Atomic Structure & Notation
1. Atoms
- Protons
- Neutrons
- Electrons
2. Atomic Number
3. Mass Number
4. Atomic Symbols
5. Calculations
ATOMS
Atoms are the building blocks of all matter.
An atom is made up of smaller bits called subatomic
particles which include...
Name
Charge
Protons
+
neutral
Electrons
Neutrons
Position
Nucleus
Outside
Nucleus
Mass
1
0
1
Atoms – Bill Nye
• Complete the Bill Nye Atoms Worksheet
What is the structure of an atom?
Nucleus
• Where the positive charge
resides
–Later called the proton
• Later evidence showed the
existence of a neutral particle
(no charge)
–Later called the neutron
What is the structure of an atom?
–Proton (p+)
•Has a positive (+) charge
•Has a relative mass of 1
•Determines the atomic
number
•Found inside the nucleus
What is the structure of an atom?
–Neutron (n0)
• Has no (0) charge
• Has a relative mass of 1
• Determines the isotope
–Isotopes are two of the same
element with different masses
• Found inside the nucleus
What is the structure of an atom?
• Electron (e-)
–Has a negative (-) charge
–Has a relative mass of 0 (zero)
–Determines the ion (more
later)
–Found outside the nucleus
Atomic Number
• Each element is different because they
contain different numbers of PROTONS
• The “atomic number” of an element is
the number of protons in the nucleus
• # protons = # electrons
Atomic Number
Atomic number of an element is the
number of protons in the nucleus of each
atom of that element.
Element
Atomic #
# of protons
Carbon
6
6
Phosphorus
15
15
Gold
79
79
Mass Number
Atomic Mass (Mass Number) is the
number of protons and neutrons in the
nucleus of an atom:
Mass # = p+ + n0
p+
n0
8
8
8
16
Arsenic - 75
33
42
33
75
Phosphorus - 31
15
16
15
31
Atom
Oxygen - 16
e- Mass #
Atomic Notation
• Contains the symbol of the element,
the mass number and the atomic
number.
Superscript →
Symbol
Subscript →
Atomic number
X
Mass number
Atomic Notation Example
2
Symbol
He
4
Atomic number
the number of
protons in an atom
Atomic mass
the number of protons
and neutrons in an
atom
What is a chemical symbol?
• The chemical symbol tells us what atom it is
• It’s a short form (C = Carbon)
• sometimes it’s the Latin short form
(Au = Gold)
• Always a CAPITAL LETTER, sometimes
followed by one or more lower case letters
• Ex: Oxygen O, Nitrogen N, Sodium Na
• *Cobalt – Co *Carbon Monoxide – CO
• (Co is an element, CO is a compound)
Atomic Number
Symbol
Atomic Mass
Recall - How are P, N, e- related?
• atomic number = # protons
• # protons = # electrons in a neutral atom
• # protons + # neutrons = atomic mass
# neutrons = atomic mass - # protons
# protons = atomic mass - # neutrons
Example 1
• How many protons, neutrons and electrons are
found in an atom of Cs?
• Atomic number = protons
– There are 55 protons
55
Cs
133
• Mass number = sum of protons and neutrons
• Therefore, # Neutrons = Mass Number - # Protons
# Neutrons = 133 – 55
# Neutrons = 78
Hydrogen
1
H
Protons: 1
Neutrons: 0
Electrons: 1
1
# protons + # neutrons = atomic mass
Sodium
11
Na
Protons: 11
Neutrons: 12
Electrons: 11
23
# protons + # neutrons = atomic mass
Example 2

If an element has an atomic number of 34
and a mass number of 78, what is the:
a) number of protons = 34  # p+ = Atomic Number
b) number of neutrons
= 44 # n0= Atomic Mass - # p
c) number of electrons = 34  # e- = # p+
34
d) complete symbol
=
Se
78
 Atomic #
 Symbol
 Atomic Mass
Example 3

If an element has 91 protons and 140
neutrons what is the
+ = Atomic Number
=
91

#
p
a) Atomic number
b) Mass number
= 231
Atomic Mass = # n0 + # p
c) number of electrons = 91  # e- = # p+
d) complete symbol
91
=
Pa
231
 Atomic #
 Symbol
 Atomic Mass