Periodic Trends for Chem II

Disclaimer Atomic theory is based on the best indirect evidence we have, and not on direct observations. What scientists know has changed and evolved rapidly over the last 200 years. It could change again as new evidence is produced from new experimentation.

Periods Each row in the table of elements is a period. The periods correspond closely to the energy levels of electrons.

As the atomic number increases… -The number of protons increases -The number of electrons increases

Groups Each column on the periodic table is called a group Properties of elements repeat in a predictable way when atomic numbers are used to arrange elements into groups

Trends Mass increases down a group Variation across a period…as you move from left to right across a period there are certain trends Metals become less reactive Nonmetals become more reactive

Trends Another way is to say that reactivity increases as you move further away from “the stairstep”

Trends Noble gases are the least reactive of all elements That is the exception to the trend in reactivity

Coulomb's Law The force between subatomic particles is proportional to their charge and inversely proportional to their distance.

Coulomb's Law The more protons there are, the greater their attraction to the electrons. The more electrons there are, the more they repel each other. The more energy levels there are, the further the valence electrons are from the protons.

Trend: atomic radius Atomic radius is the approximate size of the atom as measured from the nucleus to the outer (valence) electrons. This is a very difficult thing to measure since you can never know where electrons actually are at any given time. We use approximations based on the distance between atoms/ions in compounds.

Trend: atomic radius Atomic radius tends to increase down groups and towards the left of periods.

Trend: Atomic radius The atomic radii for ions are different from atomic radii, because electrons have been lost (metals) or gained (non-metals).

Trend: atomic radius Remember: ions always involve a transfer of electrons, never shared like in covalent bonds.

Trend: atomic radius Atomic radii are usually measured in picometers (10^-12m or 0.000000000001m) Atomic radii generally decrease from left to right in a period and generally increase as you move down a group. Why? Why do we see these trends in atomic radii? What has Coulomb’s Law got to do with it?

TEM imagery of graphene

Trend: ionization energy Ionization is the energy required to remove an electron from a (gaseous) atom. This is just for the first ionization energy.

Trend: ionization energy Ionization energies generally increase from left to right in a period and generally decrease as you move down a group. How can Coulomb’s Law help explain this trend?

Trend: electronegativity Electronegativity indicates the relative ability of its atoms to attract electrons. In a chemical bond, the element with the greater electronegativity more strongly attracts the shared electrons. Which is the most electronegative element?

Trend: electronegativity In the following compounds, which atom(s) will attract the electrons more? H2O CO2 HCl CaCl2

Trend: electronegativity Electronegativity differences lead to polar compounds.

Questions?

I have some questions for you! Describe one of the trends on the periodic table. (yes, I know this one isn’t a question) Which would be more reactive, copper (Cu) or potassium (K)? Why? Which would have a larger radius, Zr or Sb? Explain why using the principles of Coulomb's Law. Name one polar compound. C’mon, just one!?