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Types of Atoms
While all atoms are made of the same three sub-atomic particles, the quantity of these sub-atomic particles can vary, causing key differences between atoms. The number of protons in the nucleus determines the type of atom. This is what differentiates one element from another; each element has a unique number of protons, known as the atomic number. For example, carbon has an atomic number of 6, which means that every carbon atom contains 6 protons in its nucleus. The number of protons in an atom cannot change without changing the identity of the element.
However, this is not true of neutrons; the number of neutrons that reside in the nucleus can differ between atoms of the same element. Atoms with the same number of protons (the same element) but different numbers of neutrons are referred to as isotopes. For example, carbon has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14. The numbers here refer to the mass number, the total number of sub-atomic particles within the nucleus (protons and neutrons). So, carbon-12 has twelve sub-atomic particles in its nucleus (six protons and six neutrons) while carbon-13 has thirteen sub-atomic particles (six protons and seven neutrons) and carbon-14 has fourteen sub-atomic particles in its nucleus (six protons and eight neutrons). The atomic mass is calculated from the average of all of the known isotopes of each element, taking into account the abundance of each isotope. This is why the atomic mass listed for each element on the periodic table is rarely a whole number.
The number of electrons in an atom can also vary without changing the identity of the element. When an atom has an equal number of protons and electrons, we call it a neutral atom. However, sometimes atoms will gain or lose electrons causing them to become electrically charged. Atoms that have a negative or positive charge as a result of gaining or losing electrons are called ions; negative ions have more electrons than protons, while positive ions have fewer electrons than protons.