Section 3: Periodic Trends

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.

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.

Atomic Radius

• Definition: Half of the distance between nuclei in covalently bonded diatomic molecule

ElectronicStructureof Atoms

Atomic Radius

-due tomore

shielding(more energy levels)

increases d

ow

n a g

rou

p

-due to greater nuclear charge(more protons)

decreases across a period

Atomic Radius

Example of Atomic Radius Trend

• Arrange the following elements in order of increasing atomic radii: Cs, F, K, Cl

ElectronicStructureof Atoms

Cations from metals:e–’s lostpositive (+) chargesmallerouter shell lost

Anions from nonmetals:e–’s gainednegative (–) charge larger less nuclear charge

Ionic Radius

Ions

• Here is a simple way to remember which is the cation and which the anion:

This is a cat-ion.This is Ann Ion.

He’s a “plussy” cat!

She’s unhappy and negative.

+ +

Cation Formation

11p+

Na atom

1 valence electron

Valence e- lost in ion formation

Nuclear charge on remaining electrons increases.

Remaining e- are pulled in closer to the nucleus. Ionic size decreases.

Result: a smaller sodium cation, Na+

Anion Formation

17p+

Chlorine atom with 7 valence e-

One e- is added to the outer shell.

Nuclear charge is reduced and the e- cloud expands.

A chloride ion is produced. It is larger than the original atom.

Ionic Radius

Notice that the anions are larger than the cations!

Example of Ionic Radius Trend

• Arrange the following ions in order of increasing ionic radii:

Ca2+, K+, Al3+, S2-, Cl-, Te2-

ElectronicStructureof Atoms

1. Atoms get ___ down a group due to ___.

A. bigger, greater nuclear charge (protons)

B. smaller, less nuclear charge (protons)

C. bigger, more shielding (levels)

D. smaller, less shielding (levels)

Quick Quiz!

ElectronicStructureof Atoms

2. Atomic radii ___ across a period due to ___.

A. decrease, greater nuclear charge (protons)

B. decrease, less nuclear charge (protons)

C. increase, more shielding (levels)

D. increase, less shielding (levels)

Quick Quiz.

ElectronicStructureof Atoms

3. Which of the following has the largest radius?

A. Na

B. Mg

C. K

D. Ca

Quick Quiz. WHY?most

shielding(levels)

least nuclear charge

(protons)

ElectronicStructureof Atoms

4. Metals tend to ________ electrons to form _________ cations.

A. gain, positive

B. gain, negative

C. lose, negative

D. lose, positive

Quick Quiz.