A TOMS AND THE ELECTROMAGNETIC SPECTRUM : A H ISTORY Physics of Atomic Nuclei - Lecture 1 Peggy Norris, [email protected]

ATOMS AND THE ELECTROMAGNETIC

SPECTRUM: A HISTORY

Physics of Atomic Nuclei - Lecture 1

Peggy Norris, [email protected]

DEMOCRITUS (460-370 BC)

First to propose existence of smallest particle

“Atomos” used to describe particle

Judy Vondruska (SDSU)

ARISTOTLE(384-322 BC)

Believed in the four elements of air, earth, water and fire

Felt that regardless of the number of times you cut a form of matter in half, you would always have a smaller piece of that matter. this view held for 2000

years primarily because Aristotle was the tutor of Alexander the Great.


JOHANN BECHER (1635-1682) AND GEORG STAHL (1660-1734)

Developed the Phlogiston theory which dominated chemistry between 1670 and 1790. when something burned, it lost phlogiston to the air

problem with the theory was that burning of metals resulted in an increase in the mass. This problem was solved by assigning negative mass to phlogiston.


ANTOINE LAVOISIER (1743-1794)

Proposed the Combustion Theory which was based on sound mass measurements

Proposed the Law of Conversation of Mass which represents the beginning of modern chemistry

Sidebar: was associated with a tax-collecting firm and was married to the daughter of the one of the firm's executives. This relationship with the tax firm led to Lavoisier's beheading at the guillotine in 1794.


MODEL OF ATOM


JOSEPH PROUST (1754-1826)

Proposed the the Law of Constant Composition in 1799

the composition of a substance is always the same, regardless of how the substance was made or where the substance is found there are always 2 atoms of

hydrogen and 1 atom of oxygen in a molecule of water.


JOHN DALTON (1776-1844)

Developed the concept of the mole and proposed a system of symbols to represent atoms of different elements

Proposed the Law of Multiple Proportions. assumed that simplest ratios would be used in

nature and came up with a formula for water of HO

this law led directly to the proposal of the Atomic Theory in 1803


JOSEPH GAY-LUSSAC ( 1778-1850)

Announced Law of Combining Volumes in 1808. showed that at the same temperature and

pressure, two volumes of hydrogen gas reacted with one volume of oxygen gas to produce two volumes of water (as a gas).


AMADEO AVOGADRO (1776-1856)

Proposed what is now known as Avogadro's Hypothesis in 1811 at the same temperature and pressure, equal

volumes of gases contain the same number of molecules or atoms

when combined with Gay-Lussac's Law of Combining Volumes, the only possible formulas for hydrogen, oxygen and water are H2, O2 and H2O, respectively

hypothesis not widely accepted for another 50 yrs


DIMITRI MENDELEEV (1834-1907)

Proposed the periodic law and developed the first periodic table in 1869. Medeleev's table was arranged

according to increasing atomic weight and left holes for elements that were yet to be discovered


J. J. THOMSON (1856-1940) Identified the negatively charged electron in the cathode ray

tube in 1897 deduced that the electron was a component of all matter and

calculated the charge to mass ratio for the electron. e/m = -1.76 x 108 coulombs/g

Thomson and others also studied the positive rays in the cathode ray tube and discovered that the charge to mass ratio depended on filling gas in the tube. The largest charge to mass ratio (smallest mass) occurred when hydrogen was the filling gas. This particle was later identified as the proton. e/m = +9.58 x 104 coulombs/g

Proposed the "plum pudding" model of the atom. the volume of the atom is composed primarily of the more massive (thus larger)

positive portion (the plum pudding). The smaller electrons (actually, raisins in the plum pudding ) are dispersed throughout the positive mass to maintain charge neutrality.


https://www.youtube.com/watch?v=_nLESblUAHY

NEW MODEL OF ATOM


ROBERT MILLIKAN (1868-1953)

Determined the unit charge of the electron in 1909 with his oil drop experiment at the University of Chicago. Thus allowing for the calculation of the mass of the electron and the positively charged atoms. e = 1.60 x 10-19 coulombs


https://www.youtube.com/watch?v=XMfYHag7Liw

ERNST RUTHERFORD (1871-1937)

Proposed the nuclear atom as the result of the gold-foil experiment in 1911 (Video) proposed that all of the positive charge and all of

the mass of the atom occupied a small volume at the center of the atom and that most of the volume of the atom was empty space occupied by the electrons Developed planetary orbit model of atom


https://www.youtube.com/watch?v=5pZj0u_XMbc

NEW MODEL OF ATOM


MAX PLANCK (1858-1947)

Made observations of the radiation of heated materials

Showed (from purely formal / mathematical foundations) that light must be emitted and absorbed in discrete amounts (quanta) if it was to correctly describe observed Prior to then light had been considered as a continuous

electromagnetic wave, thus the discrete nature of light was completely unexpected

the energy of each quantum is equal to the frequency of the radiation multiplied by the universal constant: E=f*h, where h is 6.63 x 10-34 Js.


