This worked chemistry problem demonstrates how to determine the symbol for the ion when given the number of protons and electrons.
Problem: Give the symbol of an ion that has 10 e- and 7 p+.
Solution: The notation e- refers to electrons and p+ refers to protons. The number of protons is an element's atomic number. Use the periodic table to find the element with an atomic number of 7. This element is nitrogen, which has the symbol N. The problem states that there are more electrons than protons, so we know the ion has a negative net charge. Determine the net charge by looking at the difference in the number of protons and electrons: 10 - 7 = 3 more electrons than protons, or a 3- charge.
Conventions for Writing Ions
When writing the symbol for an ion, the one or two letter element symbol is written first, followed by a superscript. The superscript has the number of charges on the ion followed by a + (for positive ions or cations) or - (for negative ions or anions). Neutral atoms have a charge of zero, so no superscript is given. If the charge is +/- one, the "1" is omitted. So, for example, the charge on a chlorine ion would be written as Cl-, not Cl1-.
General Guidelines for Finding Ions
When the numbers of protons and electrons are given, it's easy to figure out the ionic charge. More often, you won't be given this information. You can use the periodic table to predict many ions. The first group (alkali metals) usually have a +1 charge; the second group (alkaline earths) usually have a +2 charge; halogens usually have a -1 charge; and noble gases typically don't form ions. The metals form a wide variety of ions, usually with a positive charge.