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Chapter 3 Electronic Structure and the Periodic Law

3.2 Electronic Arrangements in Atoms

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Bohr Model or the Solar System Model

Bohr Model or the Solar System Model Niels Bohr in 1913 introduced his model of the

hydrogen atom.Electrons circle the nucleus in orbits, which are

also called energy levels.An electron can “jump” from a lower energy level

to a higher one upon absorbing energy, creating an excited state.

The concept of energy levels accounts for the emission of distinct wavelengths of electromagnetic radiation during flame tests.

Niels Bohr (1885-1962)

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Bohr’s Orbit ModelDefinitions

Quantum is the smallest increment of energy, for example, in an atom emitting or absorbing radiation.

Ground state is the condition of an atom in which all electrons are in their normal, lowest energy levels.

Excited state is an unstable, higher energy state of an atom.

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The Atomic Orbital Model Orbits are replaced with atomic orbitals,

volumes of space where the electrons can be found.

The arrangement of electrons in the orbitals is the electronic configuration of an atom, which determines the chemistry of an atom.

Bohr’s Orbit Model (continued)

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Definitions

Electrons in the highest occupied energy level are the greatest stable distance from the nucleus. These outermost electrons are known as valence electrons to be discussed in section 3.3.

Shell is a principal energy level defined by a given value of n, where n can be 1,2,3,4 etc… and is capable of holding 2n2 electrons.

An orbital is a region of three-dimensional space around an atom within which there is a significant probability (usually shown as 90%) that a given electron will be found.

Subshells have different energy levels (orbitals) within a given shell that is designed by a letter from the group s,p,d,& f

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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 ground.

n = 4

n = 3

n = 2

n = 1

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Energy levels are spaced differently, like ladder rungs

Credit: Foto-Search.com

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Atomic energylevels are like floors of a house

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State transitions for hydrogen

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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

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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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s Orbitals

An s orbital • has a spherical shape

around the nucleus.• is found in each energy

level.

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

n = 1

n = 2

n = 3

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p Orbitals

A p orbital • has a two-lobed shape.• is one of three p orbitals in each energy level from n = 2.

Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings

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Atomic Orbitals.

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Electrons in Energy Levels n = 1- 4

Energy Orbitals Maximum Total Level No. of Electrons Electrons1 1s 2 22 2s 2 8

2p 63 3s 2 18

3p 6 3d 10

4 4s 2 32 4p 6 4d 10

4f 14

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The number of

A. electrons that can occupy a p orbital is

1) 1. 2) 2. 3) 3.

B. p orbitals in the 2p sublevel is

1) 1. 2) 2. 3) 3.

C. electrons in the n =3 level

1) 3. 2) 6. 3) 18.

Learning Check

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The number of

A. electrons that can occupy a p orbital is

2) 2.

B. p orbitals in the 2p sublevel is

3) 3.

C. electrons in the n =3 level

3) 18.

Solution