Chapter 17:
Isoelectronic ions

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1. What is the trend in atomic radii of atoms as one moves across a period from left to right? Why?

Smaller! Because as we move from one atom to the next, one proton (+ charge) is added to the nucleus while one electron (- charge) is added to the outer shell. In the same period, the electrons go into the same outer shell. As the shell fills, it gains more negative charges and is attracted more to the nucleus. The increased attraction pulls the electrons in closer (Radius decreases)

2. What is the trend in atomic radii of atoms as one moves down a group? Why?

Up! Because moving down a group as we pass from one row to the next the outer shell is one shell further out from the nucleus, thus, radius increases.

3. Which atom is more metallic (i.e. more metallic means loses electrons easier)? Why? Na K Cs Fr

Because as one moves down a group electrons in outer shell are held less tightly. Less tightly means they lose electrons more easily in reactions, thus are more metallic. The order Fr >Cs >K >Na. reflects the fact that Fr is lowest in Group IA, Cs is 2nd lowest, etc. The lower in a group, the more metallic

4. Which atom is more metallic? Why? F Cl Br I

I > Br > Cl > F (see question #3 for reasons)

5. Which atom is more metallic? Why? K Ca As Br

K > Ca > As > Br
Here we move across a period. Atoms with only one to a few outer shell electrons lose those electrons easily (are metallic) in reactions (K and Ca) while those with almost a full octet in the outer shell hold their electrons tightly (As and Br).
Metals: left side of the chart
Non-metals: right side of the chart

6. What is the trend in 1st ionization potential as one moves across a period from left to right?

1st. I.P. increases as one moves from left to right across a period because radius decreases and thus electrons are held tighter (see question #1).

7. What is the trend in 1st ionization potential as one moves down a group?

1st. I.P. decreases as one moves down a group because radius increases and thus electrons are held less tightly. (see question #2)

8. Which atom has the highest 1st I.P.? Why? Mg Si P Cl

Cl > P > Si > Mg
As one moves to the right across a period 1st I.P.increases because radius decreases.

9. Which atom has the highest 1st I.P.? Why? O S Se Te

O > S > Se > Te
As one moves higher in a group 1st I. P.increases because radius decreases.

10. Magnesium and calcium both have two electrons in their outer shells. Which would you predict to have the higher 1st I.P.?

Mg would be higher in 1st I.P. than Ca because its higher in group IIA (smaller radius).

11a. Which would you have predicted to have the higher 1st I.P:

a. N or O

O > N (O further to right-smaller r)

b. P or S.

S > P (S further to right - smaller r)

c. As or Se

Se > As (Se further to right- smaller r)

11b. Look up the 1st I.P. values on Figure 17.2. Were you correct in your predictions ?

The above are probably what you predicted - the answers are wrong as a glance at Figure 17.2 shows (see question #13).

12a. Which would you have predicted to have the higher 1st I.P:

a. Be or B

B > Be (Be further to right- smaller r)

b. Mg or Al

Al > Mg (Al further to right -smaller r)

c. Ca or Ga

Ga > Ca (Ga further to right- smaller r)

12b. Look up the 1st I.P. values on figure 17.2. Were you correct?

Again, a glance at Figure 17.2 shows these answers to be wrong (see question #13).

13. If you look at the data on figure 17.2 closely, you will see that although the broad trend in 1st I.P. is up as we move across a period, there are distinct intervals where 1st I.P. actually decreases. These drops in 1st occur when moving from either a IIA to a IIIA element, or a VA to a VIA element. Can these drops in 1st I.P. be explained by the Bohr Theory of Atomic Structure?

The drops in 1st. I.P. values as we move from IIA to IIIIA or from VA to VIA cannot be explained by Bohr's theory, which simply would predict that a decrease in radius as one moves from left to right across a period would cause an increase in 1st I.P. This problem will be dealt with in great detail in the next two chapters which cover the Quantum Theory.

14. Which has the smallest radius:

a. Li or Li1+

rLi1+ < rLi

b. S or S2-

rS < r S 2-

c. P or P3-

rP < rP3-

d. Ca or Ca2+

rCa2+ < rCa

e. B or B3+

rB3+ < rB

f. I or I1-

rI < rI1-

For all of the above, as positive charge increases, the radius decreases.

15. Arrange these isoelectronic ions from biggest to smallest radius.

a. S2-, P3-, Cl1-

rCl1- < rS2- <rP3-

b. Br1-, Se2-, As3-

rBr1- <rSe2- < rAs3+

c. Ca2+, K1+, Sc3+

rSc3+ < rCa2+ < rK1+

d. Mg2+, Na1+, Al3+

rAl3+ < rMg2+ < rNa1+

e. Br1-, Y3+, Sr2+, Rb1+,
As3-, Se2-

rY3+ < rSr2+ <rRb1+ < rBr1- < rSe2- <rAs3-

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