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Model 1. The Electron Configurations of the Ground States (lowest energy states) of Several Elements

Model 1. The Electron Configurations of the Ground States (lowest energy states) of Several Elements

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ChemActivity 12



Electron Spin



69



3. For each case, predict the results of a Stern-Gerlach experiment on a beam of

atoms. That is, predict whether the atoms will pass through undeflected or will be

split into different components.

a) Li



b) Be



c) B



d) C



Model 2: Diamagnetic and Paramagnetic Atoms.

An atom with an equal number of spin "up" and spin "down" electrons is known as

diamagnetic, and the atom is repelled by a magnetic field. In this case we say that all of

the electrons are "paired." If this is not the case—that is, if there are unpaired electrons—

the atom is attracted to a magnetic field, and it is known as paramagnetic. The strength

of the attraction is an experimentally measurable quantity known as the magnetic

moment. The magnitude of the magnetic moment (measured in magnetons) is related to

(but not proportional to) the number of unpaired electrons present. That is, the larger the

number of unpaired electrons, the larger the magnetic moment.

Here are some simulated data concerning this phenomenon:

Table 1. Magnetic moments of several elements.

Element

Magnetic Moment

(magnetons)

H

Paramagnetic

1.7

He

Diamagnetic

0

B

Paramagnetic

1.7

C

Paramagnetic

2.8

N

Paramagnetic

3.9

O

Paramagnetic

2.8

Ne

Diamagnetic

0



Critical Thinking Questions

4. Why is the situation of equal numbers of spin up and spin down electrons referred

to as all the electrons being "paired"?



5. Based on the data in Table 1, rank the following atoms in terms of the number of

unpaired electrons in each atom: B, C, N, O, Ne. Explain your reasoning clearly.



70



6.



ChemActivity 12



Electron Spin



a)



Make a diagram for C similar to those in Model 1 that shows why C is

paramagnetic. Explain how your diagram is consistent with your answer to

CTQ 5 and the data in Table 1.



b)



Based on the data provided in Table 1, would you revise your prediction

from CTQ 3d of the results of a Stern-Gerlach experiment on a beam of C

atoms? If so, in what way? If not, why not?



7. Make a diagram for N similar to those in Model 1 that is consistent with your

answer to CTQ 5 and the data in Table 1.



8. Based on the data in Table 1 and your answers to CTQs 6 and 7, do electrons in a

given energy level tend to pair or not?



9. Based on the data in Table 1, indicate the number of unpaired electrons in

a) an O atom?



b) a Ne atom?



10. How many "pairs" of electrons are there in a "filled" p subshell?



ChemActivity 12



Electron Spin



71



Information

The model that we have developed for the structure of atoms has been further

refined. This more sophisticated model, known as the quantum mechanical model,

retains most of the general features that we have deduced for atomic structure. Within

this model, the electrons in atoms occupy specific regions of space known as orbitals,

with a maximum of two electrons occupying each orbital. There are three orbitals in a p

subshell and one orbital in each s subshell. The idea that the two electrons in a given

orbital must have opposite spins was first proposed by Wolfgang Pauli in 1925, and is

known as the Pauli Exclusion Principle. Most general chemistry texts have some

discussion of these ideas. An interesting introduction to the ideas of quantum mechanics

can be found in Sections 3.13 and 3.15 of Chemistry: Structure & Dynamics, by J. N.

Spencer, G. M. Bodner, and L. H. Rickard (Fourth Edition). You should read the

appropriate sections of your text to become familiar with the terms and basic ideas of this

model.



Exercises

1. Using grammatically correct English sentences, describe the structure of a 13C atom

as completely as you can. Both the nucleus and the electrons should be considered

in your description. You may use a diagram (or diagrams) as part of your answer,

but you should explain the significance in words.

2. Indicate whether each of the following statements is true or false and explain your

reasoning.

a)

b)

c)

d)



The oxide ion, O2–, has the same electron configuration as neon.

In the Si atom, there are no unpaired electrons.

An atom of Si and an atom of S have the same number of unpaired

electrons.

In all atoms with an even number of electrons, all of the electrons are paired.



3. The electron configuration of N can be represented as:



Why are three separate lines shown for the 2p energy level (subshell)?



72



ChemActivity 12



Electron Spin



4. The 3d subshell can "hold" ten electrons. Make a diagram, similar to those in the

model, for Ni. Predict if the Ni atom will be diamagnetic or paramagnetic.

5. Predict the magnetic moment and the number of unpaired electrons for the F atom.

6. Consider the element X. It has the following properties: X has a smaller atomic

radius than Ar; the ion X- has no unpaired electrons; the ion X+ has more unpaired

electrons than X. What is element X? Explain your reasoning.



Problems

1. Which of the following

atoms and ions will be paramagnetic? Ti, Ti2+, Na, Na+,



Sm, Sm3+, Cl, Cl .

