Chapter 4 Notes
Description of an atom
Inside the nucleus

The nucleus contains _________________ and __________________

protons
◦

neutron
◦
Outside the nucleus

The electron cloud contains the ____________________
◦
◦
Particle
Charge
Mass #
Location
Electron
Proton
Neutron
1
Then number of protons in an atom of a given element is the same as the atomic number
(Z).
◦
found on the Periodic Table, whole # for each element
carbon
phosphorus
gold

Mass number = protons + neutrons; always a whole number.

# of Neutrons = mass number - # of protons

# electrons in a neutral atom = # protons

Atomic mass – larger number in each element’s box on the periodic table. If you
round the atomic mass of an element to the closest whole number you generally get
the mass # for that element.
Example
Atomic
Mass
Number
Number
# protons
# electrons
# neutrons
Symbol
Mercury
Neon
2
Isotopes: atoms with the same number of protons but different number of neutrons
Examples:
Iron
Iron – 54
Iron – 56
Uranium
Uranium – 236
Uranium – 238
Average Atomic Mass for an Element
1. Multiple the percentage (percent abundance) of each isotope of the element by its
mass number.
2. Add the products of the multiplications together.
3. Divide by 100.
4. Your answer should be very close to the atomic mass of the element for that
element
Determine the average atomic mass of each element using the mass number of each
isotope and its percent abundance in nature.

Lithium: 7.5% Li – 6 and 92.5% Li – 7

Chromium: 83.79%
52
Cr, 9.50% 53Cr, 2.37% 54Cr and 4.34% 55Cr
3
Nuclear Chemistry
1. Nuclear reactions involve changes in particles in an atom's nucleus and thus cause a
change in the atom itself.
2. All elements heavier than bismuth (Bi) (and some lighter) exhibit natural
radioactivity and thus can "decay" into lighter elements.
3. Unlike normal chemical reactions that form molecules, nuclear reactions result in
the transmutation of one element into a different isotope or a different element
altogether (remember that the number of protons in an atom defines the element,
so a change in protons results in a change in the atom).

Parts of a Reaction
Reactants  Products
Emission =
Capture =
Nuclear Particles
Alpha=
4
2He
Beta or electron =
Positron = 0-1e
neutron =
0
-1e
1
0n
Balancing Nuclear Reactions
Mass # and the atomic # totals must be the same for reactants and the products.
1.
39
19K
2.
206
82Pb
 0-1e + ___
3.
238
94Pu
+ ___ 42He +

35
17Cl
+ ___
235
92U
4
4.
235
92U
+ 10n  ___ + 14156Ba + 310n
5. ___ +10n 
144
58Ce
+
94
36Kr
+ 210n
Types of nuclear reactions
1. Alpha emission

238
92U

2. Beta emission

234
90Th

3. Positron emission

22
11Na

4. Electron capture –

201
80Hg

5. Neutron emission

209
84Po

Writing Balanced Nuclear Reactions
1. Alpha decay of Cu-68
2. Positron emission of P-18
5
3. Astatine-210 releasing 3 neutrons
4. Electron capture of Ti-45
Half Life
1. Radioactive isotopes or nuclides all decay because they are unstable, some just
breakdown much faster than others
2. Half-life – amount of time for half of the original sample to decay
3. For two samples of the same isotope, regardless of the sample size, after one halflife, only half of the original amount of sample remains.

Isotopes
Half-Live

Carbon – 14
5730 years

Sodium – 24
15 hours

Bismuth – 212
60.5 seconds

Polonium – 215
0.0018 seconds

Thorium – 230
75400 years

Thorium – 234
24.1 days

Uranium – 235
7.0 x 108 years
6

4.46 x 109 years
Uranium – 238
Example Problems
1. Barium – 139 has a half-life of 86 minutes. If you originally have a 10 gram sample
of Barium-139, how much will be left after 258 minutes?
2. How many days will it take 50 grams of Radon – 222 (half-life of 3.82 days) to
decay to 3.125 grams?
3. If a sample of Cesium-135 decays from 10 grams to 2.5 grams over a period of 84
days, what is the half-life of Cesium-135?

Fusion – combining two smaller nuclei into one heavier, more stable nucleus.
3
2He
+ 11H 
4
2He
+ 01e
7

Fission – splitting a large unstable nucleus into two nuclei with smaller mass
numbers.
209
84Po

125
52Te
+
84
32Ge
1. Predict the number of valence electrons for each element based on its location in
the Periodic Table of Elements.
Magnesium =
Carbon =
Phosphorus =
Lithium =
2. At room temperature, what state of matter would the following elements be in?
MercuryFluorine –
oxygen –
Sodium –
3. Classify each as a metal, non-metal, or metalloid.
ironantimony –
nitrogen –
Sodium –
4. In what family would each of these elements be classified?
Strontium –
Fluorine –
lead –
8
Sodium –
How many valence electrons must atoms have to become stable?
Most atoms do not have 8 valence electrons. How do they get 8 valence electrons?
Which family on the periodic table already has 8 valence electrons?
What is the exception to the 8 valence electron rule?
Sodium (Na)
# electrons
# valence electrons
What does Na do to get 8 valence electrons? Draw a picture.
Reaction
9
Why would an atom become an ion?
•
Because it is more stable as an ion
•
As an ion, it follows the “___________”
•
Atoms will ____________________ to follow the octet rule
•
Atoms want to have outer electron arrangements like that of noble gases
•
Becoming an ion accomplishes this.
Lithium (Li)
How many electrons does Li have?
How many valence electrons does Li have?
What does Li do to become stable? Draw a picture
Reaction:
•
Remember: If there is less than 6 total electrons, it is stable with just 2 valence
electrons
Nitrogen
How many electrons does Nitrogen have?
How many valence electrons does nitrogen have?
Nitrogen can either gain three electrons or lose 5. Draw a picture
Reaction:
10
Think about it… Becoming ions
What will alkali metals do to become ions?
What will alkaline earth metals do to become ions?
What will boron family do to become ions?
What will carbon family do to become ions?
What will Nitrogen family do to become ions?
What will Oxygen family do to become ions?
What will halogens do to become ions?
Ions are NOT neutral
If an atom gives away electrons,
If an atoms accepts electrons,
Cations

_________ form cations

Cations are named the same as the metal
sodium
Na→
calcium
Ca→
11
Anions

______________ form anions

Anions are named by changing the ending of the name to ____________
fluorine
phosphorus
Finding # of particles and charge
subtract that charge from the # of protons to get the number of electrons

27
+3
13Al
=

35
-1
17Cl
=

137
=
+2
56Ba
Aluminum and Phosphorus
How to write ionic compounds
Step 1
Use the periodic table to find the charge of the cation and anion
12
Step 2
Step 3
Remove all (+, -, and 1’s)
Reduce the formula to the lowest whole number ratio
13