1. Section 3: Periodic Trends

2. Nuclear Charge
Nuclear charge is the pull that an electron “feels” from the nucleus.
The closer an electron is to the nucleus, the more pull it feels.
As nuclear charge increases, the electron cloud is pulled in tighter.
Increases across a period.

3. Shielding
As more energy levels are added to atoms, the inner layers of electrons shield the outer electrons from the nucleus.
Outer electrons are less tightly held.
Constant across a period.
Increases down a group.

4. Atomic Radius
Definition: Half of the distance between nuclei in covalently bonded diatomic molecule

5. decreases across a period
Atomic Radius
decreases across a period
-due to greater nuclear charge
(more protons)
-due to
more
shielding
(more energy levels)
increases down a group
Note: covalent or metallic radius of bonded atoms vs. van der Waals radius of nonbonded atoms (Noble Gases)

6. Atomic Radius

7. Example of Atomic Radius Trend
Arrange the following elements in order of increasing atomic radii: Cs, F, K, Cl

8. Ionic Radius Cations from metals: e–’s lost positive (+) charge
smaller
outer shell lost
Anions from nonmetals:
e–’s gained
negative (–) charge
larger
less nuclear charge

9. She’s unhappy and negative.
Ions
Here is a simple way to remember which is the cation and which the anion:
This is Ann Ion.
This is a cat-ion.
She’s unhappy and negative.
He’s a “plussy” cat!

10. Cation Formation Nuclear charge on remaining electrons increases.
Na atom
1 valence electron
Remaining e- are pulled in closer to the nucleus. Ionic size decreases.
11p+
Valence e- lost in ion formation
Result: a smaller sodium cation, Na+

11. Anion Formation
A chloride ion is produced. It is larger than the original atom.
Chlorine atom with 7 valence e-
17p+
One e- is added to the outer shell.
Nuclear charge is reduced and the e- cloud expands.

12. Ionic Radius
Notice that the anions are larger than the cations!

13. Example of Ionic Radius Trend
Arrange the following ions in order of increasing ionic radii:
Ca2+, K+, Al3+, S2-, Cl-, Te2-

14. Quick Quiz!
Atoms get ___ down a group due to ___.
bigger, greater nuclear charge (protons)
smaller, less nuclear charge (protons)
bigger, more shielding (levels)
smaller, less shielding (levels)

15. Quick Quiz.
Atomic radii ___ across a period due to ___.
decrease, greater nuclear charge (protons)
decrease, less nuclear charge (protons)
increase, more shielding (levels)
increase, less shielding (levels)

16. WHY? Quick Quiz. most shielding
(levels)
least nuclear charge
(protons)
Which of the following has the largest radius? Na Mg K Ca

17. Quick Quiz.
Metals tend to ________ electrons to form _________ cations.
gain, positive
gain, negative
lose, negative
lose, positive