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Atomic Structure
Subatomic Particles
Henry Mosley - 1913
 Mosley used the cathode ray
tube in a slightly different way.
 Substituted a denser material in
place of the glass and was able
to get even higher radiation 
He tried 30 different metal targets!
Atomic Number - 1913
 Mosley tried 30
different elements
as “STOPPERS.”
Found that the greater the atomic weight of
the “stopper,” the shorter the wavelength of
the x-rays.
What does any good scientist do?
PLOTS THE DATA!
• Moseley tried plotting the wavelength of the
x-ray vs. the atomic mass of the stopper.
Not so nice…
•Then he tried correlating the wavelength of
the x-ray with an integer, n. Got a very pretty
graph.
Mosley’s integer was the atomic number!
Atomic Number
 Size of nuclear charge.
 Atomic number = # protons in nucleus.
 Determines the identity of the element.
 Used to order the periodic table.
 For neutral atoms
 atomic number (#p) = number of electrons.
Henry Mosley
Killed in WWI at the age of 28.
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
What is an a.m.u.?
 atomic mass unit
 Defined as 1/12th the mass of the C-12 atom
 C-12 is used as the reference for atomic
masses.
 One atom of C-12 has a mass of exactly 12 amu
Location of Subatomic Particles
• Electrons located outside nucleus.
• Protons & neutrons located inside nucleus.
• Protons and neutrons combined are called:
Mass Number
 Mass number = # protons + # neutrons
 Always a whole number (no decimals)
 Mass number can be estimated from the atomic
mass by rounding to the nearest integer.
 REMEMBER: Only the # neutrons can vary for
any given element. The number of protons must
remain the same – it identifies the element!
Isotopes
 These are atoms of the same element
with a different number of neutrons. The
protons are the same!
 So Dalton’s theory has been modified in a
number of important ways.
 The atom is divisible.
 Atoms of the same element can be different.
Isotopic Notation
12C
6
•The left superscript is the mass number.
•The left subscript is the atomic number.
•Sometimes both are given; sometimes
only the mass number is given. (You
can figure out the atomic number from
the P.T.)
Notation
12C
6
12 = no. of protons + neutrons.
6 = no. of 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
15N
7
Mass number is ? 15
Atomic number is ? 7
# of neutrons is ? 15 – 7 = 8
# of electrons in atom is? 7
19F
Mass number is ? 19
Atomic number is ? 9
# of neutrons is ? 19 – 9 = 10
# of electrons in atom is? 9
Hyphen Notation
 Might also see C-12. The number
following the symbol is the mass number!
 You have to look up the atomic number.
What’s the difference between H-1,
H-2, and H-3?
 All H’s have 1 proton.
 H-1: mass # = 1. Has 1 proton, 0 neutrons.
 H-2: mass # = 2. Has 1 proton, so it must
have 1 neutron.
 H-3: mass # = 3. Has 1 proton, so it must
have 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
How many neutrons in Po-217?
 What’s the atomic number? 84
 How many protons?
84
 So the neutrons are …
217 – 84 = 133 neutrons!
IONS
Charged Particles
IONS
 Atoms  electrically neutral
# of electrons = # of protons
 IONS are atoms that have gained or lost
electrons.
 The protons and electrons don’t balance out
anymore, so ions carry a charge.
IONS
 Charge of ion =
#protons - #electrons
(We subtract the electrons since they’re negative.)
IONS
 If the atom loses electrons, it’s going to
have more positive charge than negative.
What kind of ion is it?
Positive (cation)
 If the atom gains electrons, it’s going to
have more negative charge than positive.
What kind of ion is it? Negative (anion)
Charge of Ions
 If we want to specify an ion, we write
the charge as a right superscript.
 Cl-1
 Na+1
 O-2
a chloride ion with a charge of -1.
a sodium ion with a charge of +1.
an oxide ion with a charge of -2.
 If there’s no right superscript, it’s
understood to be zero and a neutral
atom.
Putting it all together
 How many protons, neutrons, & electrons
in each of the following:
23Na+1
Lost 1
11
11 p, 12 n, 10 e
25Mg+2
12
Lost 2
12 p, 13 n, 10 e
Got 2
34S-2
16
19F-1
9
16 p, 18 n, 18 e
64Zn+2
30
Got 1
Lost 2
30 p, 34 n, 28 e
9 p, 10 n, 10 e
Got 3
13N-3
7
7 p, 6 n, 10 e