Lecture Notes: Atomic Structure and Periodic Trends

Key Concepts

Shielding Effect

• Electrons further from the nucleus experience shielding from inner electrons.
• Complete shells act as shields (e.g., noble gases).
• Shielding increases down a group, remains constant across a period.

Electron Configuration

• Write abbreviated configurations for elements (e.g., Hydrogen: 1s², Chlorine: [Ne] 3s²3p⁵).
• Identify valence electrons and their shielding effects.
• Same core electrons mean the same shielding effect.

Effective Nuclear Charge

• Charge felt by an electron due to the nucleus minus shielding effects from other electrons.
• Core electrons shield valence electrons from full nuclear charge.
• Effective nuclear charge increases across a period; increases down a group.
• Large jumps in effective nuclear charge between noble gases and the next period.

Atomic Radius

• Defined as half the distance between nuclei of identical neighboring atoms.
• Increases down a group due to added shells.
• Decreases across a period due to increased nuclear charge pulling electrons closer.

Ionic Radius

• Cations are smaller than their parent atoms (loss of electrons, less repulsion).
• Anions are larger than their parent atoms (gain of electrons, more repulsion).
• Isoelectronic ions have the same electron configuration but different sizes based on nuclear charge.

Ionization Energy

• Energy required to remove an electron from an atom or ion.
• Increases across a period due to greater nuclear charge.
• Decreases down a group as electrons are further from the nucleus.
• Successive ionization energies increase, especially when removing an electron from a stable configuration.

Electron Affinity

• Energy change when an electron is added to a neutral atom.
• Generally becomes more negative across a period (easier to add electrons) and more positive down a group (harder to add electrons).
• Noble gases have low electron affinity due to stable configurations.

Metallic and Non-Metallic Behavior

• Metals tend to lose electrons easily and act as reducing agents.
• Nonmetals gain electrons to complete their valence shells.
• Oxides of metals are basic; oxides of nonmetals are acidic.

Rare Earth Metals

• Include elements like Scandium, Yttrium, and Lanthanides.
• Despite the name, they are not particularly rare.

Practical Implications

• Understanding these trends helps predict chemical behavior and reactivity.
• Useful in explaining reactions, bonding, and material properties.