What element has 1 energy level and 2 valence electrons

When a scientific theory has been tested and proved by the scientific community, it becomes a law

What sources of error may have contributed to the percent yield not being 100 percent? think about things that may have led to inaccurate measurements or where mass of the product could have been lost if this experiment was conducted in a physical laboratory.

The earth's surface is (science) a: studied using seismic waves b: constantly changing over time c: only studied indirectly d: the same today as million of years