Download Chapter 3 Atoms and Elements

Chapter 3
Atoms and Elements
3.6
Isotopes and Atomic Mass
24Mg 25Mg 26Mg
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12
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Copyright © 2009 by Pearson Education, Inc.
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Isotopes
Isotopes
• are atoms of the same element that have different
mass numbers.
• have the same number of protons, but different
numbers of neutrons.
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Atomic Symbol
An atomic symbol
• represents a particular atom of an element.
• gives the mass number in the upper left corner and
the atomic number in the lower left corner.
Example: An atom of sodium with atomic number 11 and
a mass number 23 has the following atomic symbol:
mass number
23
Na
atomic number
11
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Information from Atomic Symbols
The atomic symbol for a specific atom of an element
gives the
• number of protons (p+),
• number of neutrons (n),
• and number of electrons (e-).
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Information
from
Atomicparticles
Symbols
Examples of number
of subatomic
for atoms
Atomic symbol
16
8
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O
8 p+
8n
8 e-
P
15
15 p+
16 n
15 e-
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Zn
30
30 p+
35 n
30 e-
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Learning Check
Naturally occurring carbon consists of three isotopes:
12C, 13C, and 14C. State the number of protons, neutrons,
and electrons in each of the following:
protons
12C
13C
14C
6
6
6
______
neutrons ______
electrons
______
______
______
______
______
______
______
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Learning Check
Write the atomic symbols for atoms with the
following subatomic particles:
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e- ___________
C. 47p+, 60 n, 47 e-
___________
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Learning Check
1. Which of the pairs are isotopes of the same element?
2. In which of the pairs do both atoms have 8 neutrons?
A.
15X
15X
8
B. 12X
6
7
14X
6
15X
C.
7
16X
8
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Isotopes of Magnesium
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Isotopes of Magnesium
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Isotopes of Sulfur
A sample of naturally
occurring sulfur contains
several isotopes with the
following abundances
Isotope % abundance
32S
95.02
33S
0.75
34S
4.21
36S
0.02
32S, 33S, 34S, 36S
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16
16
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Copyright © 2009 by Pearson Education, Inc.
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Atomic Mass
The atomic mass of an element
• is listed below the symbol of each element
on the periodic table.
• gives the mass of an “average” atom of each
Na
22.99
element compared to 12C.
• is not the same as the mass number.
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Isotopes of Some Elements and Their
Atomic Mass
Most elements have two or more isotopes that
contribute to the atomic mass of that element.
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Atomic Mass for Cl
The atomic mass of chlorine is
• due to all the Cl isotopes.
• not a whole number.
• the average of two isotopes:
35Cl
and 37Cl.
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Learning Check
Using the periodic table, specify the atomic mass of
each element.
A.
calcium
B. aluminum
C. lead
__________
__________
__________
D. barium
__________
E. iron
__________
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Calculating Atomic Mass
The calculation for atomic mass requires the
• percent(%) abundance of each isotope.
• atomic mass of each isotope of that element.
• sum of the weighted averages.
mass of isotope(1)x (%) + mass of isotope(2) x (%) +
100
100
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Calculating Atomic Mass for Cl
35Cl
has atomic mass 34.97 amu (75.76%) and 37C
has atomic mass 36.97 amu (24.24%).
• Use atomic mass and percent of each isotope to
calculate the contribution of each isotope to the
weighted average.
Atomic mass 35Cl x % abundance =
Atomic mass 37Cl x % abundance =
• Sum is atomic mass of Cl is
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Calculating Atomic Mass Mg
Isotope Mass
Abundance
24Mg
= 23.99 amu x 78.70/100
= 18.88 amu
25Mg
= 24.99 amu x 10.13/100
= 2.531 amu
26Mg
= 25.98 amu x 11.17/100 =
Atomic mass (average mass) Mg
24Mg
2.902 amu
= 24.31 amu
is the most prevalent isotope in a magnesium sample
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Atomic Mass of Magnesium
The atomic mass of Mg
• is due to all the Mg
isotopes.
• is a weighted average.
• is not a whole number.
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Learning Check
Gallium is an element found in lasers used in compact
disc players. In a sample of gallium, there is 60.10% of
69Ga (atomic mass 68.926) atoms and 39.90% of 71Ga
(atomic mass 70.925) atoms.
What is the atomic mass of gallium?
Which isotope has more atoms?
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Chapter 3 Atoms and Elements
3.7
Electron Energy Levels
Copyright © 2009 by Pearson Education, Inc.
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Energy
Levels
Energy levels
• are assigned numbers n = 1,
•
•
2, 3, 4, and so on.
increase in energy as the
value of n increases.
are like the rungs of a ladder
with the lower energy levels
nearer the nucleus.
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Energy Levels
Energy levels have a maximum number of electrons equal
to 2n2.
Energy level
n=1
n=2
n=3
Maximum number of electrons
2(1)2 = 2(1) = 2
2(2)2 = 2(4) = 8
2(3)2 = 2(9) = 18
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Orbitals
An orbital
• is a three-dimensional space around a nucleus,
where an electron is most likely to be found.
• has a shape that represents electron density (not a
path the electron follows).
• can hold up to 2 electrons.
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Orbitals
An s orbital
•has a spherical shape around the nucleus.
•is found in each energy level.
A p orbital
• has a two-lobed shape.
• is one of three p orbitals in each energy level from n = 2.
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Orbitals
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Electron Level Arrangement
In the electron level arrangement for the first 18
elements
• electrons are placed in energy levels (1, 2, 3, etc.),
beginning with the lowest energy level
• there is a maximum number in each energy level.
Energy level
Number of electrons
1
2 (up to He)
2
8 (up to Ne)
3
8 (up to Ar)
4
2 (up to Ca)
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Learning Check
Write the electron level arrangement for each:
1. N
2. Cl
3. K
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Learning Check
Identify the element with each electron level
arrangement:
1. 2, 2
2. 2, 8, 3
3. 2, 7
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Chapter 3 Atoms and Elements
3.8
Periodic Trends
Copyright © 2009 by Pearson Education, Inc.
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Valence Electrons
The valence electrons
• determine the chemical properties of the elements.
• are the electrons in the highest energy level.
• are related to the group number of the element.
Example: Phosphorus has 5 valence electrons.
5 valence electrons
P in Group 5A(15)
2, 8, 5
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Groups and Valence Electrons
All the elements in a group have the same number of
valence electrons.
Example: Elements in group 2A (2) have two (2)
valence electrons.
Be
2, 2
Mg
2, 8, 2
Ca
2, 8, 8, 2
Sr
2, 8, 18, 8, 2
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Periodic Table and Valence Electrons
Representative Elements Group Numbers
1
2
3
4
5
6
7
8
H
1
Li
2,1
Be
2,2
B
2,3
C
2,4
Na
Mg Al Si
2,8,1 2,8,2 2,8,3 2,8,4
He
2
N
2,5
O
2,6
F
2,7
Ne
2,8
P
2,8,5
S
2,8,6
Cl
Ar
2,8,7 2,8,8
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Learning
Check
State the number of valence electrons for each.
A. O
1) 4
2) 6
3) 8
B. Al
1) 13
2) 3
3) 1
2) 5
3) 7
C. Cl
1) 2
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Learning Check
State the number of valence electrons for each.
A. calcium
1) 1
2) 2
3) 3
B. group 6A (16)
1) 2
2) 4
3) 6
C. tin
1) 2
2) 4
3) 14
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Learning Check
For the element with the following electron arrangement:
1) State the number of valence electrons.
2) Identify the element.
A. 2, 8, 5
B. 2, 8, 8, 2
C. 2, 7
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Electron-Dot Symbols
An electron-dot symbol
• shows the valence electrons
around the symbol of the
element.
• for Mg has 2 valence electrons as
single dots on the sides of the
symbol Mg.
.
.
·Mg · or Mg · or ·Mg or ·Mg
·
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Writing Electron-Dot Symbols
Electron-dot symbols for
• groups 1A (1) to 4A (14) use single dots.
·
·
Na ·
· Mg ·
· Al ·
·C·
·
• groups 5A (15) to 7A (17) use pairs and single dots.
··
·P·
·
··
:O·
·
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Groups and Electron-Dot Symbols
In a group, all the electron-dot symbols have the
same number of valence electrons (dots).
Example: Atoms of elements in Group 2A (2) each
have 2 valence electrons.
· Be ·
· Mg ·
· Ca ·
· Sr ·
· Ba ·
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Learning
Check
.
A.
X is the electron-dot symbol for
1) Na
B.
..
.X.
.
1) B
2) K
3) Al
is the electron-dot symbol of
2) N
3) P
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The Orbitals

