1. Most of an atom is empty space, except for a tiny, dense core called the ___.
2. The nucleus is made up of _____ and _____ and it is surrounded by negatively charged electrons.
3. The scientist Dmitri Mendeleev classified ____ in a chart called the periodic table.
4. The columns on the periodic table are called groups or families, and the rows are called ____.
5. Elements in the same _____ on the periodic table have similar chemical properties.
