Transcript Atoms – The Basics

Atoms – The Basics
Inside the Atom
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Atoms are made up of smaller
particles
These particles are found in
different regions of the atom
Atomic Number & Mass Number
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Atomic number = # p+
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Mass number = # p+ + # n0
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For atoms atomic number also =
number of e-
Mass number is a whole number
Number on the periodic table is
average atomic mass (not a whole
number)
How can you solve for numbers of
neutrons?
How Atoms Differ: Ions
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Different elements have different numbers
of p+
Ions have charges
the number of e- ≠ number of p+
 charge = # p+ - # e
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Ions are the SAME element, but DIFFER because
they have either a positive or negative charge.
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If an atom GAINS electrons, its overall
charge becomes more negative.
If it LOSES electrons, its charge becomes
more positive
Isotopes
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Atoms with the same number of p+ but
different number of n0 are isotopes.
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Mass number of isotopes of the same
element changes
Isotopes are the SAME element, but
DIFFER because they have different
masses.
EX:
Symbols of Ions and Isotopes
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For ions chemists use the following
notation:
Mass
Number
Atomic
Number
X
Charge
57
26
Fe
Or just:
Fe
2
2
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For isotopes chemists use the
following notation:
Name of element – mass number
Examples:
carbon-12
carbon-14
uranium-236
Determine the Numbers of p+, n0,
and e- for the following symbols
given:
17
8
O
204
2
37
Hg
Cl

calcium - 46
nickel - 60
Complete the following table
Protons
Na+
Bromine- 84
O2- with an
atomic mass
of 13amu
Neutrons
Electrons
Average Atomic Mass
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The masses of p+ and no are the
same and they are actually tiny
(1.67 x 10-24 g). We round it to 1
amu.
The mass of an e- is even smaller
1/1840 of a proton or neutron so
we say it is approximately 0 amu.
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We can do this because scientists
measure the mass of an atom
relative to a standard mass and that
is carbon-12.
Scientists agreed that carbon-12
has a mass of EXACTLY 12 atomic
mass units (amu).
So, why do the elements
on the Periodic Table
have masses with
decimals???
They are Average
Atomic Masses
Average Atomic Masses
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This is the weighted average mass of all
the isotopes of an element.
Examples of weighted averages:
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Semester and yearly grades
Taxes
Budget
Calculating the Weighted Average
Atomic Mass
1.
2.
Multiply the mass of each
isotope by the % of the isotope
Add the products
Example
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Calculate the atomic mass of magnesium. The
three magnesium isotopes have atomic masses
and relative abundances of 23.985 amu
(78.99%), 24.986 amu (10.00%) and 25.982
amu (11.01%).
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24.31 amu
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You can also estimate which isotope is the
most abundant.
Ex: Fluorine has an atomic mass of
18.99840 amu. It has several naturally
occuring isotopes Fluorine-14, Fluorine16, Fluorine-19, Fluorine-21, Fluorine-22.
Guess which one is the most abundant?
Helium has two naturally occurring
isotopes: helium-3 and helium-4. The
atomic mass of helium is 4.003amu.
Which isotope is more abundant in nature
and why?