A period on the periodic table is a horizontal grouping of elements running from the left to right of the table. Several trends are observed going from the left side of the periodic table to the right.
Atomic radii decreases going across periods. As more protons are added to the atom, the force of attraction between the protons in the nucleus and the orbiting electrons increases, so electrons orbit closer to the nucleus, thus decreasing the size of the atom.
First ionization energies increase across a period. The first ionization energy is the amount of energy needed to pluck an electron from an atom's outermost energy level to make a positive ion. As atomic radius decreases from left to right, more energy is needed to remove an electron, because electrons are held more tightly to the protons in the nucleus.
Moving toward the right of the periodic table, electron affinity increases. Electron affinity is how much attraction an atom has for electrons. Elements on the left-hand side of the periodic table don't have much affinity for electrons; in fact, these elements tend to give away their electrons. Elements toward the right of the periodic table love electrons and try to capture them to fill their own outermost energy levels.