TRENDS IN THE PERIODIC TABLE

Important Definitions  Trend : predictable change in a particular direction  Electron Shielding : inner electrons shield outer electrons from the full attractive force of the nucleus  Effective Nuclear Charge (ENC) : charge felt by the valence electrons after you have taken into account the # of shielding electrons

Atomic Radius  Atomic radius: half of the distance between the nuclei of two atoms of the same element bonded together.

Atomic Radius  increases as you move down a group  electrons occupy consecutively higher energy levels, farther from the nucleus  ENC decreases due to increased shielding H Li Na K Rb

Atomic Radius  decreases as you move across a period  Electrons are in the same energy level  there is more nuclear charge.  Outermost electrons are pulled closer. Na MgAl Si PS ClAr

Atomic Radius Trends

Radius of Cations  Metals form cations.  Cations form by losing electrons.  Cations are smaller than the atom they came from – not only do they lose electrons, they lose an entire energy level.

Radius of Anions  Nonmetals form anions.  Anions form by gaining electrons.  Anions are bigger than the atom they came from – have the same energy level, and nuclear charge (ENC slightly smaller)

Ionic Radius Trends  trends follow the same pattern as atomic radius  increases as you move down a group  due to decreased ENC (increased shielding)  decreases as you move across a period  due to increasing nuclear charge, whether the ion is positive or negative

Ionic Radii Trends

Ionization Energy  Ionization: removing an electron from an atom or ion  1 st Ionization energy: energy required to remove an electron from an atom  Must overcome attraction between electron and nucleus  2 nd Ionization Energy: The energy required to remove a second electron from a cation

I.E. for the 1 st 10 elements SymbolFirstSecond Third H He Li Be B C N O F Ne Why did these values increase so much ?

Ionization Energy  Group 1: easily loses its 1 valence electron  Low first ionization energy  Second ionization energy will be very high since it now has a stable octet.  Group 2: easily lose 2 valence electrons  Low first and second ionization energies  High third ionization energy

Ionization

Ionization Energy Trends  decreases as you move down a group  due to electron shielding (decreasing ENC)  increases as you move across a period  due to the increase in nuclear charge

Ionization Energy Trends

Electron Affinity  Electron Affinity: the energy given off when an electron is added to an atom to make an anion.  elements with high electron affinities form negative ions in ionic compounds.  elements with low electron affinities form positive ions in ionic compounds.

Electron Affinity Trends  same as for ionization energy  decreases as you move down a group due to increased electron shielding (decrease in ENC)  increases as you move across a period due to increasing nuclear charge

Electron Affinity Trends