Periodic Trends Thursday, March 12th, 2015

Atomic Size – no definite size Atomic Size – no definite size. Scientist use the distance between two nuclei determined from chemical bonds as the atomic radius. Cause of trends – Period: As you go across a row the nuclear charge (# of protons) increases but no increase in Shielding (energy level remains the same) Ex) A Na atom is larger than an Ar atom because 18 protons can pull on energy levels 1 through 3 with greater strength than 11 protons can.  Group: As you go down a group the number of energy levels increases and shielding occurs. (the # protons increase but the energy levels also increase). Ex) A Na atom is smaller than an Fr atom because energy level #3 is closer to the nucleus than energy level # 7

What is shielding? Analogy: let electrons = movie goers, and protons = movie screen (more people in front, noise, distractions, visibility = other energy levels, other electrons, exposure to other atoms).

You would expect that Na <Mg <Al Ionization Energy: the amount of energy required to remove an electron from a gaseous atom.   Cause of trends – Period: ...but...sub-energy levels add an additional influence depending on whether they are full or not. Ex) compare Na, Mg and Al. You would expect that Na <Mg <Al but Na <Al <Mg WHY? because Mg’s 3s sub-energy level is full and Al’s 3p sub-energy level is not. A partially filled sub-energy level is less stable (has more energy) than a full sub-energy level and therefore it is easier to remove an electron from a partially filled sub-energy level. Group: Ex) A Na atom has a higher I.E. than a Fr atom, because Fr's electron is further from the nucleus...less proton pulling power...AND already has more Energy. Period Group

Electronegativity (electron affinity): a measure of how well an atom can attract and hold an electron from another atom. This results in the formation of an ion and it is the ionic radius that will dominate how well the atom can retain the electron.   Cause of trends – proton strength increases across row but size of atom decreases

Ionic Size Gain electron → size increase Lose electron → size decrease Anion – negative ion is larger than its neutral atom Lose electron → size decrease Cation – positive ion, are smaller than their neutral atom