Anions are formed when * O atoms lose one electron
atoms gain one electron
atoms lose their valence shell electrons
atoms add electrons to fill their valence shell

in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

explanation:

that tool would be a compass