Variation Across a Period

The properties within a period change in a similar way from left to right across the table, except for Period 1. Across a period from left to right, the elements become less metallic and more nonmetallic in their properties. The most reactive metals are on the left side of the table. The most reactive nonmetals are on the right in Group 17. The Period 3 elements shown in Figure 13 provide an example of this trend.

There are three metals, a metalloid, and four nonmetals in Period 3. If you were unwise enough to hold a piece of sodium in your hand, it would react quickly and violently with the water on your moist skin. But magnesium will not react with water unless the water is hot. Aluminum does not react with water, but it does react with oxygen.

Silicon is the least reactive element in Period 3 (except for argon). Under ordinary conditions, phosphorus and sulfur do not react with water, but they do react with oxygen. They also react with chlorine, which is a highly reactive nonmetal. Chlorine must be handled with as much care as sodium. Argon hardly reacts at all.