Periodicity in the Main-Group Elements

The chemical and physical properties of the main-group elements clearly display periodic character. For instance, as pointed out in Section 2.5, the metallic elements lie to the left of the staircase line in the periodic table (see inside front cover), non-metals lie to the right, and the metalloids (with intermediate characteristics) lie along this line. So, as you move left to right in any row of the pmiodic table, the metallic character of the elements decreases. As you progress down a column, however, the elements tend to increase in metallic character. 

These variations of metallic—nonmetallic character can be athibuted in part to variations in the ionization energies of the corresponding atoms. Elements with low ionization energy tend to be metallic, whereas those with high ionization euCTgy tend to be nonmetallic. As you saw in Ihe previous section, ionization energy is a periodic property, so it is not surprising Ihat the metallic-nonmetallic characte of an element is similarly periodic. 

The basic—acidic behavior of the oxides of the elements is a good indicator of the metaUic—nonmetallic character of the elements. Oxides are classified as basic or acidic depending on their reactions with acids and bases. A basic oxide is an oxide that reacts with acids. Most metal oxides are basic. An acidic oxide is an oxide that reacts with bases. Most nomnetal oxides are acidic oxides. An amphoteric oxide is an oxide that has both basic and acidic properties. 

In the following brief descriptions of the main-group elements, we will note the metallic—nonmetallic behavior of the elements, as well as the basic—acidic character of the oxides. Although elements in a given group are expected to be similar, the degree of 

I At very high pressures, however, hydrogen is beiieved I to have metaiiic properties. 

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