Download Atomic Structure - Science With Horne

The Atom
Lesson 2 - Isotopes
Different Forms of the
Same Element
• In any specific element, the # of
protons is always constant.
• Unlike the number of protons, the
number of electrons and neutrons can
vary within atoms of an element without
changing the identity of the element.
• Ex. Carbon (C) ALWAYS has 6 protons,
but it can have anywhere from 6-8
neutrons and 2-10 electrons
Isotopes
• Isotopes are atoms of the same element
(same number of protons) but with
different number of neutrons
• Carbon has three isotopes
12
6 C
6 p+, 6 no
13
6
C
6 p+, 7 no
14
6
C
6 p+, 8 no
Notice how the atomic #
(# of protons) does NOT
change but the mass
number does.
Practice
• A lithium atom has 3 protons, 3
electrons, and 3 neutrons.
• A= 6
• A nitrogen atom has 8 neutrons
• A = 15
• An unknown element has 92 protons
and 143 neutrons.
• Element = U
A = 235
Representing Isotopes
• Option # 1
• Top number is mass number
(A), bottom number is atomic
number (Z)
•
235
92 U
•
40
•
14
C
6
K
19
• Option #2
• U – 235
• K - 40
• C – 14
• Only the mass
number (A) is listed,
the atomic number
(Z) can be found on
the Periodic Table
Determining the mass
number of isotopes
• The atomic mass on the periodic table is
an average of all the known isotopes of
each element.
• It is not the mass of any individual atom.
• To determine the mass number of a
specific isotope you need to add the
number of protons to the number of
neutrons. This is the mass number (A).
Finding average atomic mass
• To find the average atomic mass of an
element you need two pieces of
information:
• The mass numbers of the different
isotopes (this is NOT the average mass
found on the periodic table)
• Neon has three: Ne - 20 , Ne - 21, Ne - 22
• The relative abundance of each isotope
• Of all the neon measured, 90.60% is Ne-20,
0.200% is Ne-21, and 9.200% is Ne-22
Finding average atomic mass
• Even though it is the least massive, Ne20 accounts for the vast majority of
Neon.
Isotope
Mass
Abundance
Ne-20
20 amu
90.60%
18.12
Ne-21
21 amu
0.20%
0.042
Ne-22
22 amu
9.200%
2.024
Avg. mass =
Mass
Contribution
20.19 amu
Examples
• Gallium-69 has a relative abundance of
60.11% and Gallium-71 has a relative
abundance of 39.89%. What is the average
atomic mass of Gallium?
Isotope
Mass
Abundance
Ga-69
69 amu
60.11%
41.48
Ga-71
71 amu
39.89%
28.32
Avg. mass =
Mass
Contribution
69.80 amu
Examples
• Thallium has two stable isotopes, Thallium203 and Thallium-205. Thallium-203 has a
relative abundance of 29.52%.Thallium-205
has a relative abundance of 70.48%. What is
the average atomic mass of Thallium?
Isotope
Mass
Abundance
Tl-203
203 amu
29.52%
59.93
Tl-205
205 amu
70.48%
144.5
Avg. mass =
Mass
Contribution
204.4 amu