Periodic Law & Trends: Atomic/Ionic Size, Ionization Energy, Electron Affinity, and Electronegativity

Understanding Atomic and Ionic Size

Atomic size refers to the distance from the nucleus of an atom to the outermost shell of electrons. Ionic size is similar but refers to the size of an atom's ion.

• Atomic Size: Generally increases down a group and decreases across a period from left to right.
• Ionic Size: Cations (positive ions) are smaller than their parent atoms, while anions (negative ions) are larger.

Ionization Energy (IE)

Ionization energy is the energy required to remove an electron from an atom in the gaseous state.

• Trend: Increases across a period from left to right and decreases down a group.
• Reason: Across a period, effective nuclear charge increases, making it harder to remove an electron. Down a group, increased distance from the nucleus and electron shielding make it easier.

Electron Affinity (EA)

Electron affinity is the energy change when an electron is added to a neutral atom in the gaseous state.

• Trend: Generally becomes more negative across a period, indicating a stronger attraction for added electrons.
• Reason: Increased nuclear charge across a period results in a stronger pull on added electrons.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto electrons in a chemical bond.

• Trend: Increases across a period and decreases down a group.
• Reason: Atoms with higher nuclear charge and smaller atomic radius have a stronger pull on bonding electrons.

Example: Comparing Elements

Consider the elements sodium (Na) and chlorine (Cl):

• Atomic Size: Na is larger than Cl.
• Ionization Energy: Cl has a higher ionization energy than Na.
• Electron Affinity: Cl has a more negative electron affinity than Na.
• Electronegativity: Cl is more electronegative than Na.