Transcript atoms - Holy Family Regional School

The Development of
Atomic Theory
TITLE
Early Ideas
Democritus—Greece 400 B.C.
His idea:
There was a limit to how far you
could divide matter. You would
eventually end up with a piece of
matter that could not be cut.
Called particles atoms--Greek
for “indivisible”
His theory:
 all atoms are small hard particles
made of a single material formed
into different shapes and sizes
always moving, and that they form
different materials by joining
together
Early Ideas
• Aristotle--Greece 384322 B.C.
– All matter was continuous,
that matter could be
divided into smaller and
smaller pieces forever —
did not believe in atoms
– Opinion accepted for
2000 years
2000 Years Later
John Dalton 1803
He performed many experiments
to study how elements join together
to form new substances.
He found that they combine in
specific ratios and he supposed it
was because the elements are made
of atoms.
His ideas:
 All matter is made of very
small particles called atoms.
 Atoms of one element are identical
in size, mass, and properties; atoms
of different elements differ in size,
mass, and properties
 Atoms cannot be subdivided, created,
or destroyed.
 Atoms of different elements combine
in simple whole-number ratios to form
compounds.
 Atoms are combined, separated, or
rearranged in chemical reactions.
J.J. Thomson 1897
English chemist and physicist discovered
1st subatomic particles
His theory:
negatively charged particles called
electrons and positively charged matter
created a model to describe the
atom as a sphere filled with positive
matter with negative particles mixed in
referred to it as the plum pudding
model
His ideas
He proposed the atom was a
sphere of positively charged
material. Spread throughout
the atom were the negatively
charged electrons similar to
plums in a pudding or
chocolate chips in ice cream.
Negatively
charged
Positively
charged
the atom
was a sphere of positively
charged material. Spread throughout the
atom were the negatively charged electrons
similar to plums in a pudding or chocolate
chips in ice cream.
The Plum Pudding Model
Ernest Rutherford
1911
New Zealand physicist discovered the
nucleus
Was a student of J.J. Thomson but
disagreed with the “Plum Pudding
Model”
Devised an experiment to investigate the
structure of positive and negative
charges in the atom.
Gold Foil Experiment
What did most of the particles
shot at the gold foil do?
Most of the particles traveled straight
through the gold foil
What was the surprising behavior of a few
of the particles?
 A few of the particles were deflected
and some even bounced back
His theory:
small, dense, positively
charged particle present in
nucleus called a proton
electrons travel around the
nucleus, but their exact
places cannot be described
most of the matter of the
atom is found in a very small
part of the atom. This is called
the nucleus of the atom. It is
very tiny and extremely dense
Niels Bohr 1913
Danish physicist discovered
energy levels
proposed that electrons move
in paths at certain distances
around the nucleus.
Electrons can jump from a
path on one level to a path on
another level
Bohr’s Model
James Chadwick 1932
English physicist
discovered neutrons
His theory
neutrons have no electrical
charge
neutrons have a mass nearly equal
to the mass of a proton
unit of measurement for
subatomic particles is the atomic
mass unit (amu)
Modern Theory
Atoms are composed of three main
subatomic particles: electron, proton,
and neutron.
Most of the mass of the atom is
concentrated in the nucleus of the
atom.
The protons and neutrons are located
within the nucleus while the electrons
exist outside of the nucleus.
21
In stable atoms, the number of protons
is equal to the number of electrons.
The type of atom is determined by the
number of protons it has.
The number of protons in an atom is
equal to the atomic number
The sum of the number of protons and
neutrons in a particular atom is called the
atomic mass