Periodic Trends Flashcards
Coulombic Force
Electrostatic Force
Coulomb’s Law (vocabulary)
Force of attraction between two oppositely charged particles is directly proportional to magnitudes of charges and inversely proportional to square of distance between charges
Coulomb’s Law (formula)
F attraction σ (q+)(q-) / r2
F attraction σ k(q+)(q-) / r2 where k is a constant
Two important generalizations
- Use effective nuclear charge Zeff arguments to justify trends across a period
- Use increased distance arguments (higher PEL n for valence electrons) to justify trends down a group
Atomic radius definition
Distance between nucleus and outer edge of electron cloud
Atomic radius across a period
Decreases, because greater Zeff increases attractive force of nucleus on valence electrons and pulls electron cloud closer to nucleus.
Atomic radius down a group
Increases, because the increased PELs lengthens distance over which nucleus must attract electrons and reduces electrostatic attraction for electrons
Ionization Energy Definition
Energy needed to remove an electron from an isolated gaseous atom. Always endothermic
IE across a period
Increases, because greater Zeff increases attractive force of nucleus on valence electrons. Tighter grip on valence electrons results in higher energy input needed for electron removal.
IE trend exceptions across a period
- Drop in IE between groups 2 and 13 because np electrons have weaker penetrating abilities towards nucleus of atom than ns electrons. Therefore np1 electron of group 13 is less tightly gripped by nucleus.
- Drop in IE between groups 15 and 16 because increased e- repulsions caused by pairing electrons in p orbitals of Group 16 atoms outweighs increase in Zeff across a period. Therefore less energy is required to remove np4 electron from group 16.
IE down a group
Decreases because increased number of PELs lengthens distance over which nucleus must attract electrons and therefore reduces electrostatic attraction for electrons.
Full PELs provide some extent of shielding between attractive pull of nucleus and valence electrons
Electron affinity
Attracting an electron to an isolated gaseous atom.
Exothermic means resultant anion is more stable (lower in potential energy). High magnitude exothermic EA means Zeff is strong and attracts electron effectively.
Endothermic means Zeff from nucleus is less prone to attracting new electron
Electronegativity
Assigned value that measures attraction of atom X for pair of valence electrons between atom X and atom Y in an XY chemical bond.
EN across a period
Increases because greater Zeff increases attractions between nucleus and valence electrons within isolated chemical bond
EN down a group
Decreases because increased PELs lengthens distance over which nucleus must attract electrons and therefore reduces electrostatic attraction for electrons.
Full PELs help shield attractive pull of nucleus
Ionic radius
Distance from nucleus to outer edge of electron cloud in an ion
Proton-to-electron ratio
Used for comparing ionic radii for isoelectronic ions.
Ionic radius decreases as p/e increases because of higher Zeff for smaller number of electrons.
Ionic radius increases as p/e decreases because of lower Zeff for greater number of electrons.
Isoelectronic
Ions with the same number of electrons and electron configuration
Cation size
Smaller than respective parent neutral atom because:
- loss of valence electrons can cause outermost electrons to be in lower PEL and much closer to nucleus
- P/E ratio increase, species radius decreases, and nucleus exerts stronger Zeff on remaining electrons
Anion size
Larger than respective parent neutral atom because:
- gain of valence electrons can increase number of PELs, lengthening distance that Zeff must act and increases e- repulsions which expands volume of electron cloud
- P/E ratio decreases, species radius increases, nucleus exerts weaker Zeff on more electrons
Reactivity
Depends on whether an atom reacts by oxidation or reduction
Metal Reactivity
More reactive as atomic number increases from top to bottom of a group
- metals react by oxidation so loosely gripped electrons result in greater reactivity. (valence electrons further from nucleus and higher PELs)
Nonmetal Reactivity
Nonmetals are more reactive as atomic number decreases from bottom to top of a group.
- nonmetals react by reduction so stronger Zeff that readily attracts electrons cause greater reactivity (valence electrons closer to nucleus and lower PELs)
Summary Principles
- Note group and period of species. State PEL and sub level containing valence electrons (invoke SEC/VEC and/or partial orbital diagram)
- If same PEL, focus on Zeff. If different PEL, focus on comparison of n vs. n distances
Effective nuclear charge
Net positive charge from the nucleus that attracts electrons
Shielding
Code electrons shield valence electrons from full attractive forces of protons in nucleus
Electron-electron repulsion
Due to like charges, electron pairs orient themselves as far as possible away from each other, causing electron cloud to expand
Zeff formula
Zeff = Z - α (shielding constant)
En formula
-Zeff Rhc / n^2
