Atoms are mostly empty space
Two regions of every atom:
Nucleus
- is made of
protons and neutrons
- is small and dense
Atomic Structure
n
pn p
n pn
Electron cloud
-is a region where you
might find an electron
-is made up of many “shells”
Subatomic particles;
Atomic Number (“Z”)
= the #
of
protons
in the
nucleus
Relative
Symbol Charge mass
Name
Proton
p+
+1
1
Neutron
n0
0
1
Electron
e-
-1
0.0005
Electrons weigh almost nothing at all!
Opposite charges attract each other
( + and - )
Alike (similar) charges repel each other
( - and - )
Atomic Number (“Z”)
= the #
of
protons
in the
nucleus
( + and + )
1
Bohr Model Diagrams
…show energy levels (shells)
and the electrons in them.
Electrons can only
be in certain, specific
energy
levels (“shells”)
(distances from the nucleus)
1st
The nucleus
is not shown.
2nd
3rd
electron
in the electron cloud
shell ( = “energy level” )
Valence Shell:
- Shells fill in order
- Inner shells fill first.
The outermost shell that contains electrons.
- Each shell can hold a
certain number of electrons:
Valence Electron:
1st shell gets 2 eAn electron in an atom’s valence shell
2nd shell gets 8 e3rd shell gets 8 e-
Negative electrons are found in
‘shells’ = ‘energy levels’
positive protons
& neutral neutrons
are found in
the dense core
(nucleus) of atoms.
Modern View of the atom:
• Two regions
• Nucleus- protons
and neutrons
• Electron cloudhas many shells
(energy levels)
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Counting Charge: Ions
“Atomic Number” Z
is the number of protons.
determines what element an atom is.
Ions:
Mini-Lecture
(Z) –
(# of e- )
=
net charge of the atom
Ion: An atom with a net charge
Cation: positive ion
Anion: negative ion
Counting Charge: Ions
Fe
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-
Symbols
 What
is the…
the…
– Net charge
– number of protons
– number of electrons
– Atomic number
Br
1-
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
Ca
2+
is the…
the…
– Net charge
– number of protons
– number of electrons
– Atomic number
O
2-
3
Counting Nucleons; Isotopes
“Atomic Number”
Z
is the number of protons.
determines what element an atom is.
Bohr
Diagrams
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.
Isotopes
Two ways of writing isotopes:
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+ )
(different isotopes of that element: different # of n0 )
Example: Carbon-12 (
) Carbon-13 (
are isotopes of Carbon.
Contain the symbol of the element, the
mass number and the atomic number:
Mass
number
Atomic
number
but have a different mass number.
12C
1.)
13C
2.)
X
19
K
Put the mass number after the element’s name:
)
carbon- 12
Symbols
carbon –14
uranium-235
Symbols
 Find
• Find the
– number of protons
– number of neutrons
– number of electrons
– Atomic number
– Mass Number
39
19
9
F
the
– number of protons
– number of neutrons
– number of electrons
– Atomic number
– Mass Number
80
35
Br
Mass Number – Atomic Number = # of Neutrons
4
Symbols
 if
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
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
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)
Study Guide
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.
5
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.
• 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: 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:
Average Atomic Mass
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…)
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