What is “matter?”
 Matter is anything that has mass and
takes up space
 EVERYTHING around you is matter
(including you!)
What is an “atom?”
 Atoms are the
smallest form of
matter
 Different types of
atoms exist and
combine to form
everything around us.
Atomic Structure
Electron Shells
Nucleus
Protons
Positive (+) charge
Found in the nucleus
Electrons
Negative (-) Charge
Found in electron shells
Much smaller than
protons
Neutrons
Neutrons have no
charge, so we call them
“neutral”
Located in the nucleus
How do different charges
behave?
Protons have a positive charge.
Electrons have a negative charge.
What happens when those charges come
in contact with one another?
Subatomic Particle Behavior
Describing Atoms
 Atomic Number = Number of
protons in an atom
 In a neutral atom, the number of
protons = the number of electrons.
Let’s Practice…
 An atom has 9 positive protons.
 Where are the protons located in an atom?
 The Nucleus
 How many negative electrons does it have?
 9 electrons
 What is the atom’s atomic number?
 Atomic Number = 9 (same as protons)
Let’s Practice…
 A neutral atom has 13 negative electrons.
 Where are electrons located in an atom?
 Electron Shells
 How many positive protons will this atom have?
 13 positive protons
 What is this atom’s atomic number?
 Atomic Number = 13 (same as protons)
In summary..
1. Atoms make up EVERYTHING!
2. Atoms are made of 3 smaller subatomic particles:
• Protons (+)
• Electrons (-)
• Neutrons (0)
3. The number of protons is the same as electrons.
4. We identify atoms by their atomic number, which is
the same as their number of protons.
But what about neutrons?
 The number of neutrons in an atom can change!
 Remember the charge of a neutron? Would adding or
removing neutrons do anything to the charge of the
atom?
No! The charge of a neutron is zero. We could add
as many neutrons as we wanted, but the
atom
wouldn’t become more positive or more negative.
How to find the # of Neutrons
 In addition to Atomic Numbers, atoms also
have one more number called the Atomic
Mass.
 The Atomic Mass is how heavy the nucleus
of an atom is – so how heavy the protons
and neutrons are together.
 Example: An atom with 6 protons and 7
neutrons in its nucleus has an atomic mass
of…
 13 amu.
Let’s Practice…
 An atom has 8 protons and an Atomic Mass of
19 amu.
 How many electrons will this atom have?
 8 electrons
 What is the Atomic Number of this atom?
 Atomic Number = 8 (same as the protons)
 How many neutrons will this atom have? (Hint: Atomic Mass
- # protons = # neutrons)
 19 – 8 = 11 neutrons