Chem 109 - General Chemistry - Spring 2013
Lecture Notes 23: 25 March
Atomic Structure & Chemical Periodicity
The Periodic Table
Periodic Table of the Elements
Recall the introduction to Chemical Periodicity in Lecture 6, including hydrogen combining ratios (LIH, BeH2, BH3, CH4, H3N, H2O, HF) and acid/base properties of oxides (basic for metals, acidic for non-metals)
Let's look at some of the elements and see what their properties are like:
- Group IA, on the left side of the chart, is known as the alkali metals because they react with water to produce strong bases (a base is alkaline). Note that all of them are soft (cut with a butter knife), low density (Li floats on oil, Na and K float on water), very reactive metals. All of them react with water with Li<Na<K<Rb<Cs. In each case the metal gives its electron to water leaving hydroxide ion (OH- a base) and hydrogen gas. For example with sodium:
2 Na + 2 H2O 2 Na+ + 2 OH- + H2
- Group VIIA, on the right side of the chart, is known as the halogens. The halogens form acids with water, are gases at the top of the Periodic Chart and high vapor pressure liquids, then solid going down; exist as diatomic molecules (X2), and are very reactive towards metals. For example sodium reacts violently with chlorine gas to give table salt (NaCl):
2 Na + Cl2 2 NaCl
- Group VIII is known as the Noble Gases, or sometimes the Inert Gases because until the 1960's they had no known compounds. Very unreactive. The only known compounds of the Noble gases are with very reactive elements like F and O, and even they don't form compounds with smaller Noble gases such as He and Ne.
DEMONSTRATION: Look at Alkali metals, properties and reaction with water (Li, Na, K); Look at the elements of Period 3 (Na - Cl) Note how their properties change from metallic to non-metallic; Look at Halogens (Cl, Br, I [warm to vaporize crystals]).
Trends in Chemical Periodicity
(plots ©1994 Hanson, Harper, Paselk, & Russell)
Trends:Note the trends for
atomic size: decreases going from left right and from bottom top.
- Size goes up with atomic number for any individual group.
- Size decreases irregularly as atomic number increases for any given period (more charge pulls electrons in to nucleus, but shielding reverses as subshells [s or p orbital sets] fill.
© R A Paselk
Last modified 25 March 2013