The idea of the atom as the building block of matter has developed over time. What was thought of as a single particle about 1 × 10‾¹º m across is now known to be a collection of smaller ...

For example, the atomic number of sodium is 11. Every sodium atom has 11 protons and 11 electrons. It has 11 positive charges and 11 negative charges. The mass number of an atom is its total ...

and the apparently widely different assumptions which have been introduced in various recent attempts to develop a theory of atomic constitution capable of accounting for other physical and ...