials used in the cathode ray tubes

• What does that mean?

J.J. Thomson

• He concluded that e- are a universal and fundamental part of all matter

• Thomson announced the discovery of the e- in 1897

• 1906 Nobel prize awarded

J.J. Thomson• (demo)• His experiments were

forerunners of…• TV, X-rays, Oscilloscopes and• Electron microscopes

Example of a gas discharge tube

J.J. Thomson

• He was unable to determine the mass of e- but showed the following ratio:

• e/m = 1.759 x 108C/g

• SI unit of charge is C, coulomb

Robert Millikan• Univ. Chicago, 1909-1912

• Conducted a famous experiment…called the

• Oil-drop experiment

• Millikan calculated the mass of e-

Robert Millikan• Me = 9.107 x 10-28g• Think about this mass vs. the

mass of the proton and neutron!

Model of Millikan’s Oil Drop Experiment

Also in the late 1800’s:• In 1895, Wilhelm Roentgen

discovered X-rays at the University of Wurtzburg in Germany.

• And in that same year,

• Henri Bacquerel discovered radioactivity.

Also in the late 1800’s:• This allowed Eugen Goldstein

in 1886 to study positive rays

• Goldstein worked with a positive ion, a helium atom stripped of 2 e-

• Called alpha particles

Also in the late 1800’s:• Alpha particles were used in a

crucial experiment – which provided us with the modern concept of the atom.

• J.J. Thomson concentrated on e-

• Mystery was inner atom

J.J. Thomson

• J.J. Thomson’s theory on the makeup of the atom – “plum pudding model”

• (diagram on board)

Earnest Rutherford• Ernest Rutherford born in New

Zealand and educated in England and Canada (Univ. Manchester)

• Bombarded gold foil with alpha particles

Earnest Rutherford• In 1910, he and his assistant,

Hans Geiger (remember what he is known for?), watched for changes in direction as alpha particles passed through the gold foil.

Gold Foil Experiment

• Most had no deflection, consistent with Thomson’s model, but…

• Some had massive deflections!…

Gold Foil Experiment

• Rutherford said it was as if he had fired a 15 inch shell at a sheet of paper and it rebounded into his lap!

Gold Foil Experiment• After considerable reflection, he

realized there could be only one possible explanation:

• The particles that bounced back were striking something small and dense at the heart of the atom…

Gold Foil Experiment

• While the other particles sailed through unimpeded.

• Rutherford realized that the atom was mostly empty space, with a very dense nucleus at the center.

Model of Rutherford’s Gold Foil Experiment

Diagram showing alpha deflections in Plum Pudding vs nucleus theories

Gold Foil Experiment

• This was a most gratifying discovery, but it presented one immediate problem:

• By all the laws of conventional physics, atoms shouldn’t exist!

• Why?

Rutherford• Thomson’s model was wrong

and was abandoned in favor of the nuclear atom

• nuclear atom – small massive positively charged nucleus surrounded by negatively charged electrons

Rutherford• He published his findings in

1911, most influential of scientific achievements!

• He was awarded the Nobel prize in Chemistry in 1922, a year after Einstein.

Nuclear Atom• (+) charge on proton must = (-)

charge on e-

• Therefore…

• The number of e- in a neutral atom must = number of protons in its nucleus

Nuclear Atom• This is called the atomic

number, symbolized by Z (in college)

Nuclear Atom• Rutherford suggested in 1920

that each nucleus should contain electrically neutral particles to account for missing mass…

• And why the nuclei didn’t blow up!

Nuclear Atom• He saw that the protons must be

offset by some type of neutralizing particles, which he called neutrons

• The idea was simple and appealing, but not easy to prove.

Nuclear Atom• Rutherford’s associate, James

Chadwick, devoted 11 intensive years to hunting for neutrons before finally succeeding in 1932.

• Chadwick was awarded the Nobel Prize in Physics in 1935.

Nuclear Atom• As it turned out historically, it

was a good thing it took a long time for the discovery of the neutron as it was essential for the development of the atomic bomb.

Nuclear Atom• Had it been discovered in the

1920s, its noted, it is very likely the atomic bomb would have been developed first in Europe, undoubtedly by the Germans.

• Science of chemistry deals with electrons

• Where are they in your body?• Where are the protons?• Where are the electrons?

Mass Number

• Atomic mass unit (amu) defined as 1/12 mass of carbon-12 atom

• A precise instrument called a mass spectrometer has been used since 1920 to determine masses of atoms.

Mass Number

• Practical purposes:

• 1 proton = 1 neutron =1 amu

• Cathedral vs the fly example

• p+ = 2000 lb = 1 ton

• n0 = 2000 lb = 1 ton

• e-1 = 1 lb

Mass Number• Therefore mass number = total

number of protons + neutrons

Isotopes

• All atoms of a given element have the same atomic number but they may have different mass numbers because of different numbers of neutrons

Isotopes• Atoms with same atomic

number but with different mass numbers are called isotopes

• [Apples in a grocery store example]

• Apples are alike – why?• What makes them different?

Atomic Mass• It is reasonable to assume mass

of an atom is concentrated in nucleus

• [remember the Cathedral and fly example]

Atomic Mass• Remember that 1 proton = 1

neutron = 1 amu

• But counting just the amu ignores the existence of isotopes

Atomic Mass• Atomic mass is a weighted

average of masses of all isotopes of a given element

• So, you are asking how do we calculate isotopes?

• [Example on board]

End of Ch 2 & 3