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Questions
Notes
The Guide to Atomic
Structure
What is an atom?
Atomic Structure
An atom is the
smallest part of an
element that still
has the element’s
properties.
Atoms are the
building blocks of
molecules.
What are the two
regions of the atom?
Atomic Structure
Nucleus: At the center of
each atom, small volume,
large density, positive
electric charge.
– made of protons and
neutrons
– contains 99.99% of the
atom's mass
Electron cloud: contains electrons, outside the
nucleus, negative charge, large volume, low
density, divided into energy levels.
What are the
particles in the
atom?
Atomic Structure
Atomic Structure
All atoms are made of the same 3
subatomic particles in different
Copy this table
amounts:
Symbol Charge Mass
Actual Mass
Number
Proton
p+
Neutron
n
Electron
o
e-
+1
0
-1
1
1.673* 10-27
kg
1
1.675* 10-27
kg
0
9.109* 10-31
kg
What is the charge
of an atom?
Atoms are Neutral
• Atoms are electrically neutral because
they contain equal numbers of protons
and electrons.
• A proton has a positive charge equal in
magnitude to the negative charge of an
electron.
8 + protons
8 – electrons
= neutral atom
Ions are Atoms with a Charge
An ion is an atom that has lost or gained
electrons and has a negative or positive
charge.
Atoms that lose electrons have a positive
charge. Atoms that gain electrons have a
negative charge.
If an atom is an ion the charge & number
will follow the element symbol.
O
-2
What holds the
particle in the
nucleus together?
Forces in the Nucleus
• When two protons are near each other,
there is a strong electromagnetic force
pushing them apart.
• Like charges repel.
Forces in the Nucleus
• The force that hold the protons together
is the strong nuclear force.
• The strong nuclear force is provided by
the neutrons and only acts over very
short distances.
Neutron hold
Strong Nuclear Force
How are atoms
measured?
The Sizes of Atoms
• The radius of an atom is the distance
from the center of the nucleus to the
outer portion of its electron cloud.
• Atomic radii are measured in picometer,
pm.
• 1 pm = 1 × 10−12 meter
• Mass of atoms is measured in atomic
mass unit, or amu.
• 1 amu = 1.660 540 × 10−27 kg
Counting Atoms
Mass Number
• The mass number of an atom is the
sum of protons & neutrons in the
nucleus of an atom.
• The electrons are too small to add
any measurable mass to the atom.
What are isotopes of
an atom?
Counting Atoms
Isotopes
• Isotopes are atoms of the same
element that have different masses.
• The isotopes of a particular element
all have the same number of
protons and electrons but different
numbers of neutrons.
• Most of the elements consist of
mixtures of isotopes.
The 3 Isotopes of Hydrogen:
Copy this diagram
Counting Atoms
Average Atomic Masses of Elements
• Average atomic mass is the weighted
average of the atomic masses of the
naturally occurring isotopes of an
element.
• The average atomic mass is listed on the
periodic table- it is an average of many
• The mass number is the mass of a
specific isotope- it is the exact mass for
that isotope
How are different
isotopes of an atom
written?
Counting Atoms
Designating Isotopes
• Hyphen notation: The mass number is
written with a hyphen after the name of the
element.
uranium-235
• Nuclear symbol: the mass number on top
and the atomic number on bottom followed
by the element symbol.
235
92
U
How can you
determine the
structure of an atom?
Counting Atoms
• number of protons = the atomic number
from the periodic table
• number of electrons = the number of
protons in neutral atoms
Lithium-7
Atomic number is 3
3 p+
3 e-
Counting Atoms
• number of neutrons = the mass number
minus the atomic number
mass number  atomic number = number
of neutrons
Li-7
7  3 protons = 4 neutrons
Counting Atoms
• number of electrons in an ion = the atomic
number minus the ion charge & number
atomic number  ion charge & number =
number of electrons
O-2
8  (-2) = 10 electrons
+2
Cu
29  (+2) = 27 electrons
Watch this video on Calculating
Atomic Structure
Click the button
Atomic Structure Video
Or copy & paste this address into your web browser:
http://www.youtube.com/watch?v=CokAnD1uKvA
How can you draw the
structure of an atom?
Placing the Particles in the Atom
-
-
Draw the protons
& neutrons in a
tight bunch in the
center
+
-
o
o +
-
-
Copy this diagram
-
-
Start with the
first energy
level & add up
to 2 electrons
The second
energy level
holds up to 8
electrons