Development of Atomic Theory

Atomos: Not to Be Cut

The History of Atomic Theory
from atomos to
strings

Early Atomic Theories

Atomic Structure

Timeline

Development of Atomic Theory

• originated 2000 BC

•

Thale

of

Melitus - matter was the basic
component of the universe

•

Eraclitus - matter was made of fire

• Auximeues - air was the basic component
of matter

Development of Atomic Theory

•

Empedocles

- developed the
idea that matter was made up
of four Elements - Earth,
water, fire, air

•

Leucippus - come up with
the idea of atoms as
invisible units of matter

Democritus (400 B.C.)

• proposed that matter was composed of
tiny indivisible particles that could not
be divided into smaller and smaller
pieces forever, eventually the smallest
possible piece would be obtained

• Greek: “atomos,” meaning “not to
be cut.”

• Not based on experimental data

Atomos

 small, hard particles that were
all made of the same material
but were different shapes and
sizes

 infinite in number, always
moving and capable of joining
together

Aristotle and Plato

• claimed that there
are only four
elements; the earth,
fire, air and water

This theory was ignored and
forgotten for more than 2000
years!

A Timeline of Atomic Models

Alchemy
500 BC -1720

• very old study and philosophy of how to
change basic substances (such as metals)
into other substances

• goal was two-fold:

– to create the Philosopher's Stone (which
caused the transmutation of lead into
gold)

– and the discovery of the Elixir of Life
(bestowing immortality on the person
who possessed it)

Alchemy (next 2000 years)

• Mixture of science and mysticism.
• Lab procedures were developed, but alchemists did not
perform controlled experiments like true scientists.

(12-1500 CE)

Galileo
(~1600 CE)

• birth of modern science -
combining logic,
experimenting, publishing
results

• made revolutionary telescopic
discoveries

Antoine Lavosier & Joseph Priestly
(1700’s)

• Quantitative analysis of
chemicals

Law of Conservation of Mass:
Matter can neither be created nor
destroyed

Joseph Priestly

Antoine Lavosier

Joseph Proust
(1700’s)
• Developed Law of Definite Proportions

Law of Definite
Proportions:
Different samples of the
same compound always
contain its constituent
elements in the same
proportions by mass

Copper carbonate always
contains
5.3 parts copper
4 parts oxygen
1 part carbon

by mass

Robert Boyle

• (1766-1844),English, school
teacher
– Stressed that “the basic nature of
elements changes if they
decompose chemically”.

Modern Atomic Theories

John Dalton (1808)

• British School Teacher

– based his theory on others’
experimental data

• Billiard Ball Model

– atom is a
uniform,
solid sphere

Dalton(1808)
Law of Multiple Proportions

Law of Multiple Proportions:
If 2 elements combine to form
more than one compound, the
masses of one element that
combine with a fixed mass of
the other element are in small
whole number ratios

Dalton’s Atomic Theory - 1808

1. Elements are composed of small indivisible particles called
atoms.

2. Atoms of same element are identical (size, mass, reactivity)

• Atoms of the same element are identical.

• Atoms of different elements are different.

3. Atoms of different elements combine together in simple
proportions to create a compound.(Law of Definite Proportions)

4. In a chemical reaction, atoms are rearranged, but not changed.
(Law of Conservation of Matter)

FYI……….Isotopes

• Dalton was wrong about all
elements of the same type being
identical

• Atoms of the same element can
have different numbers of neutrons.

• Thus, different mass numbers.
• These are called isotopes.

Frederick Soddy

• Frederick Soddy (1877-1956)
proposed the idea of isotopes in
1912 (note this was close to 30 years after Dalton’s original idea)

• Isotopes are atoms of the same element

having different masses, due to varying
numbers of neutrons.

• Soddy won the Nobel Prize in
Chemistry in 1921 for his work with
isotopes and radioactive materials.

Henri Becquerel (1896)

• Discovered radioactivity

– spontaneous emission of
radiation from the nucleus
• Three types:

– alpha (α) - positive
– beta (β) - negative
– gamma (γ) - neutral

J. J. Thomson (1903)

• Plum-pudding Model

– positive sphere (pudding) with
negative electrons (plums)
dispersed throughout
• Cathode Ray Tube
Experiments
– beam of negative particles
• Discovered Electrons

– negative particles within the atom
• Determined the charge-to-
mass ratio of an electron

Conclusion on Cathode Ray Tube
(CRT) Experiment

Where did they
come from?

