Download The Basic Atom

Protons, Neutrons,
Electrons/Isotopes/Average
Atomic Mass
If an atom was the size of
a football stadium …
Gnat-sized electron
• the nucleus (protons +
neutrons) would be a
marble on the 50 yard
line
• the electrons would be
smaller than gnats out in
the stands
Marble -sized nucleus
Important things to remember about
atoms!
Most of an atom is empty space
Electrons are so small, we can
pretend that their mass is zero
The mass of an atom is from its
nucleus (protons & neutrons)
Tells you the identity of the element
Atomic Mass
(Each element has a different atomic #)
Given the following atomic numbers,
determine the identity of the elements:
Atomic #
16
9
53
Element
Sulfur
Fluorine
Iodine
Also tells you the number of protons
Atomic Mass
(Each element has a different # of protons)
Determine the number of protons for each
element:
Element
chlorine
arsenic
oxygen
# Protons
17
33
8
Isotopes = Different versions of the same
element
Isotopes of Carbon
Important things to remember about the
isotopes of an element!
Same Element
Same # Protons
Different # Neutrons
Different Masses
Relative Percent Abundance = The overall
percent of each isotope of an element
Isotopes of Carbon
Mass number
OR
1 H
1
or Hydrogen-1
1 proton
0 neutrons
2
1H
or Hydrogen-2
1 proton
1 neutron
Hydrogen-3
3
1H
or Hydrogen-3
1 proton
2 neutrons
The mass number = # protons + # neutrons
(So, if you know the # protons, you can figure out the #
neutrons, and vice versa!)
Example:
3 H
1
or Hydrogen-3
# Protons = 1
Mass Number = 3
Mass number = # protons + # neutrons
3
=
1
+
x
x = # Neutrons = 2
Determine the # of protons and neutrons for each
of the following isotopes:
16
8O
8 protons, 8 neutrons
21
10Ne
10 protons, 11 neutrons
Flourine-19
9 protons, 10 neutrons
Boron-11
5 protons, 6 neutrons
Atomic Mass
The Atomic Mass is the average
weighted mass of the isotopes of an
element
To calculate the average weighted mass (atomic
mass of an element:
Step 1
Multiply each isotope’s mass x its %
abundance. (Make sure the % abundance
is in decimal form!)
Step 2
Add all of the answers from step 1 above.
Calculate the average weighted mass (the atomic
mass) of nitrogen.
Isotope
Nitrogen-14
Nitrogen-15
Step 1
Mass
Mass
14.003 amu
15.000 amu
Relative % Abund.
99.63
0.37
% Abund. (as a decimal)
(14.003)(0.9963) = 13.951 amu
(15.000)(0.0037) = 0.055 amu
Step 2
13.951 + 0.055 = 14.006 amu
Calculate the average weighted mass (the atomic
mass) of Silicon.
Isotope
Silicon-28
Silicon-29
Silicon-30
Mass
27.977 amu
28.976 amu
29.974 amu
Step 1:
(27.977)(0.9223) = 25.803 amu
(28.976)(0.0467) = 1.353 amu
(29.974)(0.0310) = 0.929 amu
Relative % Abund.
92.23
4.67
3.10
Step 2:
25.803 + 1.353 + 0.929 =
28.085 amu
Tells you the number of electrons too!
Remember! The atomic # also tells you the number of protons
(so…the # protons = # electrons in an atom)
When an atom loses or gains one or more
electrons, it becomes an ion.
There are two kinds of ions:
• Cations: Ions formed when atoms lose one or
more electrons – have a positive charge
Ex. Ca2+, Na1+, Al3+
•
Anions: Ions formed when atoms gain one or
more electrons – have a negative charge
Ex. F1-, P3-, O2-
Determine the number of electrons for each
element:
Element
S
S2Al
Al3+
# Electrons
16
18
13
10