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)