Key concepts in chemistry Revision Notes

Understanding Covalent Bonds

Covalent bonds primarily form between non-metal atoms. This type of bonding involves the sharing of electron pairs to achieve a stable electron configuration, usually a full outer shell (octet rule). Each shared pair of electrons constitutes one covalent bond.

Mechanism of Formation

When two atoms approach each other, their outer shells overlap. Instead of transferring electrons, they share electrons. The shared electrons are simultaneously attracted to the nuclei of both atoms, creating a strong electrostatic force that binds the atoms together to form a molecule.

Stability and Energy

The formation of covalent bonds leads to a more stable arrangement of electrons and a lower energy state for the combined atoms compared to isolated atoms. This energy stabilisation is the driving force behind bond formation.

Covalent Bonding Basics

Covalent bonding involves the sharing of electron pairs between non-metal atoms. The atoms achieve more stable electron arrangements by forming these bonds.

Resulting Formation of Molecules

The main result of covalent bonding is the formation of molecules. Molecules are groups of atoms covalently bonded together and have a fixed composition.

The Scale of Atoms and Molecules

Atoms and small molecules are incredibly small particles. Their sizes are so minute that we cannot see them directly with standard optical microscopes. To grasp their dimensions, we use units of length that are much smaller than a metre.

Typical Sizes and Orders of Magnitude

The typical diameter of an atom is approximately 1 x 10^-10 metres, which is also commonly expressed as 0.1 nanometres. One nanometre (nm) is 1 x 10^-9 metres. Small molecules are generally slightly larger than individual atoms, with typical diameters ranging from about 0.1 nm to a few nanometres. Understanding these orders of magnitude helps to conceptualise the invisible building blocks of matter.

Units of Measurement

The primary unit for expressing the size of atoms and molecules is the nanometre (nm). It is crucial to remember the conversion: 1 nm = 1 x 10^-9 m. Sometimes, picometres (pm) are also used, where 1 pm = 1 x 10^-12 m. Therefore, an atomic diameter of 0.1 nm is equivalent to 100 pm or 1 x 10^-10 m.

What are Simple Polymers?

Simple polymers are very large molecules (macromolecules) made by linking together many smaller, identical repeating units called monomers. This process, called polymerisation, forms long chains of atoms.

Poly(ethene) as an Example

Poly(ethene) is a common polymer. It is formed from ethene monomers (C₂H₄). During polymerisation, the carbon-carbon double bond in ethene breaks, allowing thousands of ethene units to join, creating long chains of carbon atoms. The structure of poly(ethene) is essentially a long chain of carbon atoms with hydrogen atoms attached along the chain.

Structure and Properties

The 'backbone' of poly(ethene) molecules consists of long chains of carbon atoms. These large chain molecules are responsible for the physical properties of poly(ethene), such as its flexibility, strength, and resistance to degradation, making it suitable for plastic products like packaging and containers.