Arranging elements by families and periods

The periodic table is composed of horizontal rows and vertical columns. Here’s how they’re named and numbered:

✓ Periods: The seven horizontal rows are called periods. The periods are numbered 1 through 7 on the left-hand side of the table (see below Figure). Within each period, the atomic numbers increase from left to right.

Members of a period don’t have very similar properties. Consider the first two members of period 3: sodium (Na) and magnesium (Mg). In reactions, they both tend to lose electrons (after all, they are metals), but sodium loses one electron, and magnesium loses two. Chlorine (Cl), down near the end of the period, tends to gain an electron (it’s a nonmetal).

✓ Families: The vertical columns are called groups, or families.

The families may be labeled at the top of the columns in one of two ways. The older method uses roman numerals and letters. Many chemists (especially academic ones like me) prefer and still use this method, so that’s what I use in describing the features of the table. The newer method simply uses the numbers 1 through 18.

The members of a family do have similar properties. Consider the IA family, starting with lithium (Li) and going through francium (Fr) (don’t worry about hydrogen, because it’s unique, and it doesn’t really fit anywhere). All these elements tend to lose only one electron in reactions.  And all the members of the VIIA family tend to gain one electron.