After collecting samples of igneous rocks, a student wishes to classify them as either intrusive or extrusive. which characteristic of the samples might be the most useful to use?me

The characteristic that would be most helpful would be grain sizes of the rocks. Extrusive rocks are immediately exposed to the elements, and therefore cool very rapidly, and there's not really adequate time for grain size to get that large. Intrusive rocks, on the other hand, can cool very slowly, and therefore grain size can grow to quite a large size. 

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.