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The Atom &
Subatomic Particles
Atomic Number
 atomic
equal to size of nuclear charge
equal to number protons
determines identity element
determines order on periodic table




 for

number:
neutral atoms:
# of protons equals # of electrons
What is an atom composed of?
What we knew by 1932:
electron
0e
relative charge
relative mass
-1
.0005486 amu
(or 0 amu)
+1
1.0073 amu
(or 1 amu)
0
1.0087 amu
(or 1 amu)
-1
proton
1p or 1H
1
neutron
1n
0
1
Location of Subatomic Particles
• electrons located outside nucleus
• protons & neutrons located inside nucleus
• protons & neutrons AKA nucleons
Isotopes
 atoms
of same element:
different # neutrons (mass # is different)


# protons stays same! (atomic # is same)
How Dalton’s theory was modified:
1.
2.
atom is divisible (protons, neutrons, electrons)
atoms of same element can be different
(different # neutrons)
Mass Number
 mass

number is equal to:
# protons added to # neutrons
Notation
12C
6
•left superscript is mass number
•left subscript is atomic number
Notation
12C
6
12 = # protons + neutrons
6 = # protons
How many neutrons in this atom?
12 (mass #)
- 6 (atomic #)
6 neutrons
17 O
8
mass number is ?
17
atomic number is ?
8
# of neutrons is ?
17 – 8 = 9
# of electrons in atom is? 8
15 N
7
mass number is ?
15
atomic number is ?
7
# of neutrons is ? 15 – 7 = 8
# of electrons in atom is? 7
Other Notations
 might

also see: F-19
number following symbol is mass number!
 need
to look up atomic number
19F
What’s the difference between
H-1, H-2, and H-3?

All H’s must have 1 proton
H-1: mass # = 1
1 proton, 0 neutrons
H-2: mass # = 2
1 proton, 1 neutrons
H-3: mass # = 3
1 proton, 2 neutrons
Consider U-234, U-235, & U-238
 what’s
the atomic number of U?
92
 how many protons in U?
92
 how many neutrons in U-234?
234 – 92 = 142
 how many neutrons in U-235?
235 – 92 = 143
 how many neutrons in U-238?
238 – 92 = 146
 how many electrons in U?
92
IONS
 atoms:
electrically neutral so ...
# electrons EQUAL # protons
 IONS
are atoms that gained or lost electrons
 protons
& electrons ARE NOT equal anymore,
so ions are not neutral they carry a charge (+/-)
IONS
 to
calculate charge of ion:
# protons minus # electrons
IONS
 If
atom loses electrons, it has more
positive charge than negative (#p+ > #e-)
 what is sign of ion formed?
positive
 If
atom gains electrons, it has more
negative charge than positive (#e- > #p+)
negative
 what is sign of ion formed ?
Charge of Ions

Ion charge written as right superscript

Cl-1

Na+1 a sodium ion with a charge of +1

O-2
a chloride ion with a charge of -1
an oxygen ion with a charge of -2
If no right superscript, it’s understood to
be zero and therefore a neutral atom
Putting it all together
 How
many protons, neutrons, & electrons
in each of the following
23Na+1
11
lost 1e-
11p, 12n, 10e25Mg+2
12
lost 2e-
12p, 13n, 10e-
34S-2
16
gain 2e-
16p, 18n, 18e64Zn+2
30
lost 2e-
30p, 34n, 28e-
What is an amu?
 atomic
 amu
mass unit
 1/12 mass of C-12 atom
 C-12:
used as reference for atomic mass