Periodic Trends

Question 1

Coulombic Force

Answer 1

Electrostatic Force

Question 2

Coulomb’s Law (vocabulary)

Answer 2

Force of attraction between two oppositely charged particles is directly proportional to magnitudes of charges and inversely proportional to square of distance between charges

Question 3

Coulomb’s Law (formula)

Answer 3

F attraction σ (q+)(q-) / r2

F attraction σ k(q+)(q-) / r2 where k is a constant

Question 4

Two important generalizations

Answer 4

1. Use effective nuclear charge Zeff arguments to justify trends across a period
2. Use increased distance arguments (higher PEL n for valence electrons) to justify trends down a group

Question 5

Atomic radius definition

Answer 5

Distance between nucleus and outer edge of electron cloud

Question 6

Atomic radius across a period

Answer 6

Decreases, because greater Zeff increases attractive force of nucleus on valence electrons and pulls electron cloud closer to nucleus.

Question 7

Atomic radius down a group

Answer 7

Increases, because the increased PELs lengthens distance over which nucleus must attract electrons and reduces electrostatic attraction for electrons

Question 8

Ionization Energy Definition

Answer 8

Energy needed to remove an electron from an isolated gaseous atom. Always endothermic

Question 9

IE across a period

Answer 9

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.

Question 10

IE trend exceptions across a period

Answer 10

1. 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.
2. 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.

Question 11

IE down a group

Answer 11

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

Question 12

Electron affinity

Answer 12

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

Question 13

Electronegativity

Answer 13

Assigned value that measures attraction of atom X for pair of valence electrons between atom X and atom Y in an XY chemical bond.

Question 14

EN across a period

Answer 14

Increases because greater Zeff increases attractions between nucleus and valence electrons within isolated chemical bond

Question 15

EN down a group

Answer 15

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

Question 16

Ionic radius

Answer 16

Distance from nucleus to outer edge of electron cloud in an ion

Question 17

Proton-to-electron ratio

Answer 17

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.

Question 18

Isoelectronic

Answer 18

Ions with the same number of electrons and electron configuration

Question 19

Cation size

Answer 19

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

Question 20

Anion size

Answer 20

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

Question 21

Reactivity

Answer 21

Depends on whether an atom reacts by oxidation or reduction

Question 22

Metal Reactivity

Answer 22

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)

Question 23

Nonmetal Reactivity

Answer 23

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)

Question 24

Summary Principles

Answer 24

1. Note group and period of species. State PEL and sub level containing valence electrons (invoke SEC/VEC and/or partial orbital diagram)
2. If same PEL, focus on Zeff. If different PEL, focus on comparison of n vs. n distances

Question 25

Effective nuclear charge

Answer 25

Net positive charge from the nucleus that attracts electrons

Question 26

Shielding

Answer 26

Code electrons shield valence electrons from full attractive forces of protons in nucleus

Question 27

Electron-electron repulsion

Answer 27

Due to like charges, electron pairs orient themselves as far as possible away from each other, causing electron cloud to expand

Question 28

Zeff formula

Answer 28

Zeff = Z - α (shielding constant)

Question 29

En formula

Answer 29

-Zeff Rhc / n^2