Atoms
Atoms
Atoms are the main building block of all matter
All chemical and physical properties of matter are a result
of how the atoms in the matter behave
All atoms are made of the same three subatomic particles.
What makes one atom different than another is the number
of each subatomic particle it has.
Different types of atoms are called elements
Bohr Model of Atom
Subatomic Particles
• Atoms are made of the three subatomic particles
Charge
Location
Proton
Electron
+
In nucleus
-
Orbiting
nucleus
Neutron
0
In nucleus
Atomic Number
•
Indicates the number of protons
•
The number of protons in the nucleus is
called the atomic number.
•
Electrons will always equal the same
number of protons. (in the atom!)
Atomic Number
3
Li
Lithium
What makes atoms unique?
• The number of protons they contain.
•
Ex.
A lithium atom has 3 protons, so its
atomic number is 3.
6.9
Atomic Mass
 Mass of an atom
 Measured in atomic mass units (u)
 Found on the periodic table below each element
symbol
3
Li
Lithium
6.9
Atomic Mass
Mass Number
•
•
•
represents the contents of its nucleus
(protons + neutrons).
The mass of electrons is so small it is
“negligible”
This value is called the mass number.
(round mass to the nearest whole
number)
• # of Neutrons = Mass # – Atomic #
• # of Neutrons in Lithium
Atomic Number
3
Li
Lithium
6.9
7
= 7-3
= 4
Mass Number
Isotopes
 Atoms of different elements have different numbers
of protons
 Atoms with the same number of protons but
different numbers of neutrons are called isotopes
 For example: Lithium can have 3 or 4 neutrons
 Li-6 and Li-7
 Mass number is used to distinguish between the two
isotopes of Lithium
Nuclear Notation
Mass Number
6.9
Li
Atomic Number
6.9Li
3
Li-6.9
Homework
• Subatomic Particles Worksheet
•
• Pencil only please 
Read p. 234-235