Periodic Table

There are general reactivity trends on the periodic table that are useful to know. Metals and nonmetals usually combine to form ionic compounds with the metal giving up an electron to become positively charged and the nonmetal element gaining an electron to become negatively charged. Nonmetals usually combine with one another to form covalent bonds in which the electrons are not fully transferred but are shared between the two elements. Examples of this are molecular oxygen O₂, molecular chlorine Cl₂, ammonia NH₃, and carbon dioxide CO₂.

The degree of metallic character of an element can be estimated by that element's location in the periodic table. Metallic character decreases moving from left to right across a period. This is clearly demonstrated in each of the first six periods where each period starts off with metallic elements, but ends with nonmetallic elements. Metallic character is also found to increase moving down a group. This trend is most clearly demonstrated by Groups 14-16, although it is true for the other groups as well, for example, cesium (Cs) is more metallic than sodium. Each of the Groups 14-17 begins with a nonmetal followed closely by a metalloid and eventually a metallic element.

Another trend found in the periodic table is size. The atomic radii (the scientific term for the size of an atom) of the elements increases going down a group, while it decreases going across a period. See Figure 2.

In the periodic table, the last naturally occurring element is element 92, uranium (U). Uranium is a radioactive element. Radioactive elements are unstable and break down to form lighter elements and in the process give off energy. All of the elements that occur past uranium are manmade, and are referred to collectively as the transuranium elements.