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Matter
Since the atom is
too small to be seen
even with the most
powerful
microscopes,
scientists rely upon
models to help us to
understand the
atom.
Believe it or not this is a
microscope. Even with the
world’s best microscopes we
cannot clearly see the
structure or behaviour of the
atom.
Scientific Models
Scientists create
models to help them
to visualize complex
properties,
structures or
behaviours. Since
the atom is so small,
scientists must
gather Indirect
Evidence to develop
their models.
This is a model of a very
complex molecule made of
many different kinds of atoms.
Each coloured ball represents
an atom of a different element.
What should a Model look like?
This is a painting of a young woman
by Pablo Picasso. Does it actually
look like a young woman?
Scientific models
may not always
look like the
actual object. A
model is an
attempt to use
familiar ideas to
describe
unfamiliar things
in a visual way.
Is this really an Atom?
Many of the models that you have
seen may look like the one below. It
shows the parts and structure of the
atom. Even though we do not know
what an atom looks like, scientific
models must be based on evidence.
The model above represents
the most modern version of
the atom.
(Artist drawing)
Subatomic Particles
Particle Symbol
Charge
Relative
Mass
Electron
e-
1-
0
Proton
p+
1+
1
Neutron
n
0
1
Why are all Atoms
Electrically Neutral?
Normally in an atom the
number of electrons
within the electron
cloud is equal to the
number of protons in
the nucleus. The
positive and negative
charges cancel each
other out. Therefore,
the atom is said to be
electrically neutral.
If an atom gains or loses
electrons the atom is no
longer neutral . This can
happen if the atom absorbs or
releases energy The atom is
then called an ION.
IMPORTANT
In all ATOMS the number of
positively charged protons
is always equal to the
number of negatively
charged electrons.
Location of Subatomic Particles
10-13 cm
electrons
protons
neutrons
nucleus
10-8 cm
Atomic Number
Counts the number
of
protons
in an atom
Atomic Number on the
Periodic Table
Atomic Number
11
Symbol
Na
All atoms of an element have
the same number of protons
11 protons
11
Sodium
Na
Mass Number
Counts the number
of
protons and neutrons
in an atom
Number of Electrons




An atom is neutral
The net charge is zero
Number of protons = Number of electrons
Atomic number = Number of electrons
Subatomic Particles in Some
Atoms
16
O
8
8 p+
8n
8 e-
31
P
65
Zn
15
30
15 p+
16 n
15 e-
30 p+
35 n
30 e-
Learning Check
Write the atomic symbols for atoms with
the following:
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e-
___________
C. 47p+, 60 n, 47 e-
___________
Solution
A. 8 p+, 8 n, 8 e-
16O
8
B. 17p+, 20n, 17e-
37Cl
17
C. 47p+, 60 n, 47 e-
107Ag
47
Make a
diagram
of the
Helium
atom in
your
notes.