1. Trends & the Periodic Table
Atomic size & Effective Nuclear Charge

2. What the periodic table really looks like!
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Source: Conceptual Chemistry by John Suchocki
What the periodic table really looks like!

3. Space-saver. previous | index | next
                                                                                                                                                                                                           
Source: Conceptual Chemistry by John Suchocki
Space-saver.

4. How is the periodic table arranged?
By Atomic Number!

5. Trends
More than 20 properties change in a predictable way based on location of element in P.T.
Include: density, melting point, atomic radius, ionization energy, electronegativity

6. Atomic Radius
Atomic radius: defined as half the distance between neighboring nuclei in a molecule or crystal.
“size” varies a bit from substance to substance.

7. Use X-ray diffraction to pinpoint the nuclei and measure the distance between them.
Cannot measure the electron cloud.

8. Trends:
Atoms get larger as you go down a column – more principal energy levels
Atoms get smaller as you move across a series – more “proton pulling power.”

9. Going down column 1:
Period
Element
Configuration 1 H 2 Li
2-1 3 Na
2-8-1 4 K 5 Rb 6 Cs 7 Fr
More principal energy levels as you go down, so it makes sense that the atoms get larger.

10. Li: Group 1 Period 2. Cs: Group 1 Period 6.
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Source: Conceptual Chemistry by John Suchocki
Li: Group 1 Period 2. Cs: Group 1 Period 6.

11. But why do the atoms get smaller as you go across the periodic table?
You are still adding electrons!

12. Biggest Going across row 2: Family Element Configuration IA or 1 Li
2-1
IIA or 2 Be
2-2
IIIA or 13 B
2-3
IVA or 14 C
2-4
VA or 15 N
2-5
VIA or 16 O
2-6
VIIA or 17 F
2-7
VIIIA or 18 Ne
2-8
Biggest
Atoms actually get a bit smaller as you go across a row. What’s going on?

13. What do you remember about charge?
Opposites attract and like charges repel.
The valence electrons are pulled into the atom by the positive charge on the nucleus.
The greater the positive charge, the more pulling power, right?

14. Well, almost . . .
Except for H and He, the valence electrons don’t feel the full effect of all the protons in the nucleus.
For most atoms, the inner shell electrons “shield” the valence electrons from the nucleus.

15. Effective nuclear charge
Charge actually felt by valence electrons.
= Atomic Number - # of inner shell electrons.
Not usually the same as the nuclear charge or number of protons in the nucleus.
Charge felt by valence electrons is attenuated or shielded by inner shell electrons.

16. Effective nuclear charge
= Atomic Number - # of inner shell electrons.

17. previous | index | next
                                                                                                                                                   
H and He are the only elements whose valence electrons feel the full nuclear charge.
Source: Conceptual Chemistry by John Suchocki

18. Effective nuclear charge = +1.
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Effective nuclear charge = +1.
Source: Conceptual Chemistry by John Suchocki
The valence electron in Li feels an effective nuclear charge of 3 – 2 = +1.

19. Calculating “effective nuclear charge.”
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Source: Conceptual Chemistry by John Suchocki
Calculating “effective nuclear charge.”

20. Row 2 elements: Effective nuclear charge
Configuration
Nuc. Chg
Eff. Nuc. Ch Li
2-1 3
3 – 2 = 1 Be
2-2 4
4 – 2 = 2 B
2-3 5
5 – 2 = 3 C
2-4 6
6 – 2 = 4 N
2-5 7
7 – 2 = 5 O
2-6 8
8 – 2 = 6 F
2-7 9
9 – 2 = 7 Ne
2-8 10
10 – 2 = 8

21. previous | index | next
                                                                                                                                                                                                                  
So as you go across a row the size tends to decrease a bit because of greater “proton pulling power.”
Source: Conceptual Chemistry by John Suchocki

22. Size  as you go  and size  as you go 
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Size  as you go  and size  as you go 
Source: Conceptual Chemistry by John Suchocki