Week 2 - How the elements are formed: Atoms and Elements Flashcards
• Greek Philosophers (400 B.C.) debated whether matter
was continuous or discrete, but could prove neither.
Early Concepts of the Atom
• In 1807 ____ presented evidence that matter was
discrete and must exist as particles.
• hiss major hypothesis stated that:
• Each chemical element is composed of small indivisible
particles called atoms,
• identical for each element but different from atoms of other
elements
• Essentially these particles are featureless spheres of
uniform density.
Dalton’s Model –
“The Billiard Ball Model”
Hiss 1807 “billiard ball
model”
pictured the atom as a tiny
indivisible, uniformly
dense, solid sphere.
Dalton’s Model
• In 1903 he discovered the electron.
• Further experiments by _____ and others showed that
an electron has a mass of 9.11 x 10-31 kg and a charge of
–1.60 x 10-19 C.
• he produced „rays‟ using several different gas
types in cathode-ray tubes.
• He noted that these rays were deflected by electric and
magnetic fields.
• he concluded that this ray consisted of negative
particles (now called electrons.)
Thomson – “Plum Pudding
Model”
• Identical electrons were produced no matter what gas
was in the tube.
• Therefore he concluded that atoms of all types contained
„electrons.‟
• Since atoms as a whole are electrically neutral, some
other part of the atom must be positive.
• Thomson concluded that the electrons were stuck
randomly in an otherwise homogeneous mass of
positively charged “pudding.”
Thomson – “Plum
Pudding Model” (cont.)
• his 1903
“plum pudding model”
conceived the atom as
a sphere of positive
charge in which
negatively charged
electrons were
embedded.
Thomson model
• In 1911 he discovered that 99.97% of the mass of
an atom was concentrated in a tiny core, or nucleus.
• his model envisioned the electrons as circulating
in some way around a positively charged core
Ernest Rutherford’s Model
His 1911
“nuclear model”
envisioned the atom as
having a dense center
of positive charge (the
nucleus) around which
the electrons orbited.
Rutherford’s Model
• Electromagnetic radiation that have relatively low
frequencies (about 1010 Hz)
Microwave
Because most foods contain moisture, their water
molecules absorb the microwave radiation and gain
energy
• As the water molecules gain energy, they rotate more rapidly,
thus heating/cooking the item
• Fats and oils in the foods also preferentially gain energy from
(are excited by) the microwaves
The Microwave Oven
• Paper/plastic/ceramic/glass dishes are not directly heated
by the microwaves
• But may be heated by contact with the food (conduction)
• The interior metal sides of the oven reflect the radiation
and remain cool
• Do microwaves penetrate the food and heat it
throughout?
• Microwaves only penetrate a few centimeters and therefore they
work better if the food is cut into small pieces
• Inside of food must be heated by conduction
The Microwave Oven
• In 1946 a ______ put his chocolate bar too close to a microwave
source
• The chocolate bar melted of course, and …
• Within a year Raytheon introduced the first commercial
microwave oven!
Raytheon Corporation engineer, Percy
Spencer
• Accidentally discovered in 1895 by the German physicist
• He noticed while working with a gas-discharge tube that a piece
of fluorescent paper across the room was glowing
• he deduced that some unknown/unseen radiation
from the tube was the cause
• He called this mysterious radiation “X-radiation” because it was
unknown
Wilhelm Roentgen
Electrons from the cathode are accelerated toward the
anode. Upon interacting with the atoms of the anode, the
atoms emit energy in the form of x-rays.
X ray production
• Within few months of their
discovery, X-rays were
being put to practical use.
• This is an X-ray of bird
shot embedded in a hand.
• Unfortunately, much of the
early use of X-rays was far
too aggressive, resulting in
later cancer
Early use of X-Rays
