Chapter 3
Atoms and Elements
3.5
Atomic Number and Mass Number
1
Atomic Number
The atomic number
• is specific for each element.
• is the same for all atoms of an element.
• is equal to the number of protons in an atom.
• appears above the symbol of an element.
2
Atomic Number and Protons
Examples of atomic number and number of protons:
• Hydrogen has atomic number 1; every H atom has
one proton.
• Carbon has atomic number 6; every C atom has six
protons.
• Copper has atomic number 29; every Cu atom has
29 protons.
• Gold has atomic number 79; every Au atom has 79
protons.
3
Examples
State the number of protons in each.
A. A nitrogen atom
1) 5 protons
2) 7 protons
3) 14 protons
B. A sulfur atom
1) 32 protons
2) 16 protons
3) 6 protons
C. A barium atom
1) 137 protons
2) 81 protons
3) 56 protons
4
Electrons in An Atom
An atom
• of an element is electrically neutral; the net charge
of an atom is zero.
• has an equal number of protons and electrons.
number of protons = number of
electrons
Aluminum has 13 protons and 13 electrons. The net
(overall) charge is zero.
13 protons (13+) + 13 electrons (13 -) = 0
5
Mass Number
The mass number
• represents the number of particles in the nucleus.
• is equal to the number of protons + the
number of neutrons.
6
6
Atomic Models
7
Examples
An atom of zinc has a mass number of 65.
A. How many protons are in this zinc atom?
1) 30
2) 35
3) 65
B.
How many neutrons are in the zinc atom?
1) 30
2) 35
3) 65
C. What is the mass number of a zinc atom that has
37 neutrons?
1) 37
2) 65
3) 67
8
Examples
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
B. Its mass number is
1) 14
2) 16
C. The element is
1) Si
2) Ca
3) 34
3) 34
3) Se
9
3.6 Isotopes and Atomic Mass
Isotopes
• are atoms of the same element that have different
mass numbers.
• have the same number of protons, but different
numbers of neutrons.
10
Atomic Symbol
An atomic symbol
• represents a particular atom of an element.
• gives the mass number in the upper left corner
and the atomic number in the lower left corner.
11
Information from Atomic Symbols
The atomic symbol for a specific atom of an element
gives the
• number of protons (p+),
• number of neutrons (n),
• and number of electrons (e-).
12
Information from Atomic Symbols
Examples of number of subatomic particles for atoms
Atomic symbol
16
O
8
8 p+
8n
8 e-
31
65
15
30
P
15 p+
16 n
15 e-
Zn
30 p+
35 n
30 e-
13
Examples
Naturally occurring carbon consists of three isotopes:
12C, 13C, and 14C. State the number of protons,
neutrons, and electrons in each of the following:
12C
6
protons
13C
14C
6
6
______
______
______
neutrons ______
______
______
electrons ______
______
______
14
Examples
Write the atomic symbols for atoms with the following
subatomic particles:
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e-
___________
C. 47p+, 60 n, 47 e-
___________
15
Examples
1. Which of the pairs are isotopes of the same element?
2. In which of the pairs do both atoms have 8 neutrons?
A.
15X
15X
8
7
B.
C.
12X
6
6
15X
16X
7
14X
8
16
Isotopes of Magnesium
TABLE
3.7
17
Isotopes of Sulfur
A sample of naturally
occurring sulfur contains
several isotopes with the
following abundances
Isotope %
abundance
32S
95.02
33S
0.75
34S
4.21
36S
0.02
32S, 33S, 34S, 36S
16
16
16
16
18
Atomic Mass
The atomic mass of an element
• is listed below the symbol of each element
on the periodic table.
• gives the mass of an “average” atom of
Na
22.99
each element compared to 12C.
• is not the same as the mass number.
• has the smallest unit of amu
19
Isotopes of Some Elements and Their
Atomic Mass
Most elements have two or more isotopes that
contribute to the atomic mass of that element.
8
20
Examples
Using the periodic table, specify the atomic mass of
each element.
A.
calcium
__________
B.
aluminum
__________
C.
lead
__________
D.
barium
__________
E.
iron
__________
21
Calculating Atomic Mass
The calculation for atomic mass requires the
• percent(%) abundance of each isotope.
• atomic mass of each isotope of that element.
• sum of the weighted averages.
22
Atomic Masses
Why is the atomic mass of the element carbon 12.01 amu?
carbon-12:
98.89 % natural abundance
12 amu
carbon-13:
1.11 % natural abundance
13.0034 amu
mass of carbon = (12 amu)(0.9889) + (13.0034 amu)(0.0111)
= 11.87 amu + 0.144 amu
= 12.01 amu
Atomic Mass for Cl
The atomic mass of chlorine
is
• due to all the Cl isotopes.
• not a whole number.
• the average of two isotopes:
35Cl and 37Cl.
24
Calculating Atomic Mass for Cl
35Cl
has atomic mass 34.97 amu (75.76%) and 37C
has atomic mass 36.97 amu (24.24%).
• Use atomic mass and percent of each isotope to
calculate the contribution of each isotope to the
weighted average.
Atomic mass 35Cl x % abundance =
Atomic mass 37Cl x % abundance =
• Sum is atomic mass of Cl is
25
Examples
Gallium is an element found in lasers used in
compact disc players. In a sample of gallium,
there is 60.10% of 69Ga (atomic mass 68.926)
atoms and 39.90% of 71Ga (atomic mass 70.925)
atoms.
What is the atomic mass of gallium?
26