The positive nuclear core is surrounded by a negative cloud of electrons. A neutral atom has equal numbers of electrons and protons. Almost all nuclei contain roughly equal numbers of neutrons and protons. Since all nuclei contain protons, all nuclei are positively charged. An atom with an excess or deficit of electrons is charged and is called an ion.
Ions have very different properties than the neutral atom and are very important in reactions.
A body made of sodium atoms is a highly reactive soft metal. The sodium ions in table salt have given up a single electron and have very different properties. The elementary charge in coulombs is 1.6002 x 10 -19 C, positive for a proton, negative for an electron. The various elements are distinguished by the number of protons in the nucleus of their atoms. The number of protons in the nucleus is called the atomic number.