Period 3 element density and for its ability to resist corrosion due to the phenomenon of passivation. Structural components made from aluminium and its alloys are vital to the aerospace industry and are important in other areas of transportation and structural materials. The most useful compounds of aluminium, at least on a weight basis, are the oxides and sulfates. "Silicon" (symbol Si) is a tetravalent metalloid. It is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table. Controversy about silicon's character dates to
Heart (symbol) but the "hearts" suit did not have this element since the 15th century. The heart symbol reached Japan with the Nanban trade of 1543 to 1614, as evidenced by an Edo period Samurai helmet (dated c. 1630), which includes both the rounded and indented forms of the heart symbol, representing the heart of Marishiten, goddess of archers. Since the 19th century, the symbol has often been used on Valentine's Day cards, candy boxes, and similar popular culture artifacts as a symbol of romantic love. The use of the heart symbol as a logograph for the English verb "to love" derives