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Matter,
Elements, &
Atomic
Structure
Matter
 Matter
is anything that has mass and takes
up space.
 Ex: buildings, a fork, air, your clothes, etc…
 The
ancient Greeks believed that all
matter is made up of four elements: air,
earth, fire, and water. Much later,
scientists realized that matter is composed
of many different elements.
Elements
 Elements
are the simplest pure
substances, and they cannot be broken
down into any other substances.
 Elements
are often called the building
blocks of matter because all matter is
composed of one element or a
combination of two or more elements.
What is an atom?
 An
atom is the smallest particle of an
element (or unit of matter) that
retains the identity of the
substance.
Atomic Structure
 Atoms
are composed of 2 regions:
 Nucleus: the center of the atom that
contains the mass of the atom
 Electron cloud: region that surrounds
the nucleus that contains most of the
space in the atom
Electron
Cloud
Nucleus
What’s in the Nucleus?
 The
nucleus contains 2 of the 3 subatomic
particles:
 Protons
(p+) : positively charged
subatomic particles
 Neutrons (n0): neutrally charged
subatomic particles
What’s in the Electron Cloud?
 The
3rd subatomic particle resides
outside of the nucleus in the electron
cloud.
 Electron
(e-): the subatomic
particle with a negative charge and
relatively no mass
How do these particles
interact?
 Protons
and neutrons live compacted
in the tiny, positively charged nucleus
accounting for most of the mass of
the atom.
 The negatively charged electrons are
small and have a relatively small mass
but occupy a large volume of space
outside the nucleus.
How do the subatomic particles
balance each other?
 In
an uncharged atom:
 The protons = the electrons


If 20 protons are present in an atom then 20
electrons are there to balance the overall
charge of the atom—atoms are neutral or
uncharged.
The neutrons have no charge; therefore they
do not have to equal the number of protons
or electrons.
How do we know the number of
subatomic particles in an atom?
 Atomic
number: this number indicates the
number of protons in an atom.


Ex: Hydrogen’s atomic number is 1
 So hydrogen has 1 proton
Ex: Carbon’s atomic number is 6
 So carbon has 6 protons
**The number of protons identifies the atom.
Ex. 2 protons = He, 29 protons = Cu
How do we know the number of
subatomic particles in an atom?
 Mass
number: the number of protons and
neutrons in the nucleus.
Ex: hydrogen can have a mass of 3.
Since it has 1 proton it must have 2 neutrons

#
of neutrons = mass # - atomic #
Determining the number of
protons and neutrons
 Li
has a mass number of 7 and an atomic
number of 3


Protons = 3 (same as atomic #)
Neutrons= 7-3 = 4 (mass # - atomic #)
 Ne
has a mass number of 20 and an atomic
number of 10


Protons = 10
Neutrons = 20 - 10= 10
What about the electrons?
 The
electrons are equal to the number of
protons (in an uncharged/neutral atom).

So e- = p+ = atomic #
 Ex:
Na has a mass # of 23 and an atomic #
of 11
= 11
no = 12
e- = 11
 p+


Where Do You Find “these” Numbers?
Atomic Number
Symbol
Name
Atomic Mass
(this is an average, always round)
electrons
protons
Atomic number equals the number of _________or
__________.
protons + ___________.
neutrons
Atomic mass equals the number of _________
Determine the number of
subatomic particles in the
following:
 Cl
has a mass # of 35 and an atomic # of
17
 p+ = ____,
K
no = ____, e- = ____
has a mass # of 39 and an atomic # of 19
 P+ = ___,
no = ___ e- = ____
Early Ideas about Atoms!!!
 Democritus-
a Greek Philosopher who lived about
440 B.C.
 He thought you could“cut” down matter into
pieces, until you reached the smallest possible
piece
 He named these pieces atoms which is Greek for
“uncuttable”.
Early Ideas about Atoms!!
 John
Dalton- a British school teacher
 In 1802 proposed an Atomic Theory.
 He carried out experiments and concluded that
atoms have different characteristics.
Atomic Theory grew as a series of models that
developed from experimental evidence. As more
evidence was collected, the theory and models
were revised.
 To this day, scientists have found few exceptions to
Dalton’s theories.
Dalton’s Atomic Theory
 Atoms
can’t be broken into smaller pieces.
 Atoms of the same element are exactly the
same.
 Atoms of different elements are different
 Atoms of 2 different elements combine to form
compounds.
 Atoms of each element have a unique mass.
 The masses of the elements in a compound
are always in a constant ratio.
 Ex.
Water- H2O
2 Hydrogen
1 Oxygen
J.J. Thomson
 J.J.
Thomson was a British scientist who
found that atoms contain negatively
charged particles in 1897.
 Because scientists knew that atoms didn’t
have an electrical charge, Thomson
reasoned that atoms also contained
some sort of positive charge.
 Thomson described an atom as having
negative charges scattered throughout a
ball of positive charge.
Earnest Rutherford
 In
1911, one of Thomson’s students found
evidence that went against his model.
 Rutherford’s research team aimed a beam
of positively charged particles at a thin
sheet of gold foil. They found that some
particles passed through without problem,
but some deflected strongly.
 Since like charges repel, Rutherford inferred
there had to be a cluster of positive
charges in a tiny region (called a nucleus).
Earnest Rutherford
 Scientists
knew that electrons had almost
no mass, so they reasoned that nearly all
of an atom’s mass must be located in the
tiny, positively charged nucleus.
 Later it was suggested that the nucleus
was made of one or more positively
charged particles.
 Rutherford named those particles –
protons.
What is the structure of an atom?
Bohr Model
“Planetary Model”
Bohr model – electrons are in
specific energy levels
Schrödinger Model
“Electron Cloud Model”
Electron cloud model – electrons are in a
random cloud
James Chadwick
 In
1932, British scientist James Chadwick
discovered another particle in the nucleus
of atoms.
 His discovery completed the modern
atomic model.
 It was the hardest to detect because it
had no charge.
 This electrically neutral particle was called
a neutron.