Chapter 3 Atoms and Elements

3.7 Electron Energy Levels

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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.

2

Energy Levels

Energy 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

3

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.

4

Orbitals

A p orbital

• has a two-lobed shape.

• is one of three p orbitals in each energy level from n = 2.

5

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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Examples

Write the electron level arrangement for each:

1. N

2. Cl

3. K

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Examples

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 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, 2Mg 2, 8, 2Ca 2, 8, 8, 2Sr 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 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 Ar 2,8,1 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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Examples

State the number of valence electrons for each.A. O

1) 4 2) 6 3) 8

B. Al1) 13 2) 3 3) 1

C. Cl1) 2 2) 5 3) 7

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Examples

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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ExamplesHow many electrons are in energy level 3 of

the followingA. Sulfur b.

Phosphorous

Examples

Identify the elements that have the following electron level

Energy level 1 2 3a. 2 1 0b. 2 8 6c. 2 8 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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Examples .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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ExamplesWhat is the dot Lewis structure of

CarbonMagnesiumAluminumArgon

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.

25

ExamplesSelect 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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ExamplesSelect the element in each pair with the higher

ionizationenergy.

A. Li or KB. K or BrC. P or Cl

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