Timeline Project

The Atom

By: Kassidy Yu
Period 6

General Information

The world around us is made up of what we call matter. Scientists have spent
millennia been studying their surroundings in an attempt to understand this
matter. One of the ideas that has arisen is that matter is made up of tiny,
invisible particles which we now call atoms. This booklet will go through 9
important scientists who contributed to what we know about atoms today.

Democritus - 442 BCE Analogy Each atom is unique
and create objects
Democritus was a Greek philosopher who built when put together,
on ideas from Leucippus and Anaxagoras. similar to how
Instead of conducting experiments to form his building
theory, he used logic. blocks are
individually
Democritus thought about how when you cut a stone in stacked in
half, each half still holds the properties and identity of the order to create
original stone. He reasoned that eventually, after cutting the a structure.
halves in half, there is a piece so small it cannot be divided
further.

Democritus was the one who named the atom. He referred
to these tiny pieces as atomos which means indivisible or
uncuttable in Greek.

He theorized that matter is made of Each atom is unique to material it makes up. For
different atoms with a separating them example, stones have specific stone atoms.
void. They are solid spheres with no
internal structure.

Atomic Model

Aristotle - 340 BCE Atomic Model:
Four Elements

Like Democritus, Aristotle was a Greek philosopher. Aristotle
did not conduct experiments either. He rejected Democritus’s
atomic theory and built off of Empedocles’s theory.

Instead of believing that matter is made up of tiny pieces. Aristotle Analogy
proposed that all matter is made of four main elements. He theorized
that each element has specific properties, and the ratio of these Aristotle’s atomic
elements affects the properties of matter. Fire is hot, air is light, earth theory can be
is cool, and water is wet.
compared to salad
He added that stars and planets were made of a separate, divine ingredients. The ratio
element, which he named aether.
of the ingredients
Overall, Aristotle actually hindered affects the taste of the
scientific progress. Most people
accepted his incorrect theory, salad, much like how
which delayed the development of the ratio of elements is
a new atomic theory. John Dalton
only formed his theory nearly two said to affect the
millennia later. properties of matter.

John Dalton - 1803 Atomic Model

John Dalton was an English His model was known as
chemist whose ideas became the the Billiard Ball or solid
theoretical foundation to the sphere model. Essentially,
modern atomic theory of matter. its structure was similar
Although they were not perfect, to Democritus’ theory,
many scientists built off of them. but Dalton added his own
ideas.

Dalton used experimental He had four main principles:

methods unlike Democritus and 1. Indivisibile, indestructible atoms make up all
matter.
Aristotle, who tried to reason their
2. Atoms of the same element share identical
atomic theories. He built on properties, different elements are different.

Democritus’s ideas to make them 3. Atoms of different elements can mix
physically or form compounds by chemically
a scientific theory. Analogy combining in whole number ratios.

Dalton’s second principle is 4. In chemical reactions, atoms are rearranged.
analogous to a set of colored pencils. They cannot be converted into atoms of a
Although each pencil is a different different element.
color and have differing
characteristics, they are all colored
pencils.

J.J Thomson - 1897

J.J. Thomson, an English physicist, built on

Dalton’s theory, but he discovered that atoms

were not indivisible and in fact made of smaller Thomson knew atoms had a neutral overall

parts. He used cathode ray tubes, sealed glass charge and reasoned that there must be some

with very little air. In a cathode ray tube, a source of a positive charge. He proposed that

particle beam flows from the cathode (negative atoms consist of negative particles embedded

charge) to the anode (positive charge) once a in a clump of positively charged matter. His

high voltage is applied across the two model is often called the plum pudding model.

electrodes.

He found that magnetic fields diverted the Atomic Model Analogy

cathode ray’s path. Thompson calculated the Thomson’s model
can be compared
Thomson placed a negative and mass of these cathode ray particles and
to the English
positively charged electric plate discovered that they had a much smaller desert, plum
pudding. Similar
around the cathode ray and found mass than that of any known atom. He to the plums in

that the positively charged plate repeated the experiment with different the desert,

attracted it and the negative one metals for the electrodes, and properties Thomson’s model depicts electrons as
pieces of matter within a clump of
repelled it. Opposite charges found were constant throughout. He positively charged matter.

attract and like charges repel, so concluded that these tiny, negatively charged

he concluded that the cathode ray particles he called corpuscles (known now as

must be made of negatively electrons) must be parts of an atom, and that

charged particles. all atoms consist of them.

Albert Einstein- 1905 Einstein was a German theoretical physicist. He is known
best for E=MC² and his theory of relativity. In 1905 he
mathematically proved that atoms exist, solidifying what
had already been discovered.

The atomic theory stated that liquids are made of molecules, which are always randomly
in motion. The properties come from the average of all the movement. This random
motion known as Brownian motion was discovered in 1827. Einstein found a flaw the
motion of the particles would produce statistical fluctuations. For instance, two adjacent
molecules are bound to eventually move in the same direction at the same time. This
could happen on larger scales and would be bound to eventually happen to a visible
scale, so a liquid may randomly move from one direction to the opposite.

