Chapter 3 Atoms and Elements3.6

Isotopes and Atomic Mass

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24Mg 25Mg 26Mg 12 12 12

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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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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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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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