Chemistry is the study of matter and the changes it undergoes. This science seeks to explain the properties of all substances in the universe by examining them at the most fundamental level. Chemists rely on a set of unifying concepts that provide a framework for understanding all physical and chemical phenomena.
The Particulate Nature of Matter
Every substance, whether a solid, liquid, or gas, is built from incredibly small, discrete particles, known as the particulate nature of matter. All matter is composed of atoms and molecules. Atoms are the basic units of an element, defined by having only one type of atom. When two or more atoms bond together, they form a molecule, the smallest unit of a compound, such as water or carbon dioxide.
The different states of matter are macroscopic manifestations of how these particles are arranged and move. In a solid, particles are tightly packed in fixed positions, giving the substance a definite shape and volume. Particles in a liquid are closely spaced but can slide past one another, allowing the substance to flow. Gas particles are separated by vast distances and move randomly and quickly, meaning a gas will expand to fill any volume.
Structure and Bonding Determine Properties
The arrangement of electrons around an atom governs how it interacts with other atoms to form bonds. This bonding process dictates the three-dimensional structure of a substance, which determines its properties. Understanding the forces between atoms is the primary mechanism for predicting a material’s behavior, such as its melting point, strength, or electrical conductivity.
Two primary types of chemical bonds exist based on how atoms handle their outer electrons. Ionic bonds form when one atom transfers an electron to another, creating oppositely charged ions held together by a strong electrostatic attraction. This strong force results in compounds like table salt, which are hard, brittle, and possess high melting points.
Covalent bonds form when atoms share electrons to achieve a stable configuration. The molecules formed can be small, like water, or massive, like plastics and proteins. Since the attractive forces between individual covalent molecules are much weaker than ionic bonds, these substances often have lower melting and boiling points, existing as liquids or gases at room temperature. The precise three-dimensional geometry of a molecule, determined by its bond angles, further influences its properties and how it interacts with other molecules.
Chemical Change and the Role of Energy
Chemical change is the process where substances are transformed into entirely new substances through the breaking of old bonds and the formation of new ones. A fundamental principle governing all chemical transformations is the Law of Conservation of Mass. This law states that matter is neither created nor destroyed during a reaction, meaning the total mass of the starting materials (reactants) must equal the total mass of the final products, as the atoms are merely rearranged.
Energy plays a central role in driving these changes. Energy must be supplied to break existing bonds in the reactants, and energy is released when new bonds form in the products. Reactions that release a net amount of energy into the surroundings, often as heat, are exothermic reactions. Conversely, endothermic reactions absorb energy from the surroundings, requiring a continuous input of energy.
The speed at which a chemical change occurs is described by the reaction rate, or kinetics. This rate is influenced by factors like temperature, concentration, and the presence of a catalyst. Increasing the temperature provides particles with more thermal energy, leading to more frequent and forceful collisions that initiate the reaction.
Organization and Classification Systems
Chemists have developed systematic frameworks to organize the millions of known substances. The most important is the Periodic Table of Elements, which organizes all known elements into a coherent structure. The table arranges elements in increasing order of their atomic number, which is the number of protons in an atom’s nucleus.
This arrangement represents the Periodic Law, which states that the properties of elements recur in a regular, predictable pattern. Elements placed in the same vertical column, or group, exhibit similar chemical behavior because they share a similar arrangement of outer electrons. The table’s structure allows chemists to predict the properties of an unstudied element based on its position. Compounds are also classified into broad categories such as organic (containing carbon and hydrogen) and inorganic (all other compounds).