Orbitals are
grouping in group
according to the
angularmomentum
quantum number l
is called subshells.


Types of orbitals
Notations: s, p, d, f
11.9 - Electron Arrangements in the First
Eighteen Atoms on the Periodic Table
 Recall: Atomic number (Z) = #
electrons = # protons
 Electron configuration:
describes the orbitals that are
occupied by the electrons in an
atom
 Orbital diagrams: describe the
orbitals with arrows
representing electrons
 a. Arrows are written up or
down to denote electron’s
spin
Electrons Configuration

Shorthand version – give the symbol of the noble
gas in the previous row to indicate electrons in filled
shells, and then specify only those electrons in
unfilled shells
E.g Shorthand version of P: [Ne] 3s2 3p3


The valence-shell electrons are the outer most
shell of electron
E.g
Valence electrons of P is 5
Electron Configurations and the
Periodic Table
 Write the full electron configuration
 short hand notation
 Determine the valence electrons
 Na, O, Cl, Mg, S, Cu,
Atomic Size
Atomic size is described
using the atomic radius;
the distance from the
nucleus to the valence
electrons.
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Atomic Radius Within A Group
Atomic radius increases
going down each group of
representative elements.
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Atomic Radius Across a Period
Going across a period from left to right,
• an increase in the number of protons increases attraction
for valence electrons.
• atomic radius decreases.
Copyright © 2009 by Pearson Education, Inc.
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Learning Check
Select the element in each pair with the larger atomic
radius.
A. Li or K
B. K or Br
C. P or Cl
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Ionization Energy
Ionization energy is the energy it takes to remove a valence
electron.
Na(g) + Energy (ionization) -> Na+(g) + e-
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Ionization Energy In a Group
Going up a group of
representative elements,
• the distance decreases
between nucleus and
valence electrons.
• the ionization energy
increases.
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Ionization Energy
• Metals have
•
lower ionization
energies.
Nonmetals have
higher
ionization
energies.
Copyright © 2009 by Pearson Education, Inc.
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Learning Check
Select the element in each pair with the higher ionization
energy.
A. Li or K
B. K or Br
C. P or Cl
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