Transcript atom - davis.k12.ut.us

History of Atoms Notes
Use the information on each slide to fill in the blanks
on the student notes.
History of Atoms
Democritus
Who first thought of the atom?
The Greek philosopher Democritus (about 400
BC) – he thought if you cut an object over and
over and over again, there would eventually be a
piece that you couldn’t cut anymore. He called
that piece an atom. In Greek, atom means
“indivisible”. His idea of an atom was a round,
solid mass.
Round, Solid Atom
Democritus by Agostino Carracci (1557-1602)
History of Atoms
John Dalton
The Atomic Theory
John Dalton was an English Scientist. In
1803 he proposed the atomic theory
that says:
-Atoms cannot be divided
-Atoms are invisible
-All elements are made of atoms
-Atoms of the same element are the
same
-Atoms of different elements are
different
-Molecules are formed by two or
more atoms joined together
Round, Solid Atom
Billiard Ball Model
History of Atoms
Joseph J. Thomson
What are atoms made of?
In 1897 a scientist named Joseph J. Thomson
was experimenting with gases and electricity and
discovered that atoms have negative and
positive charged particles. The negative
charged particles are called electrons and the
positive charged particles are called protons.
He imagined an atom to be similar to plum
pudding with lumpy, negative electrons in the
smooth, proton pudding.
History of Atoms
Ernest Rutherford
What else are atoms made of?
Ernest Rutherford discovered in 1908 that
atoms are mostly made of empty space while
doing his now famous GOLD FOIL experiment.
In the center of the atom is a dense nucleus of
protons surrounded by electrons. In fact, if the
nucleus of an atom was the size of a pea, the
atom would be as big as a football stadium! The
electrons would be flying around the outside of
the stadium and the rest of the atom would be
empty space! Rutherford’s electrons orbit the
center of the atom.
History of Atoms
Ernest Rutherford
Imagine one of these in the middle of there. All the rest of the stadium is empty
space and the electrons are flying around the outside.
Ernest Rutherford’s
Gold Foil Experiment
Ernest Rutherford’s biggest achievement in
atomic theory (empty space) was made during
his GOLD FOIL experiment. He shot a beam of
alpha particles (radioactive) through a thin gold
foil sheet, expecting to have them pass through
the plum pudding model of the atom. However,
a small number of positive particles were
repelled at very high angles while most were not
deflected at all, passing right through. This
showed there was a solid center to the atom
that the particles bounced off of.
Rutherford's gold foil experiment helped
describe the nuclear structure of the atom by
proving the existence of positive particles
lumped together in the center of the atom –
later called the nucleus.
Top:
Expected results - alpha
particles passing right through
the plum pudding model of
the atom undisturbed.
Bottom:
Observed results - small
portion of the particles were
deflected, and some by very
large angles, indicating a small,
concentrated positive charge.
Rutherford concluded that
the positive charge must be
all in the center of the atom.
History of Atoms
Niels Bohr
What do atoms look like?
Niels Bohr in 1913 proposed that the atoms
look like a solar system, with the protons in
the nucleus acting like the sun, and the
electrons flying around the nucleus like
planets orbiting the sun. Electrons orbit the
nucleus in shells. When drawn this way, it is
called a Bohr diagram of an atom.
Bohr Diagram: electrons are drawn in
shells around the nucleus
Bohr believed the electrons
orbit the nucleus
History of Atoms
Werner Heisenberg
What do we currently believe?
Werner Heisenberg in 1926, proposed that
electrons don’t really fly all around the atom. He
said that electrons had certain spots they floated
around in called orbitals (sort of like balloons). He
created Quantum Mechanics (mathematical
description of energy and matter) and the
Uncertainty Principle which says you cannot
know the exact position and speed of an
electron at any given moment.
Heisenberg’s atomic model uses an electron
cloud. The electron cloud maps the possible
location of electrons in an atom instead of specific
locations.
Heisenberg’s Model: Electron Cloud
(dots are possible spots electrons may be found)
History of Atoms
James Chadwick
The Last Piece of the Puzzle
James Chadwick in 1932, discovered
that atoms also contain neutrons.
Neutrons do not have a charge, they
are “neutral”. They have the same mass
as a proton. The neutrons and the
protons make up the nucleus, or the
center of the atom.
Chadwick discovered neutrons
in the nucleus of an atom
History of Atoms
Scientist Mnemonic Device
Did
Dumbledore
Try
Repelling
Behind
Hogwarts
Castle?
Democritus
Dalton
Thomsen
Rutherford
Bohr
Heisenberg
Chadwick