2. Which species has more unpaired electrons, Fe or Fe2+?



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74



ChemActivity



13



Lewis Structures (I)

(What Makes a Molecule?)



The properties of a molecule depend on how the electrons are distributed in the

molecule. For example, it takes more energy to separate an oxygen atom from a carbon

atom in a molecule of carbon monoxide, CO, than it does to separate an oxygen atom

from a carbon atom in a molecule of carbon dioxide, CO2. Another example: CO2 is a

linear molecule (the three nuclei lie in a straight line), whereas H2O is not linear (the

three nuclei do not lie in a straight line). These experimentally determined facts can be

predicted by making diagrams of molecules, called Lewis structures. The purpose of

Lewis structures is to provide a simple way for chemists to represent molecules that

allows reasonable predictions to be made about the structure and properties of the actual

molecules.



Model 1: Common Methods to Designate Atoms.

Figure 1.



Two methods to designate atoms



We have seen that a hydrogen atom has a core charge of +1 and that a neutral

hydrogen atom has one valence electron. Also, we have seen that a fluorine atom has a

core charge of +7 and seven valence electrons. Thus, we have represented these two

atoms as shown in Figure 1. Alternatively, we could represent each atom with the

appropriate atomic symbol and a dot for each valence electron, also shown in Figure 1.

The latter designations take up less space, make the atom easily identifiable, and are more

concise; the core charge is not explicit, however, and it is the responsibility of the reader

to keep the core charges in mind.



ChemActivity 13



Lewis Structures (I)



75



G. N. Lewis proposed the following as representations of the valence electrons for

the groups indicated.

IA



IIA



Li



Be



IIIA



IVA



B



C



VA



VIA



N



O



Critical Thinking Questions

1. For each of the following neutral atoms give the core charge.

a)

b)

c)



Li



Cl



He



2. Give the Lewis representation for each of the following atoms.

a)



iodine



b)



calcium



c)



phosphorus



VIIA

F



VIIIA

Ne



76



ChemActivity 13



Lewis Structures (I)



Model 2: Lewis Structures for Molecules.

The covalent bond—the sharing of two electrons in the valence shell of both atoms:



Or, more simply,

F



+



H



=



F H



for a covalent bond

Another example,

O



+



2 H



=



O H

H



O



+



2 H



=



O H



or



H



Information

Here are two rules for Lewis structures:





Hydrogen must share two electrons—a bonding pair.







The sum of the shared (bonding) electrons and the lone pair electrons for carbon,

nitrogen, oxygen, and fluorine atoms must be eight—an octet. Usually the other

elements in groups IV, V, VI, and VII also follow the octet rule.



ChemActivity 13



Lewis Structures (I)



77



Critical Thinking Questions

3. Given the shell model of the atom, suggest a possible reason that Lewis proposed a

maximum of two electrons for hydrogen and a maximum of eight for carbon,

nitrogen, oxygen, and fluorine atoms?



4. Answer the following for the nitrogen atom:

a)



What is the Lewis representation for N?



b)



How many additional electrons does one N atom require when it forms a

molecule?



c)



What is the likely formula for a molecule composed of hydrogen atoms and

one nitrogen atom? Draw the Lewis structure for this molecule.



5. What is the likely formula for a molecule composed of hydrogen atoms and one

sulfur atom? Draw the Lewis structure for this molecule.



6. Make a checklist that can be used to determine if a Lewis structure for a molecule is

correct.



7. Without attempting to draw a Lewis structure, calculate the total number of valence

electrons in each of these molecules:

a) H2CO

b) N2

c) Cl2



78



ChemActivity 13



Lewis Structures (I)



Model 3: Lewis Structures of Some Molecules.

H—C—H



N



N



Cl



Cl



O



H2CO



N2



Cl2



Critical Thinking Questions

8. What is the total number of electrons in the Lewis structure in Model 3 for each

molecule:

a)



H2CO



b)



N2



c)



Cl2



9. Compare your answers to CTQs 7 and 8. How does one determine the total number

of electrons that should be used to generate a Lewis structure?



10.



For Cl2, is the sum of the bonding electrons and the lone pair electrons (also known

as nonbonding electrons) around each Cl atom consistent with the Lewis model?



11.



For N2, is the sum of the bonding electrons and nonbonding electrons around each

nitrogen atom consistent with the Lewis model?



12.



For H2CO:

a)



Is the sum of the bonding electrons and lone pair electrons around the

carbon atom consistent with the Lewis model?



b)



Is the sum of the bonding electrons and lone pair electrons (nonbonding

electrons) around the oxygen atom consistent with the Lewis model?



ChemActivity 13



Lewis Structures (I)



79



13. Revise (as necessary) your checklist that can be used to determine if a Lewis

structure for a molecule is correct.



14.



Use your checklist to determine whether or not the following is a correct structure

for CO2:

O=C=O



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