Transcript Presentation

History of the Atom
The Model of the Atom
Part I: The Road to Modern Atomic Theory
• Early Theories - 400 B.C.
• common Greek theory was that all matter
consisted of four "elements"—earth, air,
fire, and water.
• Democritus (460-360 BC) = a philosopher
who theorized that all matter was made
of indivisible particles (atomos).
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First to use the word atom
his theory was based on logical reasoning, not data.
philosophers/scientists of this time
period did not think doing experiments was necessary—you
could reach the truth by pure logical reasoning.
Part II: Early Atomic Models
• Dalton, John—“Solid marble” model
• English schoolteacher (1808), proposed
explanation for the 3 laws above
• thought elements were composed of
atoms, which were indivisible
Part II: Early Atomic Models
• Dalton, John—“Solid marble” model
• English schoolteacher (1808),
• thought elements were composed of
atoms, which were indivisible
• Dalton’s Atomic Theory: (five parts)
1. all matter is composed of very tiny particles called atoms
2. atoms of a given element are identical in size/mass/other
properties; other elements’ atoms are different
3. atoms cannot be subdivided, created, or destroyed
4. in chem rxn, atoms are combined/separated/rearranged
5. atoms of diff. elements combine in small, whole-# ratios to form
compounds
• Even after 2000 years of the atomic concept, the
structure of the atom was still unknown.
• at this point, the atom was still thought to be the smallest
unit of matter.
• Thomson, Joseph John—“Plum Pudding”
model (or “cookie dough”)
• English physicist (1897), performed
cathode-ray tube experiments
• discovered the electron and its charge
• cathode-ray tube: glass vacuum tube (attached to
a voltage source) through which electricity flows,
producing a glow.
• electrons travel towards anode (opposite charges
attract)
• paddle wheel placement shows that particles were
moving, had mass, and had a negative charge
• this model shows the electrons on the outside of the
positively-charged atom (if there were negative parts
to atoms, then there must be positive parts as well to
balance it, because most matter is neutral).
• Rutherford, Ernest—“Electron cloud, positive
nucleus” model
• (1911) performed the gold-foil experiment,
leading to the discovery of the
positively-charged nucleus.
• experiment = bombard a thin gold foil with
alpha-particles (He+).
• they did this assuming that the
charge and mass within each
atom was uniformly distributed
throughout the atom, and
expected the alpha particles to
go right through.
• most did, but some (1/8000)
actually bounced back toward
the particle source!
• Rutherford concluded that the reason some
particles bounced back was that there was
a very dense positively-charged area that
occupied only a very small amount of space
within the core of the atom (nucleus).
• to explain this, Rutherford proposed a new
model for the atom. He imagined the atom
as a miniature solar system with a nucleus as the
sun and electrons orbiting like planets.
• although the nucleus was only one million
millionth the volume of the atom,
it had over 99.9% of the mass.
• Bohr, Neils—“Quantized energy level” model
• Danish physicist (1913), new model that
explained why negative electrons do not
fall in towards the positive nucleus
• Bohr used Rutherford's ideas to explain the
behavior of the simplest atom with one
electron—hydrogen.
he suggested that an electron was restricted to
certain allowed orbits round the nucleus.
• if it jumped from one orbit to another, the atom emitted
or absorbed light.
• thus, electrons only exist in specific energy levels (orbits
around the nucleus) similar to the rungs of a ladder.
• these levels reflect quantum amounts of energy that are
required to move electrons from one level to another
• Modern Atomic Model
• e- in the modern model are arranged in “orbitals,”
not orbits (Bohr model).
• Orbitals are cloud-like regions around the nucleus
of the atom in which 1 or 2 electrons are most
likely to be found. They are arranged about the
nucleus on the x, y, and z axis.
Part III: The Dual Wave-Particle Nature of Light
• the modern model assumes the n0 and p+ still reside in
the nucleus, and the e- reside in “orbitals,” or electron
clouds (regions of space in which there exists a high
probability of finding 1 or 2 e-).
• To explain, let’s start from the beginning:
– we know that e- can be excited, or caused
to jump to higher-energy orbits from their
lower-energy ground states, with the
introduction of energy, usually in the form
of electricity (think of the glow in a CRT).
– further experiments were performed in
the early 1900s concerning the
photoelectric effect.
• this effect was produced when e- were emitted as certain
frequencies of visible light shone on the surface of the metal
(think photovoltaic cells).
– but why only certain frequencies?
Scientists could not answer this question
until they began to think of light as having
a dual wave-particle nature, rather than
existing only as a wave.
– waves can contain energy at any level within the
spectrum; particles only carry energy with them in
small, specific amounts, called quanta.
– this explained why only certain frequencies of
light would cause the electrons to escape the
surface of the metal during the
photoelectric experiments—the light was acting
as a particle, not a wave, in this case.
• As seen in the figure
here, when hydrogen’s eare excited, they move to
higher-energy orbits. Falling
from these orbits to their
original ground state causes
them to emit photons of light.
• if this light passes through a
prism, it is split into its component wavelengths (and
colors). Each element has its own distinct line emission spectrum.
• the energy of each emitted photon corresponds to a particular
frequency of visible light (see diagram above).
• this worked nicely for hydrogen, but no so much with other
elements!