Matter &
The Atom
Matter
The term matter describes all of the
physical substances around us
Anything that has mass and volume
(takes up space)
The Universe is made up of matter and
energy (light, sound, and heat are not
made of matter)
Matter is made of atoms
Atoms are the building blocks of matter
An atom is the smallest whole particle of
matter
Sub-atomic particles are tiny particles that
make up an atom
Models
Models are often used for things
that are too small or too large to
be observed or that are too
difficult to be understood easily
Models
In the case of atoms, scientists use large
models to explain something that is very
small
How small are atoms?
http://learn.genetics.utah.edu/content/begin/cells/scale/
Models of the atom were used to explain
data or facts that were gathered
experimentally.
So, these models are also theories
Early Models of the Atom
(442 B.C.) Democritus
He asked: “What would
happen if you took
something (like a tree)
and kept breaking it into
smaller and smaller
pieces?
Would it still be a piece
of a tree?
Could you keep breaking
it?
He concluded:
Eventually you would
get to a size that could
no longer be broken.
This would be an
indivisible piece
Greek word: atomos =
indivisible
Thus: ATOM
Early Models of the Atom
John Dalton
(1803)
All elements are composed of indivisible particles.
Atoms of the same element are the same
Atoms of different elements are different.
Different elements are atoms with different
masses
Early Models of the Atom
J.J. Thomson
(1897)
Found negative and positive
charges
Plum pudding model
Atom made of a positively
charged material with the
negatively charged electrons
scattered through it.
Early Models of the Atom
Ernest Rutherford
(1899)
Mostly empty space
Small, positive nucleus
Contained protons
Negative electrons
scattered around the
outside
Early Models of the Atom
Niels Bohr (1915)
Electrons are small
and negatively
charged
Electrons move in
definite orbits around
the nucleus (energy
levels)
Protons are in the
nucleus and the
nucleus is small
compared to the atom
Early Models of the Atom
James Chadwick
(1932)
Discovered the neutron
Modern Model of the Atom
The electron cloud
Spherical cloud of
varying density
Varying density shows
where an electron is
more or less likely to
be
Atomic Structure
Nucleus
– Protons
– Neutrons
Electrons
Atomic Structure
Nucleus
The center of the atom
– Contains protons and neutrons
All particles are inside or around the
nucleus
Atomic Structure
Electrons (e-)
Tiny, very light particles
– Smallest subatomic particle
Have a negative electrical charge (-)
Found in a cloud outside of the nucleus
Orbits in energy levels
Atomic Structure
Protons (p+)
Located in the nucleus of the atom
Much larger and heavier than electrons
Protons have a positive charge (+)
Number of protons is different for each
element. If the number of protons changes,
the element changes.
Atomic Structure
Neutrons (n0)
Located in the nucleus of the atom
Large and heavy like protons, a bit bigger
than a proton
Neutrons have no electrical charge (neutral)
Relative size of sub-atomic
particles
Atomic Structure
proton (p+)
neutron (no)
electron (e-)
Describing Atoms
Atomic Number = number of protons
– In a neutral atom, the # of protons = the # of
electrons
– All atoms want to be neutral (no electrical
charge)
Element
Atomic #
# protons
Hydrogen
1
1
Gold
79
79
Oxygen
8
Iron
26
Lead
82
Helium
2
Sodium
11
Sulfur
16
26
11
Describing Atoms
Atomic Mass Number - sum of the
number of protons plus neutrons.
– Electrons are so tiny that their mass isn’t
enough to affect the atomic mass
Measured in Atomic Mass Units (amu)
Isotopes
Isotope – when an atom has a
different number of neutrons
than other atoms of the same
element
–Example: 2 Oxygen Atoms, one
has 8 neutrons one has 9.
Isotopes, continued
The number of protons for a given atom
never changes.
The number of neutrons can change.
Two atoms with different numbers of
neutrons are called isotopes
Isotopes have the same atomic #
Isotopes have different atomic Mass #’s