Key concepts in chemistry Study Guide

The understanding of the atom has progressed significantly since John Dalton proposed his atomic theory in the early 19th century. Dalton described atoms as indivisible, solid spheres, believing that all atoms of a particular element were identical. However, subsequent discoveries of subatomic particles – electrons, protons, and neutrons – necessitated a refinement of this model. The electron, discovered by J.J. Thomson, led to the 'plum pudding' model, where a sphere of positive charge contained negatively charged electrons. Ernest Rutherford's gold foil experiment identified the atomic nucleus, a dense, positively charged core. This replaced the plum pudding model with one where electrons orbit a central nucleus. Niels Bohr further refined this by proposing that electrons orbit in specific energy shells. Finally, further research revealed the nucleus itself is composed of protons and neutrons, establishing the modern understanding of atomic structure. Each discovery addressed limitations of previous models, illustrating the dynamic nature of scientific understanding.

When atoms form covalent bonds, they share pairs of electrons, resulting in the creation of molecules. Molecules are discrete groups of atoms held together by shared electrons. This is distinct from ionic bonding, where ions are formed. Covalent bonding commonly occurs between non-metal atoms, such as in water (H₂O) or oxygen (O₂). Each molecule formed by covalent bonding consists of a specific number and type of atoms joined together through these shared electron pairs.