Ionic Bonding is the attraction of positively charged ions called cations to negatively charged ions called anions

The cartoon below illustrates what ionic bonding is all about.

The electromagnetic attraction between charged atoms or groups of atoms with a charge (polyatomic ions)


An ion is a charged atom or groups of atoms. As we learned in the electrons section of the course valence electrons are defined as electrons that exist in the outer shell of the atom. Atoms transfer electrons from one atom to another in order to obtain a full valence shell or a noble gas configuration. Electrons are negatively charged therefore the addition of electrons to an atom creates an negatively charged atom or an anion. Atoms that have formed anions have their name changed, Cl-1 is no longer a chlorine atom it is now a chloride ion. The ending of the elements name is changed to -ide. Examples Chloride (Cl-1), Fluoride(F-),Hydride(H-), Oxide (O-2), Nitride (N-3).

The loss of negatively charged electrons from an atom creates positively charged ions or cations. Atoms will gain or lose electrons until they have the same number of electrons as the nearest noble gas.

Example Sodium (Na) has 11 electrons, the nearest noble gas is Neon (Ne) with 10 electrons. Sodium loses 1 electron to become a +1 ion.

Chlorine has 17 electrons, the nearest noble gas has 18 electrons(Ar). Chlorine gains 1 electron to become a -1 ion.

Click on the link below to see vision learnings atom drawings.

To see the trends for ion formation in the periodic table go back to the periodic trends page on this site.

Lewis Dots

To keep track of valence electrons, a system of symbols, called Lewis symbols was designed by G.N. Lewis in 1916. To draw a Lewis symbol, write its chemical symbol surrounded by dots representing its valence electrons.
Below is a link that will help you with lewis dot diagrams for individual elements.

Ionic Bonding

Ionic bonding is the defined as the transfer of an electron from one atom to another. Metals will transfer electron to form positive ions. Non-metals accept electrons to form negative ions. Opposite charges attract and compounds with a net 0 charge result.
Watch sodium give an electron to Chlorine

The reaction of sodium metal and chlorine gas is extremely exothermic. There is a tremendous amount of energy released when two ions come together. In this case a sodium ion combines with a chlorine ion to make sodium chloride (table salt).
While it takes energy to remove an electron from an atom a tremendous amount of energy is given off when ions come together to form compounds. As you learned in the periodic trends section the energy required to remove an electron is called ionization energy. As long as the ionization energy is not greater than the energy released when ions combine a metal will continue to lose electrons. For most metals the energy required to remove an electron becomes too much to overcome once an atom reaches a noble gas configuration. Therefore most metals will continue to lose electrons until the atom has reached the noble gas configuration. Once an arom reaches a noble gas configuration it is said to have a full outer shell. Take for example sodium. Sodium has an electron configuration of 1s22s22p63s1. It has one electron in energy level 3.
Sodium loses the 1 electron in energy level 3 to obtain a full outer shell or a noble gas configuration (10 electrons = Ne). The sodium atom now has 8 valence electrons, the same as all noble gases with the exception of He (2). The sodium atom has become a sodium ion with 11 protons, 10 electrons net charge = +1.

Chlorine on the otherhand is a non metal. Non metals accept electrons until they obtain a full outer shell. A chlorine atom has an electron configuration of 1s22s22p63s23p5. It needs one more electron to obtain a noble gas configuration (18 electrons)so 1 electron is placed in the third energy level thus giving chlorine a full outer shell and changing a chlorine atom to a chlorine ion with 17 protons 18 electrons net -1 charge.

Once the ions have formed the ions are quickly attracted to one another and the compound NaCl is formed.

Other examples of Ionic bonding

Magnesium Oxide

Magnesium oxide is formed by an identical process, but magnesium donates two electrons, because it has 2 valence electrons and oxygen receives two electrons because it has 6 valence electrons. Again the number of electrons donated by the metal is the same as the number of electrons received by the non-metal. Consequently, there is the same number of metal ions as non-metal ions. Sodium chloride has the formula NaCl, and magnesium oxide has the formula MgO.

Calcium Chloride

Things get slightly more complicated if the metal donates a different number of electrons from what the non-metal needs to fill its outer shell. There are two possibilities, either the metal gives too many electrons, or the metal does not give enough electrons.

Calcium chloride is formed by reacting calcium metal with chlorine gas. Each calcium atom donates 2 electrons. However, each chlorine atom can only receive one electron before its outer shell is full. Therefore, each calcium atom needs 2 chlorine atoms to accept one electron each. Consequently, calcium chloride contains twice as many chloride ions as calcium ions, and so the formula is CaCl2. The same happens for any Group 2 metal reacting with any halogen.

Potassium Oxide

The opposite situation arises when potassium reacts with oxygen. Potassium can only donate one electron, but oxygen needs two electrons to fill its outer shell. Consequently, each oxygen atom reacts with two potassium atoms, and potassium oxide has the formula K2O. This pattern is repeated for any Group 1 metal reacting with any Group 6 non-metal.
Take a look at these ions in action

Criss Cross Method For predicting formulas

If you fully understand the formation of the ions the predicting of chemical formulas is actually quite simple. We will employ the criss cross method for predicting formulas. The idea is that that ionic compounds are nuetral eventhough they ara a collection of positive and negative ions. Therefore the ratio of positive ions to negative ions must be based on the charge of the ions. See the microsoft word document below for a further explaination of the criss cross method.

Transition Metals

Transition metals do not form predictible ions like sodium, calcium and aluminium. The presence of d electrons allows for multiple charges (or oxidation states) to be formed. Take a look at the table below of possible ions for the first row of transition metals.
Obviously this presents a problem for us. We were using the known charge of the ions to predict the ratio of ions in a compound. The idea is that the total charge of the compound equals 0. That holds true for th transition metals however, we can not predict the charge on a transition metal ion. That is why we have to name the compounds with roman numerals. We use roman numerals to represent the charge on a transition metal ion. For example Sodium (Na) always forms a +1 charge, whereas Iron (Fe) can form either a +2 or a +3 charge. Therefore the use of Roman numerals is imperative if we are going to predict the right formula for the compound.
Iron (II) is Fe+2.
Iron (III) is Fe+3.
If we do not use Roman numerals to tell us the oxidation state of a transition metal ion there is no way of telling if iron has a +2 or a +3 charge when we are predicting the formula of the compound. See the link below for more help with the predicting of ionic compounds that have transition metals.

Polyatomic ions

Polyatomic ions are ions that are made up of a group of atoms. They act the same way normal ions act. They have either positive or negative charges and they form comounds through electrostatic attraction and neutralization of charges. Below is a link to a page with some improtant polyatomic ions.
Polyatomic ions