Explanation:
9 The center of the atom is called the nucleus. Electrons are found in areas called shells. A shell is sometimes called an energy leve
12. 6
13. The atomic mass of an element is the average mass of the atoms of an element measured in atomic mass unit (amu, also known as daltons, D).
14.Elements are arranged from left to right and top to bottom in order of increasing atomic number.
15.The vertical columns on the periodic table are called groups or families because of their similar chemical behavior. A
10.Of these 118 elements, 94 occur naturally on Earth. Six of these occur in extreme trace quantities: technetium, atomic number 43; promethium, number 61; astatine, number 85; francium, number 87; neptunium, number 93; and plutonium, number 94.