One of the most important requirements in chemistry is to become familiar with the ways in which elements react to form compounds.  We will investigate the types of chemical bonds, ways to predict compositions and the conventions of naming the compounds.

Electron transfer involves creation of ions, which bond via ionic bonds to form ionic compounds.  A Familiar compound like table salt, sodium chloride, is a classic example of an ionic compound.

The documents called Type 1 Compounds and Type 2 Compounds give information on predicting composition and naming of ionic compounds[1]

Electron sharing involves the sharing of electrons between two atoms and the creation of covalent bonds.  Covalently bonded compounds typically have very different properties from ionic compounds, and they also involve combinations of different types of elements.

The document called Type 3 Compounds gives information of the naming of covalent compounds.[2]

Although there are many elements and even more compounds to consider, familiarity with a few rules will greatly simplify the process of becoming conversant with determining compositions of compounds.

The octet rule

It is well known that the elements in group 8, the noble gases, are extremely unreactive.  Examination of the electronic structures of these elements shows that the outer shells are full of electrons; they do not lack for electrons to fill the shells.  None of the other elements has a filled outer shell.

The inference we draw from this is that the atom wants to obtain a filled shell, and this it achieves by forming bonds.  This can be done by either addition of electrons or removal of electrons. The noble gas atom already has a filled shell and does not need to indulge in bonding to achieve it. 

Elements on the left side of the table, metals, will lose electrons to form positive ions; elements on the right hand side of the table, non-metals, will gain electrons.  In both cases, a filled shell will result.

Of course, we must recognize that the atom is now charged because the electron and proton counts are not equal.  Electron loss creates positive ions, and electron gain creates negative ions.

In an ionic compound, a positive ion and a negative ion come together and form an ionic bond through the strong electrostatic interaction between the ions of opposing charge.

Predicting ion charges

It is essential to be able to predict the charge on an ion in order to predict the composition of compounds formed containing it.  We can use the periodic table to assist us in this.

The table shows the periodic table with the charges of the ions shown.  Note, that in this version, the SI scheme of 1 – 18 is used rather than the older 1A – 8A.  We find a very strong correspondence between group number (using the older scheme) and ion charge.

Rule to predict ion charge

Ionic compounds

Ionic compounds always contain a metal and a non-metal.

Predicting composition

A compound is always neutral, and so charges of the ions in the compound must balance out.  We always[3] know the charges on the ions from the periodic table.  So the next stage is to determine the correct ratio of ions that will produce charge neutrality.  Basically the total number of positive charges must equal the total number of negative charges.  The document “Type 1 Compounds” provides guidance in this.

Elements with variable ionic charge

We have shown that the periodic table can be used to predict ionic charges.  However, there are some elements that are not susceptible to this approach.  Some of the heavier A-type elements like tin and lead show two ionic charge possibilities: Sn2+ and Sn4+; Pb2+ and Pb4+.  In writing the formula we would identify the state by writing Sn(II) or Sn(IV).

The transition metals also show a high degree of variable ionic charges: Cr2+, Cr3+, Cr4+, Fe2+, Fe3+, Cu+, Cu2+ and so on.  You are not expected to remember all of these different ions, but be able to predict a composition if given the ion, and write the composition with the correct notation.

Covalent bonding

The ionic bonding model works very well for metals and non-metals, but for compounds made exclusively from non-metals, which dominate chemistry in terms of numbers, it fails completely.  This is because non-metals form negative ions and never positive ions.  It would also be impossible to describe the bond between the atoms in the diatomic elements like F2, O2 and N2 using the ionic model.

In these elements and compounds, covalent bonding operates.

A covalent H–H bond is the net result of attractive and repulsive electrostatic forces. The nucleus–electron attractions (blue arrows) are greater than the nucleus–nucleus and electron–electron repulsions (red arrows), resulting in a net attractive force that holds the atoms together to form an H2 molecule.

The sharing of electrons effectively increases the electron count around the atom.  Alone, each fluorine atom has seven electrons in the outer shell.  Sharing two electrons in a single covalent bond means that each atom now appears to have eight – it has satisfied its octet demand.

The same principle applies to describing bonds between unlike atoms, such as hydrogen and oxygen in water.

Note that the O atom has achieved its octet by sharing one electron from each of two H atoms to supplement the six valence electrons it already  has.

Multiple bonds

For some molecules, the sharing of two electrons is not sufficient to satisfy the octets of the atoms.  Consider the series F2, O2 and N2.  The elements are in groups 7A, 6A and 5A respectively.  The atoms have 7, 6 and 5 electrons in the valence shell respectively.  It seems pretty obvious that, if the sharing of two electrons in F2 satisfied the octet, then the sharing of two electrons will not do so in O2 or N2.  However, more electrons can be shared, leading to multiple covalent bonds.

Naming and writing formulas of covalently bonded compounds between non-metals is discussed in the document “Type 3 Compounds.”

Ternary compounds

So far we have discussed compounds that involve only two elements bonded by either ionic or covalent bonds.  There is a class of compounds, many of them very familiar, which contain more than two elements and also both ionic and covalent bonding.  The compound is an ionic compound which contains either or perhaps both ions in the form of a polyatomic ion held together by covalent bonds.

Most polyatomic ions are negatively charged; only the hydronium ion and the ammonium ion are positively charged.  The rules for balancing the charges in compounds containing polyatomic ions are the same as for binary ionic compounds.  The composition of the polyatomic ion does not change at all.

[1] Available in the content section of the Anlon course

[2] Available in the content section of the Anlon course

[3] There are some exceptions to this for the A group elements, and the transition metal ions cannot be predicted.