The atomic number (symbol: Z) of an atom is the number of protons in the nucleus of the atom. The atomic number of an atom identifies which element it is. In a neutral atom, the atomic number is equal to the number of electrons orbiting the nucleus. The elements of the periodic table are listed in order of increasing atomic number.
Atomic number is not the same as:
- atomic mass (symbol: ma), which is the mass of a single atom, commonly expressed in unified atomic mass units
- mass number (symbol: A), which is the sum of the number of protons and number of neutrons in the nucleus of an atom
- relative atomic mass (also called atomic weight; symbol: Ar), which is the ratio of the average mass per atom of an element from a given sample to 1/12 the mass of a carbon-12 atom.
The atomic number of the periodic table directly corresponds to the number of protons which is in the atom. Once another proton is added, it is no longer the same element. The same cannot be applied to when another neutron or another electron is added. Adding more electrons will give the atom a negative charge and removing electrons will give the atom a positive charge. Metals tend to lose electrons, which creates a positive charge. Non-metals tend to gain electrons, forming a negative charge. Electrons are the foundation for determining how compounds are formed among atoms.
Adding or removing neutrons within an atom changes its isotope. As an example, carbon-12 is the most stable isotope for a carbon atom. However, we can add two more neutrons and carbon-12 is now carbon-14, a less stable isotope of carbon. The number of an isotope directly correlates to the atomic mass of an element. The amount of neutrons in any given atom by subtracting the atomic number from the atomic mass.
For example, oxygen has 8 protons. The atomic number is the number of protons an element has. So, if an unidentified element has 8 protons, then it is oxygen. This is because each element has a certain number of protons. If an element had 9 protons, it would not be oxygen, but an element can have 8 protons, and 9 neutrons and still be considered oxygen; we call this an isotope.