As you move from left to right across the periodic table elements become?

More stable.

The periodic table is arranged such that elements with the same number of valence electrons share a column. Valence electrons are those in the outermost shell. The elements at the far left of the table (hydrogen, lithium, sodium, etc) have only one Valence electron, which makes them more prone to reaction than the "Noble Gases" (Helium, Neon, Argon, etc) which have a full Valence shell. For more information, you can visit "http://en.wikipedia.org/wiki/Valence_electron".