Chemical Bonding – It’s Not About the Octet!

Why do atoms bond? Students, when asked this question, will often answer – to complete an octet of valence electrons. Their understanding of the concept of bonding is based on the idea that atoms ‘need’ a complete octet in its outer energy level. But, why do atoms need eight (or two) electrons in the outer energy level? Is this the driving force behind bonding? Is this what causes chemical reactions to occur?

The octet is a useful guide for working out ion formation or writing chemical formulas, but should not be taught as the driving force behind bonding. Using the octet rule as a conceptual basis or explanation of chemical bonding leads to a faulty understanding of what keeps our universe together. Our focus, while teaching this very basic concept of chemistry, should be on the electrostatic interactions between atoms. Atoms don’t need an octet. Atoms are driven to lower potential energy which in turn increases stability.

“Atoms are driven to lower potential energy which in turn increases stability.”

Chemical bonding is related to energy and electrostatic interactions. By nature, we know that the preferred state of matter is to maintain the lowest possible potential energy, and by forming bonds, atoms release energy, and thereby obtain a more stable state.

Why is it so important to make this distinction while teaching the concept? What difference does it make?

Chemical bonding is one of the most challenging topics for chemistry students. The octet rule can be problematic, but at the same time very helpful. Teaching and the understanding of bonding based on the idea that atoms ‘need’ a full outer energy level is presenting the octet rule as an explanatory principle rather than a rule-of-thumb, and leads to a faulty understanding of the concept. When teachers focus on the electrostatic interactions between atoms, and use the octet rule as a helpful tool, students will generally develop a well-constructed conceptual understanding of bonding based on electrostatics.

“The ‘octet rule’ can be problematic.”

The universe is not held together by atoms transferring or sharing electrons! Forces – opposing electrostatic forces – hold the universe together. Let’s start with what holds the atom together. If we look at the atom, it should not be able to exist! The positively charged nucleus should ‘pull in’ all of the electrons and the atom should collapse! So how does the atom exist – perfectly balanced opposing electrostatic forces?

Let’s consider some student misconceptions when it comes to chemical bonding, and how teachers can address these misconceptions in order to lead students to a conceptual understanding of chemical bonding based on electrostatics.

Student Misconceptions – Why Do Atoms Bond?

  • Students often think that atoms ‘need’ to acquire a full outer energy level, and associate this ‘need’ with what causes chemical reactions to occur.
  • Students often conclude that reactions happen because atoms are ‘trying’ to gain full valence energy levels and that there are just two ways of doing this: transferring electrons to form ionic bonds, or sharing electrons to form covalent bonds.
  • Students may agree that the Na+ ion is more stable than the neutral ion, but some will argue that the Na7- anion is just as stable.

To get an idea of what your students are thinking, ask your students the follow question. (Hand out a slip of paper with the following question and responses to each student. This is not graded. Check as many responses as you agree with.)

Why do chloride ions and sodium ions bond – what is the driving force?
Check all responses that you agree with.

____a)   Chloride ions and sodium ions bond because an electron has been transferred between them.

____b)   A chloride ion only bonds to the sodium ion it accepted an electron from.

____c)   Chloride ions and sodium ions bond due to opposing electrostatic forces.

____d)   A sodium ion is attracted to one chloride ion by a bond and is attracted to other chloride ions by  forces.

____e)  Chloride ions and sodium ions do not form molecules.

____f)   Chloride ions and sodium ions bond because they want to complete an octet of valence electrons.

Answer: c & e

How can teachers address these misconceptions?

We all need to have a critical look at our understanding of bonding. Do we consider the role of electrostatics and energy in determining the bonding in a substance? Then, do we relate the fact that bonded atoms do generally have full outer energy levels.

  • Teach and use the octet rule as a useful rule-of-thumb for working out ion charges and writing chemical formulas, but that it should never be used as a conceptual explanation.
  • Introduce all forms of bonding as electrostatic phenomena. Students are familiar with electrostatic phenomena (balloons sticking to walls, hair standing on end), the ‘invisible forces’ of attraction and repulsion, negative and positive poles attracting, and can apply these ideas to help them understand chemical bonding

“Introduce all forms of bonding as electrostatic phenomena.”

