The Atom &
Subatomic Particles
Atomic Number
 atomic




= size of nuclear charge
= number of protons in nucleus
determines identity of element
used to give order to periodic table
 For

number:
neutral atoms:
atomic number = number of electrons
What is an atom?
Number of Protons = Number of Electrons
Number of Protons = Atomic Number
Atomic Mass = Protons + Neutrons
What we knew by 1932:
Relative Charge
Relative Mass
Electron, 0e
-1
.0005486 amu
(or 0 amu)
Proton, 1p or 1H
+1
1.0073 amu
(or 1 amu)
0
1.0087 amu
(or 1 amu)
-1
1
1
Neutron, 1n
0
Location of Subatomic Particles
• Electrons located outside nucleus
• Protons & neutrons located inside nucleus
• Protons & neutrons = nucleons
Isotopes
 atoms
of same element with
different # neutrons (mass # is different)


# protons stays same! (atomic # is same)
Dalton’s theory modified in number of
important ways:


the atom is divisible (protons, neutrons, electrons)
atoms of same element can be different (different
# neutrons)
Mass Number
 Mass
number = # protons + # 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 – 6 = 6 neutrons
17O
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 ?
Atomic number is ?
15
7
# of neutrons is ? 15 – 7 = 8
# of electrons in atom is?
7
19
F Mass number is ? 19
Atomic number is ? 9
# of neutrons is ? 19 – 9 = 10
# of electrons in atom is? 9
Other Notations
 Might

also see: C-12
number following symbol is mass number!
 Need
to look up atomic number
What’s the difference between
H-1, H-2, and H-3?

All H’s have 1 proton

H-1: mass # = 1


H-2: mass # = 2


1 proton, 0 neutrons
1 proton, so it must have 1 neutron
H-3: mass # = 3

1 proton, so it must have 2 neutrons
Consider U-234, U-235, & U-238
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
 What’s
How many neutrons in Po-217?
 What’s
 How
 So
the atomic number? 84
many protons?
84
the neutrons are …
217 – 84 = 133 neutrons!
IONS
 Atoms:
electrically neutral so ...
# electrons = # protons
 IONS = atoms that gained or lost electrons
 protons and electrons aren't = anymore, so ions
are not neutral they carry a charge (+/-)
IONS
 Charge
of ion:
electrons
= # protons minus #
(subtract the electrons since are negative)
IONS
 If
atom loses electrons, it has more
positive charge than negative
 What kind of ion is it?
positive
 If
atom gains electrons, it has more
negative charge than positive
negative
 What kind of ion is it?
Charge of Ions

Ion charge written as right superscript
Cl-1 a chloride ion with a charge of -1
 Na+1 a sodium ion with a charge of +1
 O-2
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
1
Lost 1
34S-2
16
11 p, 12 n, 10 e
25Mg+2
12
Lost 2
12 p, 13 n, 10 e
Got 2
16 p, 18 n, 18 e
Lost 2
64Zn+2
30
30 p, 34 n, 28 e
What is an amu?
 equal
to the atomic mass unit
 amu
 1/12 the mass of the C-12 atom
 C-12
is used as the reference for atomic mass
Atomic Weight or Mass
 Atomic
mass is relative
 1st H-1 used as standard:

1 atom of H = 1 amu
 next


O combines with many elements
1 atom of O = 16 amu's
 Now

O-16 used as standard:
C-12 is the standard:
1 atom of C-12 has a mass of exactly 12 amu's