A light-year is a unit of distance. It is the distance that light can travel in one year. Light moves at a velocity of about 300,000 kilometers (km) each second. So in one year, it can travel about 10 trillion km. More p recisely, one light-year is equal to 9,500,000,000,000 kilometers.
Based on its position in the periodic table, you can predict how many electrons it has, how many valence electrons, how many levels of electrons, and its atomic number and mass.
