chapter 3atoms and elements 3.6 isotopes and atomic mass 1 24 mg 25 mg 26 mg 12 12 12 copyright ©...
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Chapter 3 Atoms and Elements3.6
Isotopes and Atomic Mass
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24Mg 25Mg 26Mg 12 12 12
Copyright © 2009 by Pearson Education, Inc.
IsotopesIsotopes
• 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 SymbolAn 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 113
Information from Atomic SymbolsThe 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 Atomic SymbolsExamples of number of subatomic particles for atoms
Atomic symbol
16 31 65
O P Zn 8 15 30
8 p+ 15 p+ 30 p+
8 n 16 n 35 n 8 e- 15 e- 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:
12C 13C 14C 6 6 6
protons ______ ______ ______
neutrons ______ ______ ______
electrons ______ ______ ______
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Learning CheckWrite 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 Check1. 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 7
B. 12X 14X 6 6
C. 15X 16X 7 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 naturallyoccurring sulfur
containsseveral isotopes with thefollowing abundancesIsotope %
abundance 32S 95.02 33S 0.75 34S 4.21 36S 0.02
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32S, 33S, 34S, 36S16 16 16 16
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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 element compared to 12C.
• is not the same as the mass number.
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Na22.99
Isotopes of Some Elements and Their Atomic Mass
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Most elements have two or more isotopes that contribute to the atomic mass of that element.
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 CheckUsing the periodic table, specify the atomic
mass ofeach 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 Cl35Cl 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 amu25Mg = 24.99 amu x 10.13/100 = 2.531 amu26Mg = 25.98 amu x 11.17/100 = 2.902 amu
Atomic mass (average mass) Mg = 24.31 amu
24Mg is the most prevalent isotope in a magnesium sample
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Atomic Mass of MagnesiumThe 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 CheckGallium 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
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Energy LevelsEnergy 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 LevelsEnergy levels have a maximum number of electrons equal to 2n2.
Energy level Maximum number of electrons n = 1 2(1)2 = 2(1) = 2 n = 2 2(2)2 = 2(4) = 8 n = 3 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
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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An s orbital
•has a spherical shape around the nucleus.
•is found in each energy level.
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 Elements3.8
Periodic Trends
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Copyright © 2009 by Pearson Education, Inc.
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 ElectronsAll the elements in a group have the same
number ofvalence electrons.
Example: Elements in group 2A (2) have two (2) valence electrons.
Be 2, 2Mg 2, 8, 2Ca 2, 8, 8, 2Sr 2, 8, 18, 8, 2
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Periodic Table and Valence ElectronsRepresentative Elements Group Numbers 1 2 3 4 5 6 7 8
H He 1 2
Li Be B C N O F Ne 2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8
Na Mg Al Si P S Cl Ar2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8
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Learning CheckState the number of valence electrons for each.A. O
1) 4 2) 6 3) 8
B. Al
1) 13 2) 3 3) 1
C. Cl
1) 2 2) 5 3) 7
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Learning CheckState 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 CheckFor the element with the following electron
arrangement:1) State the number of valence electrons.2) Identify the element.
A. 2, 8, 5B. 2, 8, 8, 2C. 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 thesame 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 2) K 3) Al
. .B. . X .
. is the electron-dot symbol of
1) B 2) N 3) P
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The Orbitals Orbitals are
grouping in group according to the angular-momentum 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
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Atomic size is
described using the
atomic radius; the
distance from the
nucleus to the
valence electrons.
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.
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Learning CheckSelect the element in each pair with the larger
atomicradius.
A. Li or KB. K or BrC. 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 ofrepresentative
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.
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Copyright © 2009 by Pearson Education, Inc.
Learning CheckSelect the element in each pair with the higher
ionizationenergy.
A. Li or KB. K or BrC. P or Cl
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