CHEM 102 Lecture 3: Periodic Trends and Intermolecular Forces

Vocabulary flashcards covering atomic radius, ionic radius, effective nuclear charge, ionization energy, electronegativity, chemical bond types, and intermolecular forces.

Atomic Radius

One half the distance between the nuclei of two adjacent atoms in a substance.

Effective Nuclear Charge ($Z_{\text{eff}}$)

The nuclear charge felt by an electron when both the actual nuclear charge ($Z$) and the repulsive shielding effects of other electrons ($S$) are taken into account, calculated as $Z_{\text{eff}} = Z - S$.

Shielding Constant ($S$)

A parameter in effective nuclear charge calculations that represents the repulsive screening effect of electrons, usually close to the number of inner (core) electrons.

Cation Radius Trend

A cation is always smaller than its parent neutral atom because both have the same number of protons while the cation has fewer electrons, causing the nucleus to attract the remaining electrons more strongly and shrink the electron cloud.

Anion Radius Trend

An anion is always larger than its parent neutral atom because it possesses more electrons than protons, leading to increased electron-electron repulsion that enlarges the electron cloud.

Ionization Energy (IE)

The minimum energy required to remove an electron from a gaseous atom in its ground state. Because energy must be absorbed, ionization energies are always positive quantities.

First Ionization Energy ($I_1$)

The energy change required to remove the outermost electron from a neutral gaseous atom to form a $+1$ ion (e.g., $\text{Al}_{(g)} \rightarrow \text{Al}^+_{(g)} + e^-$, with $I_1 = 580\,\text{kJ/mol}$).

Group 2A to 3A Ionization Energy Anomaly

An exception to the periodic ionization trend where Group 3A elements have a lower first ionization energy than Group 2A elements in the same period (e.g., $\text{B}$ vs. $\text{Be}$) because electrons in the filled $2s$ orbital provide shielding for the single electron in the higher-energy $2p$ orbital.

Group 5A to 6A Ionization Energy Anomaly

An exception to the periodic ionization trend where Group 6A elements have a lower first ionization energy than Group 5A elements in the same period (e.g., $\text{O}$ vs. $\text{N}$) due to extra electron-electron repulsions created by pairing two electrons in a doubly occupied $2p$ orbital.

Electronegativity

The tendency of an atom to attract electrons towards itself within a chemical bond.

Covalent Bond

A chemical bond characterized by the equal sharing of two electrons between atoms that exhibit no difference in electronegativity.

Polar Covalent Bond

A chemical bond formed by the unequal sharing of two electrons due to a moderate difference in electronegativities between the bonded atoms, creating partial charges ($\delta^+$ and $\delta^-$).

Ionic Bond

A chemical bond characterized by the complete transfer of electrons between a metal and a nonmetal, arising from a large difference in their electronegativities.

Intramolecular Forces

The strong chemical bonds (such as covalent or ionic bonds) that hold individual atoms together within a molecule or compound.

Intermolecular Forces

Attraction forces existing between separate molecules that determine many macroscopic physical properties and are substantially weaker than intramolecular chemical bonds.

Dipole-Dipole Forces

Electrostatic intermolecular attraction forces that occur when polar molecules with permanent dipole moments align with their positive and negative ends adjacent to one another.

Hydrogen Bonding

A strong dipole-dipole attraction occurring among polar molecules in which hydrogen is covalently bonded to a small, highly electronegative atom ($\text{N}$, $\text{O}$, or $\text{F}$).

London Dispersion Forces

Relatively weak and short-lived attractive forces present in all molecules (both polar and nonpolar) caused by temporary, instantaneous dipoles that induce dipoles in neighboring molecules.

Ice Density Anomaly

The phenomenon whereby solid water (ice) is less dense than liquid water because each $\text{H}_2\text{O}$ molecule forms hydrogen bonds with four neighboring molecules in an open, highly ordered crystalline framework that contains open cavities.

Ion Propulsion Engine

A spacecraft propulsion engine that creates thrust according to Newton's third law by ionizing gas atoms (specifically Xenon) and accelerating them outward.