Average mass of all the isotopes of an element.

Properties of the elements repeat in each period-or row- of the table.

1 or 2-letter symbols that identify most elements.  Some periodic tables also list the names of the elements.

The number of proton's in an atom's nucleus.

The average mass of an element's atoms.  Atomic masses in parantheses are those of the most stable isotope.

The table is organized in horizontal rows called periods. 

Elements of the modern periodic table fall into 18 vertical columns, or groups.

1) atomic number

2) chemical symbol

3) name

4) atomic mass

State of matter that consists of a gas-like mixture of free electrons and nuclei of atoms that have been stripped of electrons.

Process in which two atomic nuclei combine, forming a larger nucleus and releasing huge amounts of energy.

Nuclear fission, which occurs in stars on a huge scale, combines smaller nuclei into larger nuclei, creating heavier elements.