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Atomic Models and Theories

Atomic Models and Theories: How Our Current Understanding of Atoms Came to Be
Because atoms are so small it is difficult to study them
o If the (nitrogen) atom were the size of a blueberry, and we wanted to see how many
atoms were in a grapefruit, the grapefruit would have to be the size of planet Earth to fit
all those blueberries.
 Put another way, the number of atoms in a grapefruit is about equal to the
number of blueberries you would need to fill up the entire sphere of planet
 Can you imagine counting the number of blueberries it would take to stuff the
entire sphere of the earth? That number is so huge that it’s hard to relate to, so
the analogy helps us better understand how big it is.
Scientists have created models to help us visualize what an atom looks like & these models have
changed many times.
Atomic Models
Democritus “Atomic Theory” (456 BC)
o The atomic theory stated that the universe is composed of two elements: the atoms and the
void in which they exist and move
o According to Democritus atoms were miniscule quantities of matter. Democritus
hypothesized that atoms cannot be destroyed, differ in size, shape and temperature, are
always moving, and are invisible. He believed that there are an infinite number of atoms.
 This hypothesis was created in 465BC
o All matter consists of invisible particles called atoms.
o Atoms are indestructible.
o Atoms are solid but invisible.
o Atoms are homogenous.
o Atoms differ in size, shape, mass, position, and arrangement.
o Solids are made of small, pointy atoms.
o Liquids are made of large, round atoms.
o Oils are made of very fine, small atoms that can easily slip past each other.
John Dalton’s “Atomic Theory” (1803)
o Using evidence from many experiments John Dalton came up with his atomic theory
 All elements consist of atoms that cannot be divided
 All atoms of the same element are exactly alike & atoms of different elements are
 An atom of one element cannot be changed into an atom of a different element
by a chemical reaction
 Compounds form when atoms of more than one element combine in a specific
o Dalton thought that atoms were like smooth, hard balls that could not be broken into
smaller pieces
J.J. Thomson discovered that atoms contain negatively charged particles called electrons using a
cathode ray tube (1897)
o At this time scientists knew that atoms had no electrical charge (neutral), so Thomson
reasoned that atoms must also contain some sort of positive charge that balances out the
charge of the negative electrons
o Thomson proposed a model that had electrons scattered throughout a ball of positive
 Ernest Rutherford was one of Thomson’s former students and he found evidence that challenged
Thomson’s model (1911)
o The experiment Rutherford conducted shot a beam of positively charged alpha particles
at a thin sheet of gold
If Thomson’s model was correct the charged particles would be deflected
(repelled) because Thomson proposed that the positively charged cloud was
evenly spread out in an atom
To everyone’s surprise only a few particles were deflected
 Based on these results Rutherford suggested that the atom is mostly empty space
but has a positive charge in the center
 Rutherford concluded that an atom’s positive charge must be packed within a
small region in its center (the nucleus) and come from particles called protons
Niels Bohr was a Danish scientist who revised the atomic model again (1913)
He suggested that electrons are found only in specific orbits around the nucleus
o The orbits in Bohr’s model look like planets orbiting a Sun
Erwin Schrodinger changed the model again and introduced what he called the “cloud model”
He determined that electrons do not orbit the nucleus like planets but instead moved rapidly
within a cloud like region around the nucleus
James Chadwick of England showed that another particle existed inside the nucleus along with
protons, the neutron (1932)
o The neutron was hard to find because it does not have a charge
The standard model was developed in the 1970’s and is currently the accepted model for atomic
The Standard Model of physics is a theory of the elementary particles, which are
either fermions or bosons. It also explains three of the four basic forces of nature. The four
fundamental forces are: gravity, electromagnetism, the weak force, and the strong force.
Gravity is the one the model does not explain.
In search of Giants:
What are the 4 forces of nature?
There are 4 forces that govern our universe…these forces were created by the Big Bang
4 Forces:
o Strong Nuclear Force- holds the nucleus (protons and neutrons) of an atom
 Particle: Gluon
o Electromagnetic Force- attraction between positively and negatively charged
 This allows:
o Atoms to form bonds
o Electrons to orbit the nucleus
 Particle: Photon
o Weak Nuclear Force- turns neutrons into protons during radioactive decay
Particle: W boson
o Gravity- everything with mass has an attraction to anything else with mass…weakest of
the 4 forces
 Particle: Graviton (theoretical)