Modern Model of the Atom:
“Electron Cloud Model”
Atomic
Structure
Two regions of every atom:
Nucleus
- is made of
protons and neutrons
- is small and dense
n
pn p
n pn
Electron cloud
-is a region where you
might find an electron
-is made up of many “shells”
Subatomic particles;
Relative Actual
mass (g)
Symbol Charge mass
Name
Proton
p+
+1
1
1.67 x 10-24
Neutron
n0
0
1
1.67 x 10-24
Electron
e-
-1
1/1840 9.11 x 10-28
Opposite charges attract each other
( + and - )
Alike (similar) charges repel each
other
( - and - )
( + and + )
Atomic Number (“Z”)
= the #
of
protons
in the
nucleus
Counting Charge: Ions
“Atomic Number” Z
is the number of protons.
determines what element an atom is.
(Z) –
(# of e- )
=
net charge of the atom
Ion: An atom with a net charge
Cation: positive ion
Anion: negative ion
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Counting Charge: Ions
Fe
Ions:
Mini-Lecture
Ca
S
Mg
If you see an element symbol alone, assume it
has an equal number of p+ and e-, and therefore
no net charge.
(Z) –
Ti 3+
(# of e- )
Cl 1-
=
net charge of the atom
Fe 2+
O 2-
Cations and anions are indicated by writing the
net charge of the ions in superscript to the right
of the element symbol.
Symbols
is the…
the…
– Net charge
– number of protons
– number of electrons
– Atomic number
Symbols
 What
 What
Br
1-
is the…
the…
– Net charge
– number of protons
– number of electrons
– Atomic number
Ca
2+
Symbols
 What
is the…
the…
– Net charge
– number of protons
– number of electrons
– Atomic number
O
2-
Practice
2
Isotopes
Counting Nucleons; Isotopes
“Atomic Number”
Z
is the number of protons.
determines what element an atom is.
if the atom is neutral, Z = # of e“Mass Number”
the number of protons + neutrons.
called the mass number, because p+ and n0
are what make up the mass of atoms; the
mass of electrons is insignificant.
Atoms of the same element can have differing
masses due to differing numbers of neutrons.
Isotopes:
Atoms that share the same atomic number
(same element; same # of p+ )
but have a different mass number.
(different isotopes of that element: different # of n0 )
Example: Carbon-12 ( 12C ) Carbon-13 ( 13C )
are isotopes of Carbon.
Two ways of writing isotopes:
1.)
Contain the symbol of the element, the
mass number and the atomic number:
Mass
number
Atomic
number
2.)
39
X
19
K
Symbols
• Find the
– number of protons
– number of neutrons
– number of electrons
– Atomic number
– Mass Number
19
9
F
Put the mass number after the element’s name:
Mass Number – Atomic Number = # of Neutrons
carbon- 12
carbon –14
uranium-235
Symbols
the
– number of protons
– number of neutrons
– number of electrons
– Atomic number
– Mass Number
Symbols
 Find
 if
80
35
Br
an element has an atomic
number of 34 and a mass number
of 78 what is the
– number of protons
– number of neutrons
– number of electrons
– Complete symbol
3
Symbols
 if
an element has 91 protons and
140 neutrons what is the
– Atomic number
– Mass number
– number of electrons
– Complete symbol
Symbols
 if
an element has 78 electrons and
117 neutrons what is the
– Atomic number
– Mass number
– number of protons
– Complete symbol
Atomic Mass
• How heavy is an atom of oxygen?
• It depends on number of protons and neutrons:
– Remember: the mass of an electron is negligible.
Study Guide
• There are different isotopes of oxygen.
• So, we take an average based on how common
each isotope of oxygen is and calculate the…
Average Atomic Mass
Measuring Atomic Mass
• When measuring the mass of atoms, we
do not use grams because the numbers
would be too small; instead we use the
Atomic Mass Unit (amu)
Why use amu?
Because an amu is basically defined as the
mass of a proton or a neutron,
The mass number of an atom
is
the mass of the atom in amu.
1 amu
= 1/12 the mass of a carbon-12 atom.
( official definition)
= basically, it is the mass of one proton
or one neutron
A Carbon-12 atom has a mass of 12 amu.
A Potassium-40 atom has a mass of 40 amu.
A Uranium-235 atom has a mass of 235 amu.
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Average Atomic Mass: A Summary
• Definition: the weighted average of all naturally
occurring isotopes of an element.
• Units of atomic mass are the amu.
• Average atomic masses are not whole numbers
because they are averages.
Sneak Preview:
The average atomic mass of an element (in amu)
is also numerically equal to the mass of one ‘mole’ of
that element in grams.
(More on this later…)
A Block on the Periodic Table
Cl
name Chlorine
atomic number 17
average atomic mass 35.453
symbol
You will find these differently arranged in different Periodic tables.
The Atomic Number gives the elements position in the table.
Modern View
Introduction:
Bohr Diagrams
Electrons are found in
‘shells’ = ‘energy levels’
can hold 8 + 10 electrons
can hold 8 electrons
can hold 2 electrons
• The atom is mostly
empty space
• Two regions
• Nucleus- protons
and neutrons
• Electron cloudhas many shells
(energy levels)
Bohr said:
Electrons can only
be at certain, specific
energy
levels
(distances from the nucleus)
n=1
n=2
n=3
“Electron energy is quantized”
5
Bohr Model Diagrams
…show energy levels (shells) and the electrons in them.
nucleus is not shown.
Inner shells fill first;
Valence Shell:
The outermost shell that contains electrons.
Valence Electron:
1st shell (n = 1) gets 2 eAn electron in an atom’s valence shell
2nd shell (n = 2) gets 8 e3rd shell (n = 3) gets 8 e-
…before 4th shell starts to fill
Modern View
• The atom is mostly
empty space
• Two regions
• Nucleus- protons
and neutrons
• Electron cloudhas many shells
(energy levels)
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