The nucleus makes up blank of the mass of an atom

In a ball-and-stick molecular model, what do the sticks represent?

What are fossils of organisms that existed over a wide area but only for a limited time period called?

Select the statement that describe chemical properties a. antacid tablets neutralize stomach acid b. helium is the lightest monatomic element c. water freezes at 0 celsius d. mercury is liquid at room temperature