  • Describe differences, such as melting points or boiling points in terms of the particles having different strengths of bonding.
  • Explore how an atom’s ability to attract electrons relates to its atomics structure.
  • Explore the forces of attraction and repulsion within the atom.
  • Extend these ideas to develop a model that explains bonding in terms of electrostatic interactions.
  • During the periodic trends unit (which should precede bonding) have students think about why compounds with ions such as Na2+don’t occur based on energetics. The energy required to remove a second electron far outweighs any gain in lattice enthalpy based on ionization energy. Ask students: would it be easier for a sodium atom to lose one electron, or gain seven? Again, related to energy. The idea of a sodium atom gaining seven electrons is just not feasible. Relate to periodic trends.
  • The octet rule is a guideline that suggests that atoms tend to bond in such a way that they have a full outer electron shell, typically containing eight electrons. While the octet rule is a useful guideline for understanding many chemical reactions, it is difficult to think of it as a rule. But, as with a lot of rules, there are several instances where atoms can have more or fewer than eight electrons in their outermost shell.

Phenomena discussion

Why do hydrogen and fluorine react?

Consider pure hydrogen and fluorine. If these diatomic molecules are “happy” with their complete outer energy level, why would they form hydrogen fluoride? Students are aware that the reacting elements exist as diatomic molecules, but will struggle to explain the driving force behind the formation of hydrogen fluoride. Hydrogen bonding between fluorine and hydrogen is the main driver for the production of HF molecules.

When studying the double replacement reaction later, ask students: “If atoms of the reactants are “happy” with an octet, why would they react to form new products”? The driving force behind this exchange is the formation of products that are more stable or have a lower energy state than the reactants.

The primary reason for the ion exchange is the formation of new combinations of ions that result in the production of a precipitate, gas, or a molecular compound that is insoluble in the reaction medium. The specific driving force depends on the nature of the reactants and the solubility of the products. Overall, the rearrangement of ions in a double replacement reaction is driven by the tendency of the system to achieve a more stable state, often through the formation of products with lower energy or increased solubility.

Ask students over and over throughout the year: “Why do atoms bond”? to decrease potential energy and increase stability. Matter exhibits a natural tendency to reach a state of lower energy in order to create a more stable state. This is achieved by a more stable configuration which lowers the overall energy.

“Chemical bonds form as a result of electrostatic forces between atoms.”

The driving force behind chemical bonding is to achieve a more stable and lower-energy state. Atoms are most stable when they achieve an electron configuration in such a way that minimizes their potential energy.

The lower potential energy associated with chemical bonding arises from the fact that the bonded system has a lower total energy compared to the isolated atoms. When atoms come together to form bonds, they release energy, and this energy is typically lower than the sum of the energies of the individual atoms.

This lower potential energy is a result of the electrostatic attractions that hold the atoms together in a more stable configuration.

Why is this important? What difference does it make?

Although clearly not the whole picture, the electrostatics approach can support more effective progression for our high school chemistry students, and help them appreciate related concepts, such as: why some bonds are stronger, bond enthalpies, lattice structures, molecular formation, polar covalent bonds and intermolecular forces. The idea that bonds form as chemical systems move toward a lower energy is closely related to the idea that electrical forces act to give chemical structures equilibrium. The ’lowest energy’ point is the equilibrium configuration where the forces are balanced.

“Surely, we don’t want our students to think that everything is held together by octets!”

The universe, everything in our world, is not held together by octets. The driving force of everything that we know is stability, a reduction of energy. When chaos ensues, are we not driven to work towards stability and a reduction of energy? And so, we see this same phenomenon applies to the very basic level of our universe – the atom.

I am not proposing that we not teach the Octet Rule! Quite the opposite! The octet rule is a very useful tool for students to use as they write chemical formulas and balance chemical reactions. I am proposing the purposeful attention to teaching the electrostatic attraction that is the driving force behind bonding in concurrence with the octet rule as a guideline.

In summary, chemical bonding, whether through ionic or covalent bonds, occurs to achieve a more stable and lower-energy state than that of individual atoms. The octet rule provides a useful guideline, but exceptions exist when atoms have more or fewer than eight electrons in their outermost shells due to expanded or incomplete octets. By teaching the energetics of chemical bonding in concurrence with the octet rule, students will be able to move through a more effective progression in their chemistry studies and understanding the world around us.