Why mass numbers cannot be used to arrange
elements in the periodic table.

Because the number of protons, which is the same as the atomic number, is what distinguishes various elements, not the atomic mass.

An element's atomic mass is made up of two parts: protons and neutrons. While atoms of the same element, such as C-14 and C-12, might have various numbers of neutrons, the number of protons in a carbon atom is always 6.

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: