Why do you think the mass of the carbon atom shown is 12 atomic mass units (amu)?

The mass of Carbon-12 is exactly 12 amu. But, that is not the number that is reported in your periodic table. Carbon as it exists in nature is not all carbon-12. There is some small amount of carbon-13 and even traces of carbon-14 in there. The periodic table reports the average mass of the carbon as it is found in nature, not that of pure carbon-12. The average mass of carbon is slightly heaver than carbon-12, because of the slight amounts of those heaver isotopes of carbon in nature.

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Explanation:

a rain shadow is a dry region of land on the side that is protected against the prevailing winds of a mountain range. the prevailing winds are the winds that occur most of the time in a particular place of the earth

prevailing winds carry air toward the mountain range. as the air rises up over a mountain range, the air cools, water vapor condenses, and clouds form. on this side of the mountains, called the windward side, precipitation falls in the form of  rain  or snow. this dry air produces a  rain shadow.