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Atomic Structure
Modern Atomic Theory
Modern Atomic Theory
The atom consists of positive protons, negative
electrons, and neutral neutrons.
Protons and neutrons are located in the nucleus of the
atom, which is small and massive.
Electrons are located outside of the nucleus, which
creates the volume of the atom.
The Modern View of Atomic Structure
Subatomic particles
Name
Actual
Relative
mass (amu) mass (g)
Symbol Charge
Electron
e-
Proton
p+
Neutron
n0
-1
1/1840
9.11 x 10-28
≈0
+1
1
1.67 x 10-24
0
1
1.67 x 10-24
amu = atomic mass unit
Atomic Number
Henry Moseley discovered that each
element has a unique positive charge.
 Atomic Number or Z



Number of Protons
For neutral atoms
# protons = # of electrons
 How
many protons and electrons are in Si?
•
 What
•
14 p and 14 e
element contains 8 protons?
oxygen
Ions


Most atoms are not neutral.
An ion forms when an atom loses or gains
electrons.
 Lose electrons = positive charge
 Gain electrons = negative charge

Ion charge is written as a superscript.

Charge = # protons - # electrons
 What is the element with 16 protons and 18
electrons?

S-2
 What is the element with 20 protons and 18
electrons?

Ca+2
Atomic Mass

Atomic mass or A is the number of
protons and neutrons in an atom.
Atomic Mass Unit or amu
 based on carbon-12, exactly 12 amu
 We always round atomic mass to the
nearest whole number.

 Why?
 Atomic mass is the weighted average of
all of isotopes for that element based on
abundance.
Isotopic Notation
• We write symbols for elements as follows:
Mass number
A
Z
Atomic number
X
Element symbol
Isotopes

Atoms with the same number of protons
different number of neutrons, therefore
different atomic masses.
1
1
H
2
1
H
3
1
H
Isotopes
Naming Isotopes
 We
can also put the mass
number after the name of the
element.
 carbon- 12
 carbon -14
 uranium-235
Calculating averages
Multiply the atomic mass of each isotope by
it’s abundance (expressed as a decimal),
then add the results.
 Calculate the atomic mass of copper if
copper has two isotopes. 69.1% has a
mass of 62.93 amu and the rest has a
mass of 64.93 amu.
 62.93 amu x .691 = 43.5 amu
64.93 amu x .309 = 20.1 amu
Total = 43.5 + 20.1 = 63.6 amu

Atomic Mass
What is the atomic mass of Na? 23 amu
 What is the atomic number of Na? 11 amu
 Write this in isotopic notation: 23

Na
11

To find the neutrons subtract the two
numbers. 23 -11 = 12 amu
Symbols

Find the
– number of protons 35 p
– number of neutrons 45 n
– number of electrons 35 e
– Atomic number 35
– Mass Number 80 amu
80
35
Br
Symbols

Find the
– number of protons 35 p
– number of neutrons 45 n
– number of electrons 36 e
– Atomic number 35
– Mass Number 80 amu
80
35
Br
-1
Symbols
 if
an element has an atomic
number of 34 and a mass number
of 78 what is the
–number of protons 34 p
–number of neutrons 44 n
–number of electrons 34 e
–Complete symbol 78
34
Se
Symbols
 if
an element has 91 protons and
140 neutrons what is the
–Atomic number 91
–Mass number 231 amu
–number of electrons 91
–Complete symbol 231
91
Pa
Summary
Mass number
Element charge
1
1
Atomic number
H
+1
Element symbol
Symbols

Find the
– number of protons 56 p
– number of neutrons 79 n
– number of electrons 54 e
– Atomic number 56
– Mass Number 135 amu
– Is this an ion? Yes
– Is this an isotope? Yes
135
56
+2
Ba