The periodic table is arrange in order of increasing atomic number. The horizontal rows are called periods and the vertical columns are called groups. As you move from left to right across a group each element has one more proton and one more electron than the one before it. The number of valence (outer) electrons that an atom has determines its chemical properties, so there are several left-to-right trends that result from the succesive change in the number of electrons. These patterns start over and repeat aftert the noble gases (group 8A), which is why a new row starts after each noble gas.
1. The ionization energy increases from left to right within a period. This is the energy required to remove an electron. The metals, which are on the left side of the periodic table, lose electrons easily and form positive ions. The non-metals, on the right, tend to gain rather than lose electrons because of their high ionization energies.
2. The electronegativity, which is the attraction for shared electrons in a covalent bond, increases from left to right within a period.
3. The atomic radius decreases from left to right within a period. As each additional proton is added the outer electrons are more attracted and move in closer.
The repeating or periodic nature of these properties results in element in the same group having similar chemical properties.
If you compare the periodic table to a calendar, the days of the week would be considered a periodic property. They repeat every 7 days, and a month is arranged into weeks 7 days in length such that all days in the first column are Sundays, all days in the second column are Mondays, and so forth.