Trends in the Periodic Table Honors chemistry
Periodic Trends
• Trend = predictable change in a particular direction
• Example: reactivity of alkali metals with water as you move down Group 1
• Greater reactivity moving down the group
Na in water
K in water
Cs in water
Periodic Trends
• Ionization energy
• Electronegativity
• Atomic radius
• Ionic size
• Electron affinity
• Melting/boiling point
Ionization
Ionization of Na
Neutral Na atom loses 1 VE
Left with full shell of 8 outer electrons
Becomes positive Na ion (Na+)
• Atoms normally neutral
• Electrons can be removed from atoms!
• Excess protons positive charge
Electron Shielding
= negative charges of the inner electrons reduces attractive force between a positively-charged nucleus and outermost electrons
+
Ionization Energy
• Ionization energy = energy required to remove an electron from an atom or ion
• Decreases going down a group:
• Valence electrons farther from nucleus
• Coulomb’s Law
• Electron shielding by inner electrons
• Increases from left to right in a period:
• Greater nuclear charge (more “pull” from nucleus)
• Electron shielding stays the same
Practice
• Which will probably have the greater ionization energy? Why?
• Na or K?
• Na (ionization energy decreases down a group)
• Na or magnesium (Mg)?
• Mg (ionization energy increases left to right across a period)
• Boron (B) or fluorine (F)?
• F (ionization energy increases left to right across a period)
• O or radium (Ra)?
• O (ionization energy decreases down a group, increases left to right across a period)
Atomic Radius
• Exact size hard to determine
• Electrons in cloud without defined boundaries
• Atom can lose/gain electrons
• Calculate based on bond radius
• Bond radius = half the distance from center to center of two like atoms that are bonded together
Atomic Radius Trend
• Increases down a group:
• n increases
• More energy levels
• Orbitals farther away
• Electron shielding
• Decreases from left to right a period:
• Increasing nuclear charge
• Greatest change across groups 1-14
• Less change across groups 14-18
• Electrons already very close to nucleus
Practice
• Which element will have the larger atomic radius?
• Lithium (Li) or N?
• Li (atomic radius decreases left to right across a period)
• K or francium (Fr)?
• Fr (atomic radius increases down a group)
• Selenium (Se) or Ra?
• Ra (atomic radius increases down a group, left to right across a period)
Done for the day!
Electronegativity = measure of atom’s ability to attract electrons
in a chemical compound
• Developed by American chemist Linus Pauling
• Fluorine has greatest electronegativity
• Arbitrary values based on F (4.0)
• High electronegativity = strongly attracts electrons to atom
• Elements that don’t form compounds have electronegativity = zero
Electronegativity Trend
• Decreases down a group:
• Electron shielding
• Increases from left to right across a period:
• Nuclear charge increases
• Bigger change than moving down a group
Practice
• Which element has the greatest electronegativity?
• F
• Rubidium (Rb) or iodine (I)?
• I (electronegativity increases left to right across a period)
• Calcium (Ca) or Se?
• Se (electronegativity increases left to right )
• Strontium (Sr) or Bromine (Br)?
• Br (electronegativity increases left to right and up)
• K or argon (Ar)?
• K (Ar is a noble gas, doesn’t form compounds, electronegativity = 0)
Ionic Size
• Atoms can lose/gain electrons ions
• Metals lose electrons form positive ions
• Nonmetals gain electrons form negative ions
• Ionic radius increases down a group, and decreases across a period (similar to atomic radius)
Practice
• Which ion is larger?
• Li+ or N3-?
• Li+ (ionic radius decreases left to right across a period)
• K+ or Fr+?
• Fr+ (ionic radius increases down a group)
• Se2- or Ra2+?
• Ra2+ (ionic radius increases down a group, left to right across a period)
Electron Affinity
• Neutral atoms can gain electrons to reach 8 electrons in outer shell
• Electron gain is accompanied by release of energy called electron affinity
• Increases up a group
• Increases left to right across a period
Electronegativity = attraction of electrons
Electron affinity = energy released when electron is gained
Electron Affinity Trend
Practice
• Which element has the greatest electron affinity?
• Rubidium (Rb) or iodine (I)?
• I (electron affinity increases left to right across a period)
• Calcium (Ca) or F?
• F (electron affinity increases left to right and up)
• Strontium (Sr) or Bromine (Br)?
• Br (electron affinity increases left to right and up)
Melting/Boiling Point
• Increase and decrease across a period (sometimes twice!)
• Higher when valence shell ~half-filled
• Higher when d-orbital ~half-filled
• Lower when p- or d-orbitals completely filled
Summary Io
niz
atio
n e
ner
gy
Ionization energy
Atomic / Ionic Radius
Ato
mic
/ Io
nic
Rad
ius
Electronegativity
Elec
tro
neg
ativ
ity
Electron Affinity
Elec
tro
n A
ffin
ity
Increasing
Review Questions
• What periodic trends exist for ionization energy?
• Decreases down a group (electron shielding); increases across a period (greater nuclear charge)
• Why don’t chemists define atomic radius as the radius of the electron cloud?
• Cloud has no definite boundary; use half bond length instead
• How does the trend of atomic radius relate to position on periodic table?
• Down group Increase: addition of principal energy level
• Across period Decrease: more protons, same electron shielding
• What is the trend for electronegativity? Which element is most electronegative?
• Increases as you go to right, up. Fluorine (F)