Timeline of an Atom

The History o

By Prasanth De

of the Atom

endukuri

What is an A

- The smallest piece of an element that maintains the
Atoms are the basic units of matter and the defining

- In the center of an atom, the
nucleus is located

- the nucleus consists of protons
and neutrons, subparticles of an
atom

- In the outer orbits/rings, lie
electrons, another type of sub
particle in an atom

Atom?

e identity of that element is called an atom.
g structure of elements.

Proton - A proton is a subatomic 2
particle with a positive electric
charge
Electron- a stable subatomic
particle with a charge of negative
electricity,
Neutron- a subatomic particle
without an electric charge, neither
positive nor negative

Democritus

Thought experiment:
- Knew that if you take a

substance and keep cutting it

in half, each half would have

the same properties as

original substance

- If you keep cutting the

substance in half, there will be

a point where you will reach a An
is
point where you can no longer a
wh
divide the substance in half
Hi
because of it being so tiny

- He called these indivisible

pieces “atomos” which

literally mean indivisible His atomic model,
atomos, shaped like
balls

Fun fact:

Democritus also dabbled in mathematics, and is credited with
being the first to notice that the volume of a cone or pyramid is a
third that of a cylinder or prism with the same measurements.

442 BC

nalogy: An atom
represented by
basketball,
hich is a sphere

is atomic theory:
- Everything is made of atoms.
- They are the most basic unit of matter
- Atoms cannot be divided any further

Aristotle

Thought experiment: Aristotle's atomic th

- Rejected Democritus’ atomic Even though Aristotle’s theory
theory completely off to the modern
atomic theory, it lasted for aro
- Believed that matter was 2000 years!
made up of 4 elements:
Earth, Water, Air, and Fire

- believed that all matter was
made up either of one of the
elements or combinations of
these four elements

- The fifth element, Aether, or
quintessence, is the
materials that fill the
universe outside of what
exists on a planetary level.

- According to Aristotle, if a substance happened to be wet and
cold, then it was more likely to contain more of the water and
earth elements than fire and air elements. If something was
dry and cold, then it was more likely to contain more fire and
earth elements.

360 BC

heory Analogy: If matter was a salad, the croutons in the
salad are Earth, lettuce is water, tomatoes are Fire,
y was corn is Air and the ranch dressing is Aether. All of
ound these ‘elements’ make up matter, which is the total
salad.

Interesting fact: Aristotle created the world's first library. Aristotle had
established a philosophical school known as the Lyceum in 335 B.C. The
Lyceumhoused a collection of manuscripts comprising one of the world’s first
great libraries.

John Dalton

Dalton’s Atomic Theory: ● Dalton based his theory partially
- Theorized all matter is conservation of mass
made of atoms
- Performed experiments
to prove existence of
atoms

1. All matter is made of

atoms. Atoms are

indivisible and

indestructible.

2. All atoms of a given

element are identical in

mass and properties

3. Compounds are formed

by a combination of two

or more different kinds of

atoms.

4. A chemical reaction is a

rearrangement of atoms.

1803

y on the law of Fun Fact: John Dalton was color blind!

Analogy: Each atom is unique, and can be
bonded to each other to create objects,
similar to attaching lego blocks to each other
to create a structure

JJ Thomson

Cathode Ray Tube
Experiment

- On one side of the inside of the tube

there's a cathode and an anode. The

cathode is a negatively-charged

conductor, and the anode is a

positively-charged conductor.

- The tube is a vacuum

- This causes a particle beam to go from a

positive charge to a negative charge

- He tried many different experiments

using magnets and reversing charges on

both sides of the tube Atomic Model
- Thomson tried changing the cathode

material, which was the source of the ● Thomson proposed a model of t

particles. Since the same particles were consisting of positive and negat

emitted even when the cathode materials charges, present in equal amoun

were changed to different metals, an atom would be electrically ne

Thomson concluded that the electron was ● He proposed the atom was a sphe
a fundamental part of all atoms. positive and negative charges wer
- embedded within it

1897

Fun Fact: Thomson was given the 1906 Nobel
Prize in Physics for the finding of the electron
and for his study on the conduction of electricity
in gases.

