In chemistry, an atom is the smallest particle of an element, and it is made up of smaller elementary particles called protons, neutrons and electrons. Protons are positively charged, neutrons have no charge, and electrons are negatively charged. Ions are atoms or molecules that have lost or gained electrons and thus have a positive or negative charge. Isotopes are atoms of the same element that have different numbers of neutrons.
Use your periodic table to determine the number of protons. The number of protons in an atom defines its elemental identity. For example, any atom with 11 protons is called sodium, no matter the number of electrons or neutrons that it carries. You can determine the number of protons in an atom by looking at the atomic number, which is just above the atomic symbol on the periodic table. For example, if you want to know the number of protons for magnesium, then you look for its atomic symbol, Mg, on the periodic table, and just above it you will find the atomic number, 12, which is equal to the number of protons in magnesium.
Use the number of protons to determine the number of electrons. In an atom with no charge, the number of electrons is equal to the number of protons. For example, an uncharged magnesium atom will have 12 electrons. If the atom carries a positive or negative charge, add one electron to the original number of electrons for every negative charge, or subtract one electron from the original number of electrons for every positive charge. For example, if you have a magnesium ion with a +2 charge, then you must subtract two electrons from the original 12, giving you 10 electrons. Ions are the only forms of atoms in which the number of electrons does not match the number of protons.
Subtract the number of protons from the atomic mass to determine the number of neutrons. Different isotopes have different numbers of neutrons. Look at the atomic mass of the isotope, which will be listed after the name of the isotope or in the upper left corner of the atomic symbol. Subtract the number of protons for that element from the atomic mass to get the number of neutrons. For example, carbon-13 is a carbon isotope with an atomic mass of 13. Using the periodic table you can determine that carbon has six protons. Subtract six from 13 to get 7 neutrons present in carbon-13.