- Predict and draw the Lewis structure of simple covalent molecules and compounds
- The octet rule says that the noble gas electronic configuration is a particularly favorable one that can be achieved through formation of electron pair bonds between atoms.
- In many atoms, not all of the electron pairs comprising the octet are shared between atoms. These unshared, non-bonding electrons are called ‘lone pairs’ of electrons.
- Although lone pairs are not directly involved in bond formation, they should always be shown in Lewis structures.
- There is a logical procedure that can be followed to draw the Lewis structure of a molecule or compound.
- Lewis structureFormalism used to show the structure of a molecule or compound, in which shared electrons pairs between atoms are indicated by dashes. Non-bonding, lone pairs of electrons must also be shown.
- covalent bondTwo atoms share valence electrons in order to achieve a noble gas electronic configuration.
- exceptions to the octet ruleHydrogen (H) and helium (He) only need two electrons to have a full valence level.
- octet ruleAtoms try to achieve the electronic configuration of the noble gas nearest to them in the periodic table by achieving a full valence level with eight electrons.
The Octet Rule
Noble gases like He, Ne, Ar, Kr, etc., are stable because their valence level is filled with as many electrons as possible. Eight electrons fill the valence level for all noble gases, except helium, which has two electrons in its full valence level. Other elements in the periodic table react to form bonds in which valence electrons are exchanged or shared in order to achieve a valence level which is filled, just like in the noble gases. We refer to this chemical tendency of atoms as ‘the octet rule,’ and it guides us in predicting how atoms combine to form molecules and compounds.
Covalent Bonds and Lewis Diagrams of Simple Molecules
The simplest example to consider is hydrogen (H), which is the smallest element in the periodic table with one proton and one electron. Hydrogen can become stable if it achieves a full valence level like the noble gas that is closest to it in the periodic table, helium (He). These are exceptions to the octet rule because they only require 2 electrons to have a full valence level.
Two H atoms can come together and share each of their electrons to create a ‘covalent bond.’ The shared pair of electrons can be thought of as belonging to either atom, and thus each atom now has two electrons in its valence level, like He. The molecule that results is H2, and it is the most abundant molecule in the universe.
The Lewis formalism used for the H2 molecule is H:H or H—H. The former, known as a ‘Lewis dot diagram,’ indicates a pair of shared electrons between the atomic symbols, while the latter, known as a ‘Lewis structure,’ uses a dash to indicate the pair of shared electrons that form a covalent bond. More complicated molecules are depicted this way as well.
Now consider the case of fluorine (F), which is found in group VII (or 17) of the periodic table. It therefore has 7 valence electrons and only needs 1 more in order to have an octet. One way that this can happen is if two F atoms make a bond, in which each atom provides one electron that can be shared between the two atoms. The resulting molecule that is formed is F2, and its Lewis structure is F—F.
After a bond has formed, each F atom has 6 electrons in its valence level which are not used to form a bond. These non-bonding valence electrons are called ‘lone pairs’ of electrons and should always be indicated in Lewis diagrams.
Procedure for Drawing Simple Lewis Structures
We have looked at how to determine Lewis structures for simple molecules. The procedure is as follows:
- Write a structural diagram of the molecule to clearly show which atom is connected to which (although many possibilities exist, we usually pick the element with the most number of possible bonds to be the central atom).
- Draw Lewis symbols of the individual atoms in the molecule.
- Bring the atoms together in a way that places eight electrons around each atom (or two electrons for H, hydrogen) wherever possible.
- Each pair of shared electrons is a covalent bond which can be represented by a dash.
Multiple bonds can also form between elements when two or three pairs of electrons are shared to produce double or triple bonds, respectively. The Lewis structure for carbon dioxide, CO2, is a good example of this.
In order to achieve an octet for all three atoms in CO2, two pairs of electrons must be shared between the carbon and each oxygen. Since four electrons are involved in each bond, a double covalent bond is formed. You can see that this is how the octet rule is satisfied for all atoms in this case. When a double bond is formed, you still need to show all electrons, so double dashes between the atoms show that four electrons are shared.
Boundless vets and curates high-quality, openly licensed content from around the Internet. This particular resource used the following sources:
Steve Lower’s Website
CC BY-SA 2.5.
CC BY-SA 3.0.