Chemistry 4.3 Notes
Distinguishing Among Atoms
What we know…
Pg. 110 – Atoms are composed of electrons,
protons, and neutrons. Protons and
neutrons make up the small, dense
nucleus. Electrons surround the nucleus
and occupy most of the volume of the
atom.
**How do atoms differ from each other then?
I. Atomic Number
A. Def – the # of protons found in an atom.
1. Found in the nucleus
2. Above symbol on P.T.
3. Atoms differ from each other due to
the number of protons.
4. Ex: Hydrogen  1 proton
5. Ex: Oxygen  8 protons
6.
Atomic Number
2
He 
4
Be 
10
Ne 
Mo  42
Pb 
82
Sn 
50
Ba 
56
II. Mass Number
A. Def – the total # of neutrons and protons
in the nucleus of an atom.
1. Found above chemical symbol on P.T.
2. Formula: Mass # = protons + neutrons
3. Ex: Na  23
4. Ex: Al  27
5.
Mass Number
11
B
7
Li 
Rn  222
Cr 
52
Ti 
48
Zn 
65
III. What about Electrons?
A. **Assume electrons = the # of protons in
an atom**
1. All atoms have a charge of zero
2. Complete the following table in your
notes.
Name
Scandium
Copper
Krypton
Neon
Phosphorus
Gallium
Manganese
Symbol
Atomic
Mass
Protons Neutrons
Number
Number
Electrons
Atomic
Mass
Protons Neutrons
Number
Number
Name
Symbol
Electrons
Scandium
Sc
21
21
24
45
21
Copper
Cu
29
29
35
64
29
Krypton
Kr
36
36
48
84
36
Neon
Ne
10
10
10
20
10
Phosphorus
P
15
15
16
31
15
Gallium
Ga
31
31
39
70
31
Manganese
Mn
25
25
30
55
25
Symbol Protons Electrons Neutrons Atomic # Mass #
W
76
128
45
41
57
150
III. Isotopes
A. Def – same atom but different # of
neutrons.
1. This alters the mass #
2. Chemically the same
3. Ex: Hydrogen
 3 isotopes = H-1, H-2, H-3
4. Drawing an isotope
1st put in # of protons
2nd use mass # to determine the # of
neutrons
5. Draw the 3 isotopes of hydrogen
6. Draw the 3 isotopes of Neon
Ne-20
Ne-21
Ne-22
7. Draw the 2 isotopes of Chlorine (35 and
37)
IV. Atomic Mass
A. Def – a weighted average of all the isotopes in a sample
of a given element.
1. reflects mass
2. reflects relative abundance
B. Atomic Mass Unit
1. Def – 1/12th of a carbon-12 atom
2. Prevents using scientific notation
ex: 3.155 x 10-22 g = fluorine
3. Why is the atomic mass not a whole number?
isotopes
4. There are isotopes for almost every element
5. Ex: Chlorine 35.453
2 isotopes (Cl-35 and Cl-37)
What would the atomic mass be?
36.000 right? Wrong
we need the relative abundance
6. Cl-35 has an abundance of 75.77%.
Cl-37 has an abundance of 24.23%.
35 x .7577 = 26.52
37 x .2423 = 8.965
 35.485
7. Hydrogen has 3 isotopes, H-1, H-2,
H-3. H-1 occurs 99.99% of the time,
H-2 occurs 0.015% of the time, and H3 is negligible (can only be made in the
lab). What is the atomic mass?
 1.030