An isotope, cesium-137, has a half-life of 30 years. Starting with 40 grams, how much is left in 60 years?

You can use a very simple formula for first order decay...

Fraction Remaining (FR) = 0.5n where n is the number of half lives that have elapsed.

In the current problem we want to find the FR, and we know n = 2 half lives elapsed (60 yr/30yr = 2)

FR = 0.52 = 0.25

Since we started with 5.0 g, and we have 0.25 (1/4) left, that would be 5.0 g x 0.25 = 1.25 g

each chemical family is represented as a group or column on the periodic table. based on chemical properties, there are 6 types of chemical families. group 1 represents alkali metals, group 2 represents alkaline earth metals, groups 3 to 12 represent transition metals, groups 13 to 16 represent non-metals and other metals, group 17 represents halogens and group 18 represents noble gases.