NIELS BOHR(1913-1963)

Proposed improvement to Rutherford atomic model for this reason, the planetary model of the atom

is sometimes called the Rutherford-Bohr model. Bohr added the idea of fixed orbits, or energy

levels for the electron traveling around the nucleus his model allowed for the idea that electrons can

become "excited" and move to higher energy levels for brief periods of time


PROBLEM WITH RUTHERFORD-BOHR MODEL According to the electromagnetic theory, if a

charged particle were accelerated around another charged particle then there would be a continuous radiation of energy the loss of energy would slow down the speed of the electron

and eventually the electron would fall into the nucleus but such a collapse does not occur. Rutherford's model was

unable to explain why there was no collapse The attraction of the electron to the nucleus would cause

it to spiral into the nucleus as well


WILHELM ROENTGEN (1845-1923)

Attention was drawn to a glowing fluorescent screen on a nearby table determined that the fluorescence was caused by

invisible rays originating from the partially evacuated glass Hittorf-Crookes tube he was using to study cathode rays (i.e., electrons)

these rays penetrated the opaque black paper wrapped around the tube.

Roentgen had discovered X rays


ANTOINE HENRI BECQUEREL(1852-1908)

Observed that fluorescent materials like potassium uranyl sulfate, K2UO2(SO4)2 emitted mysteries rays like those described by Roentgen Found that uranium emitted radiation without an

external source of energy such as the sun. Becquerel had discovered radioactivity, the

spontaneous emission of radiation by a material. Demonstrated that the radiation emitted by

uranium shared certain characteristics with X rays but, unlike X rays, could be deflected by a magnetic field and therefore must consist of charged particles.


http://en.wikipedia.org/wiki/File:Becquerel_plate.jpg

PIERRE CURIE (1859-1906)MARIE CURIE (1867-1934)

Marie coined the term radioactivity After chemical extraction of uranium from the ore,

Marie noted the residual material to be more "active" than the pure uranium She concluded that the ore contained, in addition to

uranium, new elements that were also radioactive. This led to their discoveries of the elements of polonium and radium

Sidebar: After being killed crossing the street, Pierre's teaching position at the Sorbonne was given to Marie. Never before had a woman taught there in its 650 year history!


HENRY MOSELEY (1887-1915)

Discovered that the energy of x-rays emitted by the elements increased in a linear fashion with each successive element in the periodic table proposed that the relationship was a function of

the positive charge on the nucleus this rearranged the periodic table by using the atomic

number instead of atomic mass to represent the progression of the elements

this new table left additional holes for elements that would soon be discovered


FRANCIS ASTON (1877-1945)

Invented the mass spectrograph in 1920 First person to observe isotopes.

observed that there were three different kinds of hydrogen atoms. While most of the atoms had a mass number of 1, he also observed hydrogen atoms with mass numbers of 2 and 3.

Modern atomic masses are based on mass spectral analysis

His work led Rutherford to predict the existence of the neutron


Isotope: one of two or more atoms having the same number of protons but different numbers of neutrons

or Na-23 or Na-24


JAMES CHADWICK (1891-1974)

Discovered the neutron in 1932 Was a collaborator of Rutherford's Scientists realized neutrons could be used to probe atomic nuclei

lead to bombardment of Uranium with neutrons (Fermi)

observation of nuclear fission


NEW MODEL OF ATOM


ERWIN SCHRÖDINGER(1887 – 1961) Combined some particle behavior with wave

behavior as suggested by deBroglie and formulated mathematical model for hydrogen atom

Derived equation which gives the probability of finding the electron at some point in a 3-dimensional space at any given instant; gives no information about the path the electron follows solutions from equation yield information about

probability maps or shapes on different energy levels MODEL: "wave/mechanical" : basic layout of atom

similar to Bohr and Rutherford, but electrons do not follow simple orbits; their position can only be predicted in terms of probability


http://en.wikipedia.org/wiki/File:Erwin_Schrodinger2.jpg

WERNER HEISENBERG (1901-1976) De Broglie, Schrödinger, and Heisenberg showed that

classical mechanics had to be abandoned in order to describe the subatomic world adequately stated that particles don't really have a trajectory or an orbit,

much less do they behave like a ball that is shot through a corridor or is whirled around on the end of a cord.

Develop the wave-particle duality of subatomic particles Electrons that orbit around the nucleus interact with each

other by showing interference patterns, not unlike those of wave interference. If the velocity of the electron is thought of as its wavelength, the crests of neighboring electron waves amplify or cancel each other, thereby creating a pattern that corresponds to Bohr's allowed orbits.

Seemingly they exist in different places at different points in time, but it is impossible to say where the electron will be at a given time

NEW MODEL OF ATOM Electrons do not travel in definite paths around the nucleus The exact path or position of moving electron cannot be

predicted or determined Rather, there are regions inside the atom were electrons are likely to

be found. Electron clouds – Regions inside an atom where electrons are

likely to be found.

Documents

A TOMS AND THE ELECTROMAGNETIC SPECTRUM : A H ISTORY Physics of Atomic Nuclei - Lecture 1 Peggy Norris, [email protected]