Development of the Atom

By: Cindy Nguyen

Early Greeks

332 BCE - 450 BC

Democritius

400-450 BC

Came up with the theory that
all materials
are made up of
indivisibly small "atoms"

Aristotle

332 BCE

Believed that all matter was made up
of the 5 basic elements:
earth, fire, air, water and ether

Didn't believe in the theory
that all matter is made of atoms

1800's

John Dalton

1808

Stated 3 things:

1. all matter was made of small,
indivisible particles called "atoms
2. the atoms in a given element all have
unique characteristics and weight
3. there are 3 different types of atoms that
exist: simple (elements), compound (simple
molecules), and complex (complex molecules)

Came up with the "Marble Model"

--> Also known as the "Billiard Ball
Model"

Michael Faraday

1839

Discovered that atoms had an electrical
component to them

Julius Plucker

1859

A pioneer in the investigations of
cathode rays that eventually led
to the discovery of the electron

A cathode ray is a stream of
electrons that are seen in
vacuum tubes

Dmitri Mendeleev

1869

Created the periodic table

The elements where arranged in order
of increasing atomic weight

George Johnson Stoney

1879

Determined the charge of the atom

--> also known as the "particle of
electricity"

Introduced the term "electron"

Eugen Goldstein

1886

Discovered positive particles by using a tube
filled with hydrogen gas

The positive particle that he discovered
is now known as "proton"

Henri Becqurel

1896

Discovered radioactivity by accident

Radioactivity: The release of energy
from the decay of the nuclei of
certain types of atoms and isotopes

J.J Thomson

1897

Created the "plum pludding model"

The model contains:
- negatively-charged electrons
- a positively charged "soup"

Discovered the electron

Marie and Pierre Curie

1898

Discovered Polonium and Radium

"Polonium" is named after Poland, which was
Marie's homeland

Marie Curie concluded that
radioactivity originates
within the atom itself
(something was happening inside
the atom itself)

Radioactivity isn't dependent on the way that
the atoms are arranged to form molecules

1700's

Isaac Newton

1704

Suggested that atoms are held together
with attractions

These attractions are known as "forces"

1900-1920's

Max Planck

1900

presented a theory known as
"Planck's Quantum Theory"

Hypothesized that matter emits/absorbs
energy in small packets of energy called
"quantum"

Nagaoka Hantaro

1904

Came up with the Saturnian model

Contains:
- small nucleus (orbited by electrons)

It was also the first ever model to include
the nucleus core

Robert Millikan

1910

Discovered the charge of an individual electron

Also discovered the fact that all electrons carry the
same amount of negative charge

Ernest Rutherford

1911

Discovered atoms contained a positively charged
nucleus that is much smaller than the actual atom

Also discovered about the existence
of gamma, alpha, and beta rays

His model is known as the nuclear model

Conducted a goid foil experiment where a piece of
gold foil was hit with alpha particles

--> alpha particles have a positive charge

Rutherford did not expect the particle to go through gold,
however they did.

--> He concluded that the gold atoms were
mostly empty space

This means that there is large empty space around the nucleus, meaning that the nucleus is very small!

Neils Bohr

1913

Modified Rutherford's model to create his own
representation of the model

Modified Rutherford's model to create his own
representation of the model

Discovered that the electrons don't fall into the nucleus

Added a condition stating that the shells have
fixed energy. Due to this condition, he solved the
stability problem of the electrons

His theory was that the electrons orbit the nucleus
at fixed energy levels

Henry Moseley

1913

Came up with "Moseley's Law" where the law
gave scientists a more accurate way to
organize elements

Discovered an accurate mathematical connection
between the frequency of x-ray radiation
and an element's atomic number

Frederick Soddy

1913

Announced the concept that atoms can
have be the same chemically, but have
different atomic weights

These related atoms are known as
"isotopes"

Francis William Aston

1919

Developed the mass spectrograph to map
the different isotopes

Discovered the isotopes of the light elements
using his spectrograph

A spectrograph is a device that separates a signal into its component wavelengths (think of a prism splitting white light into colours)

1921-1929

James Franck

1920-1933

The Franck-Hertz experiment
gave proof for Bohr's theory that
an atom can absorb internal
energy (quanta) in only
precise and definite amounts

The experiment consists of a vacuum
tube designed to study the energetic
electrons that flew through a
thin vapor of mercury atoms

The experiment demonstrated the
existence of excited states in
the mercury atoms

Arthur Compton

Founder of the Compton Effect

Demonstrates the particle nature
of electromagnetic radiation

Electromagnetic radiation is made when
an atom absorbs energy which allows
for one or more electrons
to change their place within the atom

Louis De Broglie

1923-1924

Introduced the idea that particles
(such as electrons) could be described as
waves

Proposed the wave-mechanical model (1920s)
with Erwin Schrodinger to determine
that Bohr's model wasn't useful to determine
the locations of the electron

Model proposes that each electron circling an atom's
nucleus occupies a specific orbital (s,p,d,f)
and spins in a certain direction

Werner Heisenberg

1925

Contributed to atomic theory by
formulating quantum mechanics
in terms of matrices (sets of numbers)

Founded the uncertainty principle

States that we can't know both the position
and speed of a particle

Max Born (1925)

After Heisenberg's work, he contributed to
the further development of
quantum mechanics

Proved that Schrodinger's wave equation
could be interpreted by giving statistical
predictions of variables rather than
exact numbers

Erwin Schrodinger

1926

Formulated a wave equation that accurately calculated
the energy levels of the electrons in the atom

Proposed the wave-mechanical model
with Louis de Broglie in the 1920s

Paul Dirac

1928

Developed a relativistic quantum theory that
produced results that he believed
were produced by an electron-like particle
with a positive charge

Equation described how particles move slower
than the speed of light

Egil Hylleraas

1929

Applied Schrodinger's equation to
the Helium atom (that has 2 electrons)

--> Bohr's model couldn't explain for atoms
that had more than 1 electron

Hylleraas was also able to explain the
electron's energy

1930-1949

James Chadwick

1932

Known for his discovery of the neutron
(proved their existence)

Enrico Fermi

1942

Showed that almost every chemical element
could be made radioactive

His work on radioactivity soon led to the
discovery of nuclear fission

During nuclear fission, an atom's nucleus is
split apart

--> the split causes energy to be released

Maria Goeppert-Mayer

1949

Developed the nuclear shell model where
nucleons (neutrons + protons) were
distributed in shells with diff energy levels

Developed the model with her friend,
Hans Jensen

Hans Jensen

Model explains the distribution of energy
levels into various atom shells

Displays observations of the direction where
the nucleons (protons + neutrons) rotate their
own axes and around the nucleus' center

The atomic nucleus is thought to have
a structure of shells/spherical layers
that contains those nucleons