Atomic Size, Ionization Energy, and Electron Affinity

Periodic Trends

Atomic Radius (Down a Group)

Increases

A larger principal quantum number (n) means adding a new, larger electron shell.

Atomic Radius (Across a Period)

Decreases

The effective nuclear charge (Zeff​) increases, pulling the outermost electrons closer to the nucleus.

Cation Size vs. Neutral Atom

Cation is Smaller

Anion Size vs. Neutral Atom

Anion is Larger

Gain of electrons increases electron-electron repulsion, causing the electron cloud to expand

Size in an Isoelectronic Series

Decreases as Nuclear Charge (Z) Increases

All species have the same number of electrons, so size is determined by the number of protons in the nucleus (higher Z = smaller size)

First Ionization Energy (Down a Group)

Decreases

Valence electrons are farther from the nucleus, reducing the attraction and making them easier to remove.

First Ionization Energy (Across a Period)

Increases (generally)

The effective nuclear charge (Zeff​) increases, holding the valence electrons more tightly

Successive Ionization Energies (I1​,I2​,I3​)

Always Increase

Core Electron Ionization Energy

Much Greater than Valence Electron IE

Core electrons are much closer to the nucleus and shielded less effectively than valence electrons.

Isoelectronic Series (Ionization Energy)

Increases as Nuclear Charge (Z) Increases

The species with the greatest number of protons (nuclear charge) will have the highest IE.

Electron Affinity

Energy change when an atom gains an electron to form an anion.

Favorable Electron Affinity

Negative (Exothermic) energy change

Generally favorable for elements on the right side (nonmetals)

Reactivity of Alkali Metals (Group 1A)

Increases Down the group

Reactivity of Halogens (Group 7A)

Decreases Down the group

Dominated by the formation of X− ions

Reactivity of Alkaline Earth Metals (2A)

Increases down a group.

Ionic radii (down a group)

increases