Is it true that the periodic table shows the most common elements in the earths crust and the approximate percentage by weight that each one represents?
yes this is true.
What is the name given to a row of elements in the periodic table whose densities and melting points change gradually and predictably?
The Periodic Table is a chart of all known elements. There is no terrestrial significance of the chart, it represents all known elements in the universe. As the materials that make up the Earth, and everything that is on it, originate as debris from exploding stars billions of years ago we can safely say that all the elements on the Periodic Table are extraterrestrial in origin. [To be picky, there will some nuclear decay products…
The vertical columns. For example the elements in the first column starting from the left of the periodic table are called "alkali metals" (with the exception of hydrogen) whereas the elements in the last column to the right are called "noble gases". The elements that belong to a group share the same chemical properties because they have the same structure of valence electrons in their atoms.