Question

how can you predict which elements are reactive under normal conditionsand which are unreactive

Answer 1

i think everything in the 8 group is unreactive thats why their called noble gases
*i'm not too sure because i dont have a table in front of me

Answer 2

Yup the noble gases in group 8 are non-reactive because they have a full valence shell.

The alkali metals (group 1) tend to be quite reactive. In fact, they react with water. As you go down group 1, they become more and more reactive.

Apart from those 2 you can't really predict the reactivity of elements

Answer 3

Very few elements, other than the noble gases, the noble metals (mostly Au and Pt, Ag and Cu to some extent), and nitrogen are unreactive under normal conditions. You know that because only those elements are easily found in their pure form on the Earth's surface. Almost all other elements are found combined into compounds of various sorts.

Most elements are metals. Metals tend to lose electrons to form cations in ionic compounds, or at least lose control of electrons to form complexes and compounds in which they are surrounded by more electronegative elements (those at the far upper right of the table). Generally the most common reactive electronegative nonmetals on the Earth's surface are fluorine, chlorine, bromine, oxygen, sulfur and phosphorus. Not surprisingly, the most common substances on the Earth's crust are compounds made of these elements and the metals: oxides of metals, like FeO and Fe2O3, TiO2, ZnO, sulfides of less reactive metals, like CuS or PbS, carbonates of more reactive metals, like NaCO3, MgCO3. Many fluorides, some bromides. Many phosphates (compounds with PO3 in them)_ Silicon is a very common element, and easily forms extended networks with oxygen, so quite a lot of the Earth's crust is silicate rocks, made of SiO2.

Here I've interpreted "reactivity" as "reactivity in the natural setting," outside in the world. "Reactivity" in the laboratory setting tends to depend greatly on what you've got available -- what can react with what. You can make specific predictions if you have specific examples, but very general rules are not easy to come by.