Teaching post-16 chemistry through electrostatics

Our molecular world is based on charges and how these charges are regionalised over a molecule. Most post-16 students know that water and oil don’t mix, fizzy drinks go flat when left in a glass and substances exist in different states of matter at room temperature.

Indeed, immiscible liquids and solubility are fascinating and essential to understanding how cleaning products work. And inkjet printers work by directing a spray of ink using electrical charges. It is therefore important to embed learning that electrostatic charges influence states of matter, bond strength and molecular interactions.

What students need to know

Matter is a deceptively simple topic for junior pupils in terms of how we define a solid, liquid or gas. However, what we don’t mention is that charges determine the characteristics and interactions of compounds and bond strengths in the microscopic world where gravitational forces are too weak to have any effect.

Chemistry is all about charge and polarity

Put simply, chemistry is often the movement of electrons from one nucleus to another, or the attraction of opposing charges. Effective nuclear charge, the ability of a nucleus to attract (valence) electrons, is hugely important in understanding chemistry. Ionisation energy, electronegativity and intermolecular forces (and therefore states of matter, chromatography and solubility) and organic reactions mechanisms are driven by nuclear charge.

Common misconceptions

When asked why solids are solid, students typically say that ‘the particles are close together’ and not that there are strong forces between the particles. Students often fail to understand that the forces between particles (atoms, ions, molecules) influence the state at room temperature, and that these are determined by electrostatics. By revisiting the basic concept of electrostatic forces, you can open the door to more complex understanding of molecular interactions based on polarity.

 A covalent bond is an electrostatic sandwich formed by electrons being held between two positive centres

Explaining differences in the ionic radii of phosphorus (P^3-) and aluminium (Al³⁺) can be tricky, but if students understand charge on the nucleus and the attraction it has for the electrons then these questions are more straightforward. Curly arrows depicting electron movement in (organic) reaction mechanisms can also be confusing, but it is often simply electrostatic attraction that determines electron movement.

Ideas for your classroom

Although ionic bonding is a relatively easy fit in terms of opposite ion electrostatic attraction (use static electricity to demonstrate attraction and repulsion of charges), covalent bonding is conceptually more difficult. A covalent bond is an electrostatic sandwich formed by electrons being held between two positive centres (nuclei).

Covalent molecules are a collection of positive nuclei surrounded by negatively charged electrons. Thus, all simple covalent molecules should repel each other and be gases. So, how are some of them liquids (bromine, hexane, water) or solids (iodine, fats, sugar) at room temperature? Non-polar particles, such as diatomic elements or monatomic gases, have weak intermolecular forces based on a momentarily uneven distribution of electrons.

However, polar molecules, which have a larger electronegativity difference, have a permanently uneven electron distribution that leads to permanent partial charges within the molecule. A polar molecule will have stronger intermolecular forces as a result of permanent dipole–permanent dipole interactions relative to a non-polar molecule of similar mass.

Demonstrating London dispersion forces

Checking for understanding

When you are teaching periodicity, such as atomic size, ionisation energy and covalent bond strength (or length), make sure you ask your students which factors are causing these effects and relate it back to effective nuclear charge.

Remind students that the increasing nuclear charges across a period lead to decreasing atomic sizes. And, that the increasing number of electron shells down a group mean that, despite increasing nuclear charge, size increases because each outer electron experiences less attraction from the nucleus.

Check your students’ understanding by asking why heterolytic bond fission is occurring rather than homolytic, or why electrons are being ‘pushed’ in the direction they are by curly arrows in a reaction mechanism.