The periodic table of chemical elements, often called the periodic table, organizes all discovered chemical elements in rows (called periods) and columns (called groups) according to increasing atomic ...

For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2. Since electrons have almost no mass, it seemed that something besides the protons in the nucleus ...

Each energy level holds a certain number of electrons before electrons go into the next level. 1st Period: Hydrogen and helium—Electrons go into the first energy level. After the first level has two ...