1-Talc and graphite are two of the lowest minerals on the hardness scale. They are also described by terms like greasy or soapy. Both have a crystal structure characterized by sheet-structures at the atomic level, yet they don't behave like micas. What accounts for their unusual physical properties

They have weak chemical bonds between them therefore their sheets can easily slip.

Explanation:

Talc is structurally similar to the micas, having weakly bonded sheets.  These sheets are held together only by van der Waals forces therefore, they can slip past each other easily. Hence, talc is extremely soft,  greasy, soapy, and can function as a high-temperature lubricant.

Graphite, consists of carbon atoms arranged in a hexagonal network to form sheets that are about one atom thick. The sheets are held together by weak Van Der Waals forces hence they can slide over one another when subjected to a small  force. So graphite is not hard and is slippery.

Therefore, the  unusual properties of these substances stems from the fact that the bonds between the sheets are weak and a very low stress can cause a slip between the sheets.

part 1: the half-life of the element is 22 years.

part 2: it will take 132.02 year for the sample to decay from 308.0 g to 4.8125 g.

explanation:

part 1: there are two ways to determine the half life time.

first way: the half life time can be determined directly from its definition as it is the time required for the sample to decay to its half concentration.

from the data the initial concentration was 45.0 g and decayed to its half value (22.5 g) in 22.0 years.

second way: the radio active decay is considered first order reaction so we can use the laws of first order reactions.

k = (1/t) ln (a/a-x), where k is the rate constant of the reaction,

t is the time of the reaction

a is the initial concentration of the reactant

a-x is the remaining concentration at time (t)

a = 45.0 g the initial concentration at t = 0 year.

at t = 22 year, a-x = 22.5 g

then k = (1/22 year) ln (45.0 g/ 22.5 g) = 0.0315 year-1.

the half-life time (t1/2) = ln 2/ k = 0.693/ k =0.693/ 0.0315 = 22 years.

part 2:

using the same law k = (1/t) ln (a/a-x),

using the given data; a = 308.0 g and a-x = 4.8125 g and the calculated k from the part 1 (k = 0.0315 year-1)

t = (1/k) ln (a/a-x) = (1/0.0315) ln (308.0/ 4.8125) = 132.02 years