What is a scientific theory? a. a reasonable guess based on instinct b. an imaginative idea explaining natural phenomena c. a broad, evidence-based explanation for a phenomenon d. a tentative statement with little or no predictive power

Heat from the sun reaches earth to a. radiation b. convection c. conduction

Positive ions differ from neutral atoms in that

The radioactive isotope of lead, pb-209, decays at a rate proportional to the amount present at time t and has a half-life of 3.3 hours. if 1 gram of this isotope is present initially, how long will it take for 80% of the lead to decay? (round your answer to two decimal places.)