THEORY
EARLY THEORIES
Democritus was the first person to propose the idea that matter was not infinitely
divisible. He believed matter was made up of tiny individual particles called atomos.
EARLY THEORIES
Aristotle rejected the atomic “theory” entirely because it did not agree with his own
ideas on nature.
ATOMIC THEORY TIMELINE
1803 1897 1911 1913 1926
Solid Sphere Plum Pudding Nuclear Planetary Quantum
John J.J. Ernest Niels Erwin
Dalton Thomson Rutherford Bohr Schrödinger
His theory stated that Thomson discovered Rutherford ran Bohr modified Schödinger stated that
atoms are indivisible, electrons (he called experiments that Rutherford’s model of electrons do not move
those of a given them corpuscles) in concluded in positive the atom by stating in set paths around the
element are identical, 1897, for which he won charge concentrated in that electrons moved nucleus, but in waves.
and compounds are a Nobel Prize. the center: The Nucleus around the nucleus in It is impossible to know
combinations of orbits of fixed sizes. the exact location of
different types of the electrons.
atoms.
1803
SOLID SPHERE MODEL
ATOMIC THEORY: 5
STATEMENTS All matter is composed of extremely small particles
called atoms.
All atoms of a given element are identical, having the
same size, mass, and chemical properties. Atoms of a
specific element are different from those of any
other element.
Atoms cannot be created, divided into smaller
particles, or destroyed.
Different atoms combine in simple whole-number
ratios to form compounds.
In a chemical reaction, atoms are separated,
combined, or rearranged.
Couldn’t explain differences in atomic mass within an element
LIMITATIONS
(isotopes)
Didn’t account for the presence of subatomic particles like
protons, neutrons, and electrons
Couldn’t explain the behavior of atoms in chemical reactions
English physicist known for his work on the nature
of electrons
Proposed the Plum Pudding Model in the late 19th
century
Discovered electrons as distinct particles
J.J. THOMSON Shifted understanding from indivisible atom to
subatomic particles
1856 - 1940
ERNEST RUTHERFORD
1871 - 1937
New Zealand-born physicist known for his contributions to
nuclear physics
Introduced the Nuclear Model in the early 20th century
First model to propose a central, massive nucleus
Explained the behavior of positively charged alpha particles in
the gold foil experiment.
NIELS BOHR
1885 - 1962
Danish physicist known for his pioneering work in
atomic structure
Proposed the Planetary Model in the early 20th
century
Explained atomic spectra with precision
Introduced the concept of quantized energy levels
due electrons moving arund the nucleus, in orbits.
ERWIN SCHRÖDINGER
1887 - 1961
Austrian physicist renowned for his contributions to
quantum mechanics
Proposed the Quantum Model in the 1920s
Electrons move in waves around the nucleus.
It is impossible to know the exact location of electrons.
“clouds of probability”
Isotopes
Electron
What are isotopes?
Isotopes are atoms of the same element
with different masses.
Neutron
The isotopes of a particular element all
have the same number of protons and
electrons but different numbers of Proton
neutrons.
Hydrogen-1 Example 1
There are three isotopes of
hydrogen. All hydrogen atoms contain
Hydrogen-2
one proton (and one electron), but
they can have different numbers of
neutrons. Hydrogen-1 is the most
Hydrogen-3
commonly found isotope of hydrogen.
Formulas
The atomic number (Z) of an element is the number of protons of each
atom of that element.
Mass number (A) is the total number of protons and neutrons that make
up the nucleus of an isotope.
From this, an isolation process to get the # neutrons is possible.
