All atoms are made up of three subatomic particles: the proton, neutron and electron. Each determines part of how we see an atom:

The number of protons in an atom determines its atomic number. This is the atoms "identity": all atoms with one proton are hydrogen atoms, all atoms with two protons are helium atoms and so on. The atomic number (Z) is the same as the number of the element in the periodic table. The atomic number is denoted with a leading subscript: ₂He refers to a helium atom with two protons. (All of them.)

The sum of the number of protons and neutrons together determines the atomic mass. The atomic mass is denoted with a leading superscript: ⁴₂He refers to a helium atom with 2 protons (subscript = atomic number) and 4 total neutrons and protons. Thus, it has two neutrons. Changing the number of neutrons in an atom creates an isotope of that atom: the atom identity stays the same, but the atomic mass changes. A helium atom with only one neutron would have the symbol ³₂He: helium-3 is an isotope of helium with two protons and (2 protons + 1 neutron) = 3 atomic mass

The electrons of an atom exist in a cloud around the nucleus. In a neutral atom there are as many electrons as there are protons. Adding or subtracting an electron creates an ^ion of that atom. This is denoted by placing a trailing superscript with the charge on the atomic symbol. For example, an atom of helium-3 that lost an electron would have a +1 charge and a symbol ³₂He⁺

To summarize:

• Changing the # of protons changes the element
• Changing the # of neutrons changes the isotope of that element
• Changing the # of electrons creates an ion of that element

Example: How many protons, neutrons and electrons does the following atom have?

¹⁹₉F^-

Solution: The number of protons is given by the atomic number, which is the leading subscript: 9. All fluorine atoms have 9 protons.

The number of neutrons is gotten from the atomic mass, the leading superscript: 19. This is the sum of neutrons and protons, and we know we have 9 protons, so fluorine must have 19 - 9 = 10 neutrons .

The number of electrons is gotten from the charge. The number of electrons in the neutral species is equal to the number of protons, so fluorine neutral has 9 electron. Since we have a -1 charge, we must have 1 additional electron, so ¹⁹₉F^- has a total of 10 electrons

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