Key concepts in chemistry Study Guide

Atoms and small molecules are extraordinarily tiny, far too small to be seen with even the most powerful optical microscopes. Their sizes are typically measured in nanometres (nm) or picometres (pm). A nanometre is a billionth of a metre (1 x 10^-9 m), and a picometre is a trillionth of a metre (1 x 10^-12 m). The typical order of magnitude for the diameter of an atom is about 0.1 nanometre, or 1 x 10^-10 metres. Small molecules are slightly larger, but still within a similar order of magnitude, often ranging from about 0.1 to a few nanometres. For instance, a water molecule (H₂O) has a diameter of approximately 0.27 nanometres. These minuscule dimensions highlight why the study of chemistry relies heavily on models and indirect observations to understand the behaviour and properties of matter at the atomic and molecular scale.

Simple molecular covalent substances form when non-metal atoms share electrons to achieve a stable outer shell electron configuration, typically a full outer shell (like a noble gas). This sharing results in strong covalent bonds within the molecules, but weaker intermolecular forces between the molecules. Dot and cross diagrams are used to represent these shared electron pairs. Each circle in a dot and cross diagram represents an atom, and dots or crosses represent the outer shell electrons. When atoms bond, they share electrons, so an electron from one atom (e.g., a dot) and an electron from another atom (e.g., a cross) are drawn in the overlapping region of their outer shells. This shared pair counts towards the stable outer shell of both atoms. For example, in hydrogen (H₂), each hydrogen atom contributes one electron to form one shared pair. In hydrogen chloride (HCl), hydrogen shares one electron with chlorine, and chlorine shares one electron with hydrogen. Water (H₂O) involves oxygen sharing two electron pairs, one with each hydrogen atom. Methane (CH₄) has carbon sharing one electron with each of four hydrogen atoms. Oxygen (O₂) forms a double covalent bond, sharing two pairs of electrons between the two oxygen atoms. Carbon dioxide (CO₂) has carbon forming two double covalent bonds, one with each oxygen atom, resulting in linear molecules.