Conclusion on CRT Experiment

• negative charges came from within the
atom

• particle smaller than an atom had to
exist

• atom was divisible
• negatively charged “corpuscles,”
known as electrons

• reasoned that there must be positively

charged particles in the atom

Conclusions from the Study of
the Electron:

A. Cathode rays have identical properties
regardless of the element used to produce
them. All elements must contain identically
charged electrons.
B. Atoms are neutral, so there must be
positive particles in the atom to balance the
negative charge of the electrons
C. Electrons have so little mass that atoms
must contain other particles that account
for most of the mass

Mass of the Electron

1916 – Robert Millikan determines
the mass of the electron: 1/1840
the mass of a hydrogen atom; has
one unit of negative charge

The oil drop apparatus

Mass of the electron is
9.11 x 10-31 Kg
Electron’s charge:
1.60 x 10-19 C

Robert Millikan

Ernest Rutherford (1911)

• Discovered the nucleus

– dense, positive charge in
the center of the atom

• Gold Foil Experiment

• Nuclear Model

– dense, positive nucleus
surrounded by negative
electrons

Rutherford’s Gold Foil Experiment

• gold atoms in the sheet were mostly
open space

• atoms were not a pudding filled with a
positively charged material

• atom had a small, dense, positively
charged center that repelled his
positively charged “particles

• Nucleus is the center of the atom,
tiny compared to the atom as a whole

• protons have same charge as e-, but
almost 2000x more mass

Rutherford’s Gold Foil Experiment

Rutherford Nuclear Model

• atom had a dense positive
core, with the rest composed
of mostly empty space with
the occasional negatively
charged electron

note: this model completely changed the definition of atom

Rutherford’s Problems

• How is nucleus held together?
• Why don’t electrons collapse into nucleus?

• H atom has 1 proton & He atom has 2 protons, ∴
mass ratio should be 2:1; instead the ratio is 4:1
…there must be another particle

• hydrogen nucleus (being the lightest of all
nuclei) was an elementary particle named it
as proton, from the Greek word 'protos',
meaning 'first’

• predicted the existence of the neutron in
1920

James Chadwick (1932)

• Discovered neutrons

– neutral particles in the
nucleus of an atom

• Joliot-Curie
Experiments
– based his theory on their
experimental evidence

James Chadwick (1932)

Neutron Model

• revision of Rutherford’s Nuclear Model

The Neutron

• Discovered by James Chadwick in 1932.
• Neutron is electrically neutral & has slightly greater
mass than a proton

Mystery solved.

Updating Dalton’s Atomic Theory

3 major differences between modern atomic theory &

Dalton’s atomic theory:

• Atoms are NOT indivisible – they are made up of
protons, neutrons, and electrons

• Atoms of the same element are NOT exactly alike –
they can have different masses (isotopes)

• Atoms CAN be changed from one element to another,
but not by chemical reactions (nuclear reactions)

Niels Bohr (1913)

• Energy Levels

– electrons can only exist in
specific energy states

• Planetary Model

– electrons move in circular
orbits within specific energy
levels

Niels Bohr (1913)

• Bright-Line Spectrum

– tried to explain presence of
specific colors in hydrogen’s
spectrum

• Quantum Leap

– Electrons can jump between levels
with energy being added/released

Brightline spectrum

Erwin Schrödinger (1926)

• Quantum mechanics

– electrons can only exist in
specified energy states

• Electron cloud model

– orbital: region around the
nucleus where e-are likely
to be found

Erwin Schrödinger (1926)

Electron Cloud Model (orbital)

• dots represent probability of finding an
e-not actual electrons

The Wave Model

Quantum Mechanical Model
•

Electrons don’t move around the
nucleus in orbits.
•

Electrons exist in specific energy
levels as a cloud.
•

The electron cloud is the region of
negative charges, which
surrounds the nucleus.
•

Orbital : The region with a high
probability of containing electrons.

Electron Cloud:

• Depending on their energy they are
locked into a certain area in the
cloud.
–Electrons with the lowest energy are
found in the energy level closest to the
nucleus

–Electrons with the highest energy are
found in the outermost energy levels,
farther from the nucleus.

Atomic Model Through the Years

Indivisible

Electron

Nucleus

Orbit

Electron

Cloud

Greek

X

Dalton

X

Thomson

X

Rutherford

X

X

Bohr

X

X

X

Wave

X

X

X