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 ...

The method chosen depends on the type of mixture. Atomic structure - AQA Atoms consist of a nucleus containing protons and neutrons, surrounded by electrons in shells. The number of subatomic ...