Why do scientist use models to study atoms

Which set of characteristics best describes igneous rock? a) largest type of rock, made of organic matter, hardest type of rock b) least abundant type of rock, made of other rocks, made mostly of minerals c) found on all continents, contains wavy bands of stripes, contains fossils d) most abundant type in earth's crust, made of magma/lava, contains no fossils

Calculate the percent composition by mass of each element in al(oh)3. use at least three significant figures.

How many grams are in 68.2 molecules of br2?