Reading number of outer shell electrons and other properties from periodic table?

Question

How can one read from the periodic table the number of outer shell electrons that an atom has, to predict how these atoms will make bonds with other atoms? For example to see that hydrogen ($\ce{H}$) has 1 electron free, while carbon ($\ce{C}$) has 4 and oxygen ($\ce{O}$) has 2? This allows us to infer that carbon can make four bonds with 4 different hydrogen atoms. It's not clear what in the periodic table would tell us this; it is neither in the group nor in the period of an atom, it seems.

Related to this: can someone summarize concisely all the properties (like number of free outer shell electrons) that one can read off about each element from its position in the periodic table? I was having a hard time finding a concise summary of these.

Answer 1

Start with a periodic table that shows the electron configurations. You might try the Dynamic Periodic Table; as you mouse over an element, its electron configuration is presented.

Then go to Electron Configurations in the Periodic Table, Electronic Structure of Atoms or Block (periodic table) to see how shells and orbitals are added. It's pretty straightforward until the transition metals, and things get much more complicated for the lanthanides and actinides, where a new shell may start before an inner shell is complete.

The general rule is: an orbital is more "stable" when filled, or half-filled, so fluorine, for example, can readily accept one more electron to finish its outer 2s2 2p5 shell and it will then have the configuration of neon, 2s2 2p6. Many periodic tables don't present the inner shells, using instead the shorthand of [Ne] 3s2 3p5 for chlorine, rather than the full 1s2 2s2 2p6 3s2 3p5.

In fact, you're not alone in trying to understand the electron structure; see the current debate on the position of Lw in the table: New data on synthetic element trigger rethink of periodic table.

BTW, note there are different ways a shell can be completed. Carbon will share its four outer electrons with four hydrogen atoms; this is called a covalent bond. Chlorine outright "steals" an electron from sodium; this is an ionic bond. See Covalent Lewis Dot Structures and Ionic Bonding and Electron Transfer for more info.