Understanding Ions (Handout 1)
When we combine atoms, we form molecules. They can be compounds, where there are two or more
different elements (C6H12O6, glucose) or elements, where the molecule contains multiples of the same
element (O2, oxygen). In each case, we show how many of each kind of atom we have in the molecule by
using a subscript, the smaller number just after the element symbol. (If there is only one atom of a
particular type in a compound, we don't put a "1" for the subscript; we simply don't use a subscript, for
example H2O. There is only one oxygen atom in the water molecule.)
There are 7 elements that are also molecules (of 2 identical atoms) in their natural state and should be
memorized: H2, N2, O2, F2, Cl2, Br2, I2. These are called diatomic molecules. Under normal reaction
conditions, we should write the formula for oxygen gas as O 2, never as simply O. This holds true for each
of these 7 diatomic molecules.
Ions are atoms or groups of atoms with a net positive or negative charge. For example, H+, OH-, H3O+, Na+,
Mg+2, B+3, F- are all examples of ions. Ions are created by elements giving away or gaining an electron or
two. An example might be that Na (sodium) might give away an electron which would make it Na +
[Remember that Na starts out neutral and then gives away a negatively charged electron so it becomes
positive.]. Then Cl (Chlorine) might come along and pick up that extra electron, becoming Cl - [Remember
that Cl starts out neutral and then gains a negatively charged electron, so it becomes negative.] An ion with
a positive charge is called a cation. An ion with a negative charge is called an anion. (People remember
that the cation is positive because the "t" looks like a + sign, and the anion is negative because there is a "n"
in it.)
Ionic bonds involve ions of different charges. These ions pull each other together because of the opposite
charges. They interact with each other much like the north side of a magnet would react with the south side
of a magnet. An example of ionic bonding would be when Na+ and Cl- interact with each other to produce
NaCl. (Notice that there is not a charge on the final product. This is because the negative charge balances
out the positive charge to leave a charge of zero.)
You can use you periodic chart to determine what ions different elements form. The magnitude of the
charges on ions can sometimes be determined easily from the element’s position on the periodic table. This
information is essential to knowing how to write chemical formulas for compounds. The alkali metals
(group 1) atoms always lose 1 electron to form a +1 charge ion. The alkaline earth metals (group 2) always
lose 2 electrons to form a +2 charge ion. The halogens (group 17), are nonmetals and all nonmetals tend to
gain electrons during chemical reactions. The halogens always gain 1 electron to form a –1 charge ion.
Transition metals frequently form more that one ion. For example Cobalt can have a +2 or +3 charge. The
small black bold numbers above the symbol on some periodic charts show the ion or ions that the element
forms. The "Ion Chart" Handout #4 is an easy to read chart showing the ion and ions that the various
elements form.
Naming ions
When a sodium atom loses an electron, Na  Na+1 + 1e-, the resulting cation is called “sodium ion”.
Cations are named the same as the element they come from.
When a chlorine atom gains an electron, Cl + 1e-  Cl-1, the resulting anion is called “chloride ion”.
Notice that to name anions, we use the prefix of the element and then add an “ide” ending.