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# Isotopes and Average Atomic Mass Isotopes and by malj

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```									              Isotopes and Average Atomic Mass                                                                         Atomic Structure
An atom is made up of protons and neutrons (both found in the nucleus) and electrons ( in the surrounding electron cloud).
Elements come in a variety of isotopes, meaning they are made up          The atomic number is equal to the number of protons. The mass number is equal to the number of protons plus neutrons. In
atoms with the same atomic number but different atomic masses.            a neutral atom, the number of protons equals the number of electrons. The charge on an ion indicates an imbalance between
These atoms differ in the number of neutrons. the average atomic          protons and electrons. Too many electrons produces a negative charge, and too few, a positive charge. This structure can
mass is the weighted average of all the isotopes of an element.           be written as part of a chemical symbol.

Example
Example: A sample of cesium is 75% 133Cs, 20% 132Cs                                         (mass number)         15      +3   (charge)
and 5% 136Cs. What is its average atomic mass?                                                                 N
7
7 protons
Answer: .75 x 133 = 99.75                                                                         (atomic number)         8 neutrons
4 electrons
.20 x 132 = 26.4
.05 x 134 = 6.7
Element        Atomic    Atomic        Mass          Protons     Neutrons       Electrons
Total = 132.85 amu                                     ion             number    Mass          Number
H
Determine the average atomic mass of the following mixtures of isotopes.
127                                      1                 H+1
_______1. 80% I                       _______4. 99% H
17% 126I                              0.8 % 2H                    12
3% 128I                              0.2 % 3H                            C
6
7         +1
3    Li
35       -1
17 Cl

_______2. 50% 197Au                   _______5. 95% 14N                    39
K
50% 198Au                             3% 15N                     19

2% 16N                    24        +2
12 Mg
As +3

Ag
55                                     12
_______3. 15%          Fe             _______6. 98% C
85%     56
Fe                       2% 14C                     Ag +1

S-2

U
Electron Configuration                                                                                 Lewis Dot Diagrams
Electrons are distributed in the electron closed into principal energy levels (1,2,3,....)           Lewis Dot diagrams are a way to indicate the number of valence electrons around an atom.
sublevels (s,p,d,f), orbitals (s has 1, p has 3, d has 5, f has 7) and spin (two electrons allowed
per orbital).                                                                                                                               ..           •

Na•          :Cl:        •N:
•           •
Example: Draw the electron configuration of sodium (atomic # 11)
are all examples of this type
Answer: 1s2         2s2            2 p6              3s1                                                      of diagram

↑↓        ↑↓        ↑↓ ↑↓ ↑↓               ↑_                             Draw the Lewis Dot Diagrams for the elements below.
1. calcium                                                6. carbon

Draw the electron configurations of the following elements.
1. Cl

2. potassium                                              7. helium

2. N

3. argon                                                  8. oxygen

3. Al

4. aluminum                                               9. phosphorous

4. O

5. bromine                                               10. hydrogen

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