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CHEM Unit 4 Notes
Basics of atomic
structure
Atoms
 basic
building blocks of ordinary
matter
 the smallest representative
particle of an element
 can join together to form
molecules
Molecules
 groups
of atoms joined by
chemical bonds
 are more common than free
atoms because most unbonded
atoms aren’t stable
Atoms contain
smaller particles called subatomic particles
 three kinds

proton
 neutron
 electron

Measuring mass and charge

u (or amu) = atomic mass unit = 1/12th of
the mass of a carbon-12 atom ; measure mass
of atoms and subatomic particles

fundamental charge units measure
electrical charge
Arrangement of the atom

Nucleus







central part of the atom
contains protons and neutrons
small
massive
dense
positively charged
Electron cloud





area outside the nucleus
where electrons are found
large
mostly empty/almost no mass
negatively charged
Summary of subatomic particles
Particle
Location
Charge
Mass
Proton
Nucleus
1+
~1 u
Neutron
Nucleus
0
~1u
Electron
Electron cloud
1-
~.001 u
(usually ignorable)
Important terms
 Atomic
number: number of protons in an
atom
 Determines
elemental identity
 Mass
number: sum of protons and
neutrons
 gives
a whole-number estimate of mass
 NOT the same as atomic mass
Elements
 Element
: substance made entirely of
atoms with the same atomic number.
 Cannot be simplified by any chemical or
physical change
How many elements are there?
 Currently
there are 118 different
elements.
 90
naturally occurring elements
 28 synthetic (man-made) elements
Periodic table (PT)

Gives information about elements based
on block and row in the table
symbol (always)
 atomic number (always)
 name (frequently)
 occasionally a variety of other info

Isotopes



variations of the same element
have same atomic number and different mass
numbers
One element can have many different isotopes



Isotopes of the same have the same number of
protons
Each isotope has a unique number of neutrons
Isotope names are the element name-mass
number.


carbon-12
uranium-236
Remember!
Changing number of neutrons does NOT
change the elemental identity of an atom.
 Changing the number of protons DOES
change the elemental identity of an atom.

Isotope example: krypton (Kr)

Krypton is a mixture of six isotopes.






48 neutrons = mass number 84: krypton-84
47 neutrons = mass number 83: krypton-83
50 neutrons = mass number 86: krypton-86
46 neutrons = mass number 82: krypton-82
44 neutrons = mass number 80: krypton-80
42 neutrons = mass number 78: krypton-78
Isotope symbols
 superscript
at left = mass number
 subscript at left = atomic number.
 This is an oxygen atom:
 Atomic
number 8
 Mass number 16 16
O
8
Ions

are charged atoms
Neutral atoms have the same number of protons
and electrons.
 Ions have different numbers of protons and
electrons


Cations = positive ions.
fewer electrons than protons
 form by losing electrons


Anions = negative ions
more electrons than protons
 form by gaining electrons

Ion charge

Charge isn’t element identity
identity depends just on protons
 gaining/losing electrons changes net charge
but not identity

Charge = # protons - # electrons
 charge represented by a right superscript.

Interpret
84
36
Kr
Atomic number = 36
 Mass number = 84
 Charge = 0
 Krypton-84
 # protons = atomic number = 36
 # neutrons = mass number – atomic number =
84 – 36 = 48
 # electrons = # protons – charge = 36 – 0 = 36

Interpret:
137
56
Ba
2
Atomic number = 56
 Mass number = 137
 Charge = 2+
 Barium-137 (2+ cation)
 # protons = atomic number = 56
 # neutrons = mass number – atomic number =
137 – 56 = 81
 # electrons = # protons – charge = 56 – 2 = 54

Interpret
127 
53
I
:
Atomic number = 53
 Mass number = 127
 Charge = 1–
 Iodine-127 (1- anion)
 # protons = atomic number = 53
 # neutrons = mass number – atomic number =
127 – 53 = 74
 # electrons = # protons – charge = 53 – -1 = 54

IF an atom has 20 protons, 20 neutrons,
and 20 electrons…







# protons = 20
# neutrons = 20
# electrons = 20
Atomic number = # protons = 20
Mass number = # protons + # neutrons = 20 + 20 = 40
Charge = # protons - # electrons = 20 – 20 = 0
Calcium-40
40
20
Ca
IF an atom has 11 protons, 12 neutrons,
and 10 electrons…







# protons = 11
# neutrons = 12
# electrons = 10
Atomic number = # protons = 11
Mass number = # protons + # neutrons = 11 + 12 = 23
Charge = # protons - # electrons = 11 – 10 = 1+
Sodium-23 (1+ cation)
23
11
Na

IF an atom has 15 protons, 16 neutrons,
and 18 electrons…







# protons = 15
# neutrons = 16
# electrons = 18
Atomic number = # protons = 15
Mass number = # protons + # neutrons = 15 + 16 = 31
Charge = # protons - # electrons = 15 – 18 = -3
Phosphorus-31 (3- anion)
31
15
P
3