Download Atoms and Atomic Theory

Atoms and Atomic Theory
At the end of this lesson, you need to draw a
diagram for each model showing the evolution
of atomic theory:
"Plum-Pudding" Model
Rutherford Atom
Rutherford-Bohr Atom
Charge-Cloud Model
Quantum Model of the Nucleus
Nuclear Shell Model
The Nature of Matter
Democritus: Matter is
comprised of small
individual units (atoms)
Aristotle: Matter is
comprised of 4
fundamental elements:
earth, water, air, and fire
Most sided with Aristotle until the Renaissance.
Then in the Early 1800s
John Dalton (1766 – 1844) noticed that substance
could be broken down into other substances.
Eventually you could not break these down into
anything else (atoms).
Example. Sugar can be broken into water and
carbon dioxide. Water can be broken into oxygen
and hydrogen. Carbon dioxide can be broken into
carbon and oxygen. Carbon, oxygen and hydrogen
cannot be broken down any further.
Also, for a particular substance the ratios of atoms
produced were always the same.
For example: Nitric Acid always gives one “unit” of
nitrogen, three “units” of oxygen, and one “unit” of
hydrogen.
Dalton’s Atomic Theory
Atomic Theory: Matter is made up of fundamental units called atoms
Dalton’s original theory had five postulates:
1) Each element is made up of tiny particles called atoms. (oxygen molecule is made of
two atoms of oxygen)
2) Atoms of a given element are identical; atoms of different elements are different in
some fundamental way. (MOSTLY TRUE)
3) Chemical compounds are formed when atoms from different elements combine with
one another.
4) A given compound always have the same relative numbers and types of atoms.
5) Chemical reactions involve the rearrangement or exchange of atoms within
compounds.
Dalton’s Atomic Theory
EXAMPLE:
2 Methane and 3 oxygen molecules always give 2 carbon dioxides and 2 waters
How the Above Example Satisfies Dalton’s postulates:
1) Each element is made up of tiny particles called atoms.
– elemental oxygen is made of two atoms of oxygen
2) Atoms of a given element are identical; atoms of different elements are different in
some fundamental way.
– all atoms of elemental oxygen are the same and are different than elements of
other atoms
3) Chemical compounds are formed when atoms from different elements combine with
one another
4) A given compound always have the same relative numbers and types of atoms.
– methane is made of one atom of carbon and four atoms of hydrogen
– carbon dioxide is made of one atom of carbon and two atoms of oxygen
– water is made of one atom of oxygen and two atoms of hydrogen
Dalton’s Atomic Theory
EXAMPLE:
2 Methane and 3 oxygen molecules always give 2 carbon dioxides and 2 waters
Dalton’s postulates:
5) Chemical reactions involve the rearrangement or exchange of atoms within
compounds.
methane
oxygen
molecule
carbon
dioxide
water
What Are These Atoms?
The existence of atoms remained controversial until ~1900!
Even people who believed atoms were real were unsure what they were.
Solid
Particles?
Something
more
ethereal?
Smallest
unit of
nature?
Can they be broken
down into smaller
parts?
What Are These Atoms?
Ludwig Boltzmann Used Math to Demonstrate Atoms Could be Treated as Particles…
his Theory explained all of the available data.
What Are These Atoms?
Can they be broken up?
Several observations pointed to YES, and all involve using “bits” of the atom!
Cathode Rays. Stream of particles
produced at the cathode (negatively
charged plate).
– The metal of the cathode produces
electrons that move towards the
positive plate (anode).
– Atoms are neutral. Negative particles
can fly off of it. That must mean there
are positive particles in atoms as well!
First Picture of Atoms: Plum Pudding
Model
Electrons (plums) in a sea of positively charged something (pudding).
Made sense based on charge repulsion.
Radioactivity: Atoms “Broke” Apart
Positively Charged He Atoms “Bounced” off
of Gold: There is a Nucleus!
In 1932 the Neutron Was Discovered By
James Chadwick
proton
neutron
electron cloud
Helium Atom
Nucleus vs. Size of Atom
Almost all of the
mass of the atom is in
the nucleus.
Helium Atom
pea
Draw a diagram for each model showing the
evolution of atomic theory:
•
•
•
•
•
•
"Plum-Pudding" Model
Based on the work of J.J. Thomson, who discovered that electrons have a
negative charge (about 1897), this model presents the atom as a collection of
negatively charged electrons mixed with positively charged particles.
Rutherford Atom
Based on the work of Ernest Rutherford in 1911, this model is the first to
establish the nucleus in the center of the atom, with negatively charged
electrons orbiting it, as planets orbit the sun.
Rutherford-Bohr Atom
In 1913, Niels Bohr improved upon the Rutherford model by stating that
electrons travel the nucleus in a fixed orbit. At times, electrons can move from
one energy level to another.
Charge-Cloud Model
With the emergence of Werner Heisenberg's Uncertainty Principle, which
states that it is impossible to know both the location and speed of an electron
at the same time, scientists developed what is known as the charge-cloud
model. According to this model, electrons move around the nucleus, but no
attempt is made to show the orbital paths of the electrons. The electrons are
depicted within a cloud to indicate that they are traveling at high speeds.
Quantum Model of the Nucleus
In 1930, James Chadwick discovered the neutron, which was found to have the
same mass as the proton. With this discovery, it became clear that the mass of
an atom came from the nucleus.
Nuclear Shell Model
This model is used to describe the behavior of electrons orbiting the nucleus of
an atom. Electrons arrange themselves in shells around the atom, with the
outermost shell called the valence shell. If this shell has eight electrons, it is
considered stable. During chemical reactions, the valence shell will gain or lose
electrons to increase its stability.