Periodic Trends

Overview

The periodic table is organized to show patterns in element properties. Understanding these trends helps predict chemical behavior and reactivity.

Major Periodic Trends

Property	Across Period (→)	Down Group (↓)
Atomic Radius	Decreases	Increases
Ionization Energy	Increases	Decreases
Electronegativity	Increases	Decreases
Electron Affinity	Increases (more negative)	Decreases
Metallic Character	Decreases	Increases

Atomic Radius

The distance from the nucleus to the outermost electron.

Trends

• Across a period: Decreases (more protons pull electrons closer)
• Down a group: Increases (more electron shells added)

Order

Li > Be > B > C > N > O > F  (across period 2)
Li < Na < K < Rb < Cs        (down Group 1)

Ionic Radius

• Cations: Smaller than parent atom (fewer electrons, same protons)
• Anions: Larger than parent atom (more electrons, same protons)

$$\text{Na}^+ < \text{Na} < \text{Na}^-$$ $$\text{Mg}^{2+} < \text{Mg}$$ $$\text{Cl}^- > \text{Cl}$$

Ionization Energy (IE)

The energy required to remove an electron from a gaseous atom.

$$\text{X}(g) \rightarrow \text{X}^+(g) + e^- \quad \Delta E = IE_1$$

Trends

• Across a period: Increases (electrons held more tightly)
• Down a group: Decreases (electrons farther from nucleus)

Successive Ionization Energies

$$IE_1 < IE_2 < IE_3 < \cdots$$

Each successive electron is harder to remove.

Exceptions

• IE dips at Group 13 (loss of p electron easier than paired s)
• IE dips at Group 16 (loss of paired p electron easier)

Electronegativity (EN)

The ability of an atom to attract electrons in a chemical bond.

Pauling Scale

• F = 4.0 (most electronegative)
• Cs = 0.7 (least electronegative)

Trends

• Across a period: Increases
• Down a group: Decreases

Electronegativity Difference and Bond Type

$\Delta EN$	Bond Type
0 - 0.4	Nonpolar covalent
0.4 - 1.7	Polar covalent
> 1.7	Ionic

Electron Affinity (EA)

The energy change when an electron is added to a gaseous atom.

$$\text{X}(g) + e^- \rightarrow \text{X}^-(g) \quad \Delta E = EA$$

• More negative EA = more energy released = more favorable
• Halogens have the most negative EA (want one more electron)
• Noble gases and Group 2 have positive EA (stable configurations)

Trends

• Across a period: Generally becomes more negative
• Down a group: Generally becomes less negative

Metallic Character

The tendency of an element to lose electrons and form cations.

Trends

• Across a period: Decreases (nonmetals on right)
• Down a group: Increases (easier to lose electrons)

Summary Diagram

                    Ionization Energy →
                    Electronegativity →
                    Electron Affinity →
    ←————————————————————————————————————
    |
    |  Atomic Radius
    |  Metallic Character
    ↓

Special Cases

Noble Gases

• Very high IE (stable electron configuration)
• No electronegativity (don't form bonds normally)
• Positive EA (don't want more electrons)

Transition Metals

• Trends are less regular
• Similar atomic radii across period
• Multiple oxidation states common

Effective Nuclear Charge ($Z_{eff}$)

The net positive charge experienced by valence electrons.

$$Z_{eff} = Z - S$$

Where:

• $Z$ = atomic number (total protons)
• $S$ = shielding constant (inner electrons)

$Z_{eff}$ increases across a period, explaining many trends.