A candidate key is a column, or set of columns, in a table that can uniquely identify any database record without referring to any other data. Each table may have one or more candidate keys, but one candidate key is unique, and it is called the primary key.
All atoms of an element have the same number of protons. The number of protons in an element is a primary identifier known as its atomic number. The number of electrons is the same as the number of protons.