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Ions
An atom that carries an electrical
charge is called an ion
If the atom loses electrons, the atom
becomes positively charged (because
the number of positively charged
protons will be more the number of
electrons)
If the atom gains electrons, the atom
becomes negatively charged (because
there are more negative charges than
positive)
Ions
The number of protons does not change
in an ion.
The number of neutrons does not
change in an ion.
So, both the atomic number and the
atomic mass remain the same.
Ions
This atom has lost an electron. Now it has
one more proton than electron.
One more proton means one more positive
charge.
This makes the total charge of the atom
POSITIVE.
This atom has gained an electron. Now it has
one less proton than electron.
One less proton means one less positive
charge.
This makes the total charge of the atom
NEGATIVE.
Isotopes
The number of protons for a given atom
never changes.
The number of neutrons can change.
Two atoms with different numbers of
neutrons are called isotopes
Isotopes have the same atomic #
Isotopes have different atomic Mass #’s
Isotopes
Isotope Notation
Isotope Notation uses a symbol
to convey information about an
isotope of a particular element.
23
Na
11
Isotope Notation
Mass Number
Is the number of
protons plus the
number of
neutrons. This
number tells us
the kind of
isotope. This
number is NOT
on the periodic
table.
23
Na
11
Atomic Number
Is the # of protons (p+) in the atom
AND
The # of electrons (e-) IF the atom is
electrically neutral
( i.e., not an ion).
This number is on the periodic table.
Isotope Notation
23
23
Na
11
Na+
11
For this isotope
For this isotope
11 p+
+ 11e0 net charge
11 p+
+ 10e1+ net charge
Isotope Notation
Mass Number
23
Na
11
Atomic Number
The # of protons + the # of neutrons
Calculate the number of neutrons for this isotope.
23 (neutrons + protons) - 11 protons = 12 neutrons
Atomic Mass
Is the weighted average mass of all the isotopes
of one element in atomic mass units (amu).
In other words, it is the mass of one average atom*.
Two measurements are needed in order to calculate this
average.
1. The percent abundance (% abundance,) also known as
how often the isotope occurs in nature.
2. The mass of one atom of one isotope.
Atomic mass is also known as
Weighted Average Atomic Mass
Average Atomic Mass
Atomic Mass
Here is the formula you need to know in order to calculate
the average atomic mass.
Average Atomic Mass =
[(Mass of Isotope 1) x (% abundance of Isotope 1)]
+ [(Mass of Isotope 2) x (% abundance of Isotope 2)]
+ and so on for the 3rd and remaining isotopes.
The average atomic mass is written on the periodic table
Mass Number vs. Atomic
Mass
Mass number = # protons + # neutrons
(This number is specific for one isotope.)
Atomic mass = weighted average mass of all the isotopes
of one element in atomic mass units (amu).
(The atomic mass is on the periodic table.)