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
• The Greek philosophers (600 – 200 B.C.) were the first
people to speculate about the basic substances of matter.
• he speculated that all matter on earth is composed
of only four elements: earth, air, fire, and water.
• He was wrong on all counts!
Aristotle
• Swedish chemist,
_______ (early
1800‟s) used one or
two letters of the
Latin name to
designate each
element
Jons Jakob Berzelius
• Since _____‟ time most elements have been
symbolized by the first one or two letters of the English
name.
• YOU are expected to know the names and symbols of the
45 elements listed on Table 10.2
Jons Jakob
Berzelius
• All matter is composed of atoms.
• An atom is composed of three subatomic particles:
electrons (-), protons (+), and neutrons (0)
• The nucleus of the atom contains the protons and the
neutrons (also called nucleons.)
• The electrons surround (orbit) the nucleus.
• Electrons and protons have equal but opposite charges
Atom
Protons and neutrons have nearly the same mass and
are 2000 times more massive than an electron.
• Discovery – Electron (J.J. Thomson in 1897), Proton
(Ernest Rutherford in 1918), and Neutron (James
Chadwick in 1932)
The Atomic Nucleus
His “plum pudding” model predicted the alpha
particles would pass through the evenly distributed
positive charges in the gold atoms.
J.J Thomson
• Only 1 out of 20,000 alpha particles bounced back.
• Rutherford could only explain this by assuming that each
gold atom had its positive charge concentrated in a very
small “nucleus.”
• Diameter of nucleus = about 10-14 m
• Electron orbit diameter = about 10-10 m
• Atomic Mass is concentrated in the nucleus (>99.97%)
Rutherford’s AlphaScattering Experiment
• Therefore the volume (or size) of an atom is determined
by the orbiting electrons.
• The diameter of an atom is approximately 10,000 times the
diameter of the nucleus.
• If only nuclear material (protons and neutrons) could be
closely packed into a sphere the size of a ping-pong ball it
would have the incredible mass of 2.5 billion metric tons
Atomic Mass is
Concentrated in the Nucleus!
• Tightly Packed
Protons and Neutrons
Visual Representation of a
Nucleus
• Atomic Number (Z) – the # of protons in the nucleus
(“defines” the element – the # of protons is always the
same for a given element)
• Atomic Number also designates the number of electrons
in an element.
• If an element either gains or loses electrons, the resulting
particle is called and ion.
• For example, if a sodium atom (Na) loses an electron it
becomes a sodium ion (Na+
.)
Atomic Designations
• Mass Number (A) – protons + neutrons, or the total
number of nucleons
• Isotope – when the number of neutrons vary in the
nucleus of a given element (always same number of
protons)
• Only 112 elements are known, but the total number of
isotopes is about 2000.
More Atomic Designations
Determine the number of protons, electrons,
and neutrons in the fluorine atom 19F
Determining the
Composition of an Atom
• Protons & Neutrons – in nucleus
• Electrons – orbit around nucleus
• Mass Number (A) = protons + neutrons
• Atomic Number (Z) = # of protons
• Neutron Number (N) = # of neutrons
• Isotope – an element with different # of neutrons (same #
of protons)
Atomic Review
The weighted average mass of an atom of the element in
a naturally occurring sample
• The Atomic Mass is measured in unified atomic mass
units (u) – basically the weight of a proton or neutron.
• The 12C atom is used as the standard, and is assigned
the Atomic Mass of exactly 12 u.
• The weighted average mass of all carbon is slightly
higher than 12 (12.011) because some is 13C and 14C.
Atomic Mass
• Mass Number (A) – protons + neutrons, or the total
number of nucleons
• Isotope – when the number of neutrons vary in the
nucleus of a given element (always same number of
protons)
• Atomic Number (Z) – number of protons
Atomic Review
______ – could be divided indefinitely
______ – ultimate indivisible particle
• Most (including Aristotle) agreed with the _____ theory.
• The ____ model of matter prevailed for 2200 years,
until 1807
Continous and discrete
