In this lesson, we will learn:
- To understand the two major types of bonding in chemical compounds
- To explain why both types exist and when they are likely to form.
- How to predict the formula of ionic and covalent compounds.
- Chemical molecules are held together by intramolecular forces - chemical bonds between the constituent atoms they are made of.
- One major type of chemical bond is the ionic bond:
- Ionic bonding is the forces of attraction caused by oppositely charged ions interacting with each other. It occurs between metal and non-metal atoms.
- In trying to obtain a full outer shell, metal atoms can transfer outer shell electrons from themselves to non-metal atoms. This creates a positive metal ion which lost electrons, and a negatively-charged non-metal ion that gained them.
- The transfer of electrons is driven by the stability of having a full outer shell, and the difference in electronegativity - the non-metal atom should have a much higher electronegativity than the metal atom.
- The ionic bond created is strong due to the oppositely charged ions attracting one another.
- Examples of simple ionic compounds are sodium chloride, NaCl, or magnesium oxide, MgO.
- Ionic bonds will form between elements on opposite sides of the periodic table.
- Because ionic bonding is built on the strong attraction of opposite ions, the greater the charge difference, the stronger the bonding.
- Periodic trends such as atomic radius and the charge of ions involved have an effect on ionic bond strength in compounds. One example is atomic radius: ions with a smaller atomic radius enable any oppositely-charged ions to come into closer contact with them, which increases ionic bond strength. In short, smaller distance between opposite ions strengthens their interaction! Another is ionic charge: The larger the charge on an individual ion, the greater the charge difference will be with the oppositely charged ion, which produces a stronger ionic bond.
- Nuclear charge helps explain trends seen in isoelectronic ions (ions with the same number of electrons, e.g. N3-, O2-, F-, Na+, Mg2+, Al3+). In a group of these ions, the ion with the greatest nuclear charge has the smallest atomic radius because the increasingly positive nuclear charge attracts the same number of electrons more strongly.
- Another major type of chemical bond is the covalent bond:
- Covalent bonding is when two atoms share a pair of electrons between them – one from each atom, between both their nuclei. It occurs between non-metal atoms with similar electronegativity.
- The force of attraction in a covalent bond is the electron pair being attracted to the positive nuclei of both atoms involved, while the electron pair being shared means both atoms have their outer shell being filled.
- Covalent bonding occurs widely between non-metal atoms. Their similarity in electronegativity leads to the electrons being shared and no single atom losing or gaining electrons, unlike in ionic bonding.
- Covalent bonds form between nonmetals and elements close to each other in the periodic table.
- Covalent bonds can vary in length (measured by the distance between nuclei), but in general the shorter the bond, the stronger the bond.
- Both ionic and covalent bonding are examples of intramolecular forces. Their forces occur between the atoms or ions that make up a chemical substance itself, not between the separate molecules of the substance. They are both strong forces – simple ionic compounds often have melting points of several hundred degrees Celsius, as do some large covalent structures.
- You can use the valence of an atom to work out the formula of covalent and ionic compounds:
- For each element, find the number of valence electrons (the number would be the group number). (E.g. N has 5 valence electrons, H has 1).
- For both atoms to make the compound, fill in the outer shell and find the number of unpaired electrons. This is the valence of the atom. (E.g. N has a valence of 3 due to 3 unpaired electrons, H has 1).
- Cross the valence of each atom with the other – this is the number of atoms of the element that will combine to form the compound. (N = 1, H = 3 makes NH, a covalent compound).