the atom Analogy: The cookie dough represents positive
tive matter spread throughout an atom. The chocolate
nts so that chips represent the negatively charged matter in
eutral. an electron. The positive and negative matter is
ere, but the evenly spread out in an atom
re

Ernest Rutherfo

His experiment: Rutherford’s Atomic Model:
- central nucleus- where are all
- Fired positively charged positively charged particles lie
particles at a gold foil (reason for deflection of alpha
particles
- Expected all particles to pass - Outer cloud of electrons
through gold foil - Most of atom is empty space
(reason for alpha particles passing
- vast majority of alpha particles through foil)
were only slightly deflected
from their paths

- Others past straight through
the foil

- This experiment negated
Thomson’s atomic model

- this meant that atoms were
mostly empty space, not like a
plum pudding as J.J. Thomson

previously thought

ord 1908

Analogy: Avocado with seed Fun Fact: Element
in the middle. The seed
represents the nucleus of an 104, Rutherfordium, is
atom named after
Rutherford!

Niels Bohr 1913

Atomic Model:

- Electrons orbit around the Fun fact: Bohr won
nucleus, which is in the center Nobel Prize at the s
time as Albert Einst
- The nucleus contains positively
charged atoms

- Niel Bohr proposed that most of the mass of an atom is contained in the
around separate orbits around the nucleus of an atom. He is also determ
properties of the element. The orbits of electrons are distant from the nu

Analogy:

n the - The sun represents the nucleus and all the
same planets represent the electrons
tein..

e nucleus. Bohr was the first to discover that electrons travel
mined that the number of outer electrons determines the
ucleus. The nucleus contains the protons and the neutrons.

Erwin Schroding

Thought experiment: ● Schrodinger’s Atomic Mod
● Electron clouds are where
- Disagreed with Bohr’s “orbit
model” electrons are most likely f
Analogy: The outer part o
- Believed that electrons were cotton ball represents the
located in electron clouds, electron pclouds of an ato
dense regions of an atom

- These dense regions are known
as “electron orbitals”

- used mathematical equations

to describe the likelihood of
finding an electron in a

certain position.

- He said that electrons were
constantly moving and didn’t
have one definite or fixed
position in the atom.

- An electron’s location is
dependent on the energy of the
electron and not completely
random

ger

1926

del Fun Fact: A Crater on the Moon is
e Named After Schrodinger!
found
of a dense
outer
om.

Albert Einstein

Thought experiment:

- Came up with the theory of relativity by

observing the movement of liquid

molecules

- Founded photoelectric effect

- “When light with energy above a certain

threshold hits a metal surface, an

electron that was previously bound to

the metal is knocked loose. Each particle - His atomic model

of light, called a photon, collides with an

electron and uses some of its energy to

dislodge it from the metal. The rest of

the photon’s energy is transferred to the

now free-roaming negative charge,

called a photoelectron.”

- This proved the existence of atoms

mathematically

- Analyzed and wrote the mathematical Analogy: Moving fan - You can coun
formula to predict the size of both atoms the fan has when when the blades o
and molecules. blades are located somewhere in th
and the blades are moving, but at a
can't be exactly sure where each bla

1905

Fun Fact: He paid his first wife his Nobel
Prize money for a divorce, which was
32,000- 10 times the salary of an average
professor!

nt the number of blades
on a fan are at rest. The
he blur when the fan is on
any particular time you
ade is located.

Werner Heisenb

Thought experiment: - His atomic model
- Different orbitals represe
- Formulated quantum
mechanics in terms of matrices different energy levels

- Discovered the Heisenberg
Principle which states that a
particle's position and
momentum during a given
moment cannot be known
exactly

- this is because an electron is
such a tiny tiny amount of
mass, the act of observing it
with any kind of light (radiation)
would move the particle in a
different direction/velocity

- Electrons have 3 dimensional
movements

An analogy for energy levels of electrons is a staircase.
The landing at the bottom of the staircase is t
Each step up represents a higher energy level
The step height represents an energy differen

berg

1926

ent

Fun Fact: Heisenberg was captured by
American Troops during World War II and
sent to England!.

.
the lowest level.
l.
nce between levels.