He realized that large, observable particles generate pressure the
same way as invisible molecules. Particles tend to move from places In 1939, Einstein warned
of high concentration to low concentration, so if the concentration President Roosevelt that
the Germans were
varies, the particles flow to even out the concentration and lower the possibly researching
pressure.
Analogy nuclear weapons and

He could then calculate the average distance a visible The flaw Einstein suggested that the US
particle would travel in a given time. Einstein proved his had found can researched as well. He
discoveries on the atom by observing and measuring the be compared to later said in an interview
motion of visible particles under a microscope. He was flipping a coin. that “had I known that the
able to calculate the size of a given atom and molecule. He Although a coin may be fair, there
proved the idea the movement of atoms and molecules are statistical fluctuations in which Germans would not
causes heat. Additionally, Einstein used probability and the coin will land on heads or tails succeed in developing an
statistics to back up his discoveries in quantum mechanics. much more.
atomic bomb, I would
have done nothing.”

Ernest Rutherford - 1911 Rutherford was one of Thompson’s
students, and supported his plum pudding
model, but ultimately proved him wrong.

Rutherford placed radium into a lead box with a small
hole. Most of the radiation was absorbed, but a small
amount of alpha particles from the escaped through
the hole. He placed an extremely thin sheet of gold in
the path of the alpha particles surrounded by a screen
to detect the particles.

Rutherford expected that the alpha particles would pass straight through with a slight
deflection. This was because, according to Thompson’s theory, the positive charge would
be spread out and too weak to deflect alpha particles, which have a double positive charge.
Instead, most of them passed straight through while a very small amount bounced back at
large angles.

Rutherford concluded that atoms are mostly empty space. Since a very small amount of
the particles bounced back more than 90 degrees, he explained that the positive charge
in an atom must be concentrated in a tiny volume and contains most of its mass.
He proposed a nuclear model, and theorized that atoms consist of a small, positively
charged nucleus in the center surrounded by negatively charged electrons.

Analogy

Rutherford’s nuclear model can be compared to
the Earth. The Earth has a very dense core, much
like an atom’s nucleus.

Niels Bohr - 1913 Atomic Model

Bohr took the
planetary model and

added a postulate:

Niels Bohr was a Danish physicist who built atomic structure is
off of Rutherford’s model and gave a more quantized, meaning
complete picture of the atom. only specific values can
be taken on. He

Bohr believed that electrons orbit the explained his model
nucleus like planets around the sun, or the with Planck’s quantum
rings around saturn. The problem was, theory of radiation.

classical physics predicted that negatively Bohr proposed that electrons circle the nucleus
charged electrons would produce in certain shells: paths with a fixed radius, given
electromagnetic energy, losing energy until it by his equation. Electrons cannot orbit between
collapses into the positively charged shells; there are only specific, fixed values of
nucleus. This meant that Rutherford’s model the radius in shells in which electrons can exist
showed that all atoms are unstable. and remain stable.

Analogy

Bohr’s model can be The idea of quantization can be

compared to the solar compared to a piano. Each key is

system. The planets orbit tuned to a specific frequency so

the sun in a fixed path that each note can be played, but

much like electrons orbit it is impossible to produce a

the nucleus. sound in between, for example, a

C and C#.

Erwin Schrodinger - 1926 Atomic Model

Erwin Schrodinger was an Austrian physicist who took According to his model, the
Bohr’s model and advanced on it. Schrodinger used location of an electron isn’t
mathematical equations to describe the movement of definite. The areas of
electrons differently than Bohr. Instead of defining the probabilities were distributed to
path of a given electron, Schrodinger’s model predicts the form what is called orbitals.
probability of the location of an electron. Each orbital is a wave function
that describes the state of an
He used his equation and ideas from Louis de Broglie to electron.
describe the motion of electrons as floating or wavelike
rather than on orbiting on a fixed path. Schrodinger
developed what is known as the Schrodinger Equation, or
“the wave equation of non relativistic quantum
mechanics”, which is still used today.

Analogy

The nucleus can be described as being
surrounded by an electron cloud.
Where the cloud is dense, the
probability of an electron being in that
given region is more likely, while in
parts where the cloud is less dense, an
electron is less likely to be found.

Werner Heisenberg - 1927

Werner Heisenberg, like Einstein, was a German
theoretical physicist and won the Nobel Prize in
1932. Unlike Einstein, who suggested that US
research nuclear weapons,Heisenberg contributed
to the German atomic program in World War II.

Heisenberg researched with Niels Bohr and expanded on Bohr’s model. He developed
the matrix mechanics formulation of quantum mechanics, which gave the full, correct
definition of quantum mechanics.

Heisenberg stated that there is a limit on how precisely both the momentum or energy

and the position of a particle at a given instance. The more precisely one is known, the

less that is known about the other. For example, if you know

the position of a particle precisely, there is less known about

its momentum. This is not based on the ability to measure

them, but instead because of the nature of the system. His

Heisenberg’s uncertainty principle principle was called the Heisenberg uncertainty principle or

can be compared to taking a “ungenauigkeit” which means imprecision.

picture of a ball. When the ball is

not blurry, the location is easily

deciphered, but you cannot If the picture is blurred, you can see its movement, but it Analogy
determine its momentum. is harder to pin its exact location.