Chemistry: The Central Science of Matter and Change

Chemistry is the scientific study of the properties and behavior of matter. As a physical science within the natural sciences, it examines the composition, structure, and properties of substances, as well as the changes they undergo during reactions. By addressing the nature of chemical bonds, chemistry provides the essential framework for understanding how the physical world is constructed.

Often referred to as the "central science," chemistry occupies a critical intermediate position between physics and biology. It provides the fundamental knowledge required for various applied disciplines. For instance, it explains plant growth in botany, the formation of igneous rocks in geology, the degradation of pollutants in ecology, and the functionality of medications in pharmacology. It even extends to cosmochemistry, studying lunar soil, and forensics, where it is used to collect DNA evidence.

Laboratory, Institute of Biochemistry, University of Cologne in Germany

Key Facts

Solutions of substances in reagent bottles, including ammonium hydroxide and nitric acid, illuminated in different colors

Scope: Studies the composition, structure, and behavior of matter and its reactions.
Centrality: Bridges the gap between physics and biology, supporting fields from geology to pharmacology.
The Mole: A standard unit containing exactly 6.022 140 76 × 1023 particles.
Economic Impact: The global chemical industry is a massive economic driver, with the top 50 producers recording sales of US$ 980.5 billion in 2013.
Foundational Laws: Governed by principles such as Boyle's Law, Charles's Law, and Le Chatelier's principle.

The Building Blocks of Matter

Atoms and Elements

At the most basic level, chemistry focuses on the atom, the smallest unit of matter. Elements are substances consisting of only one type of atom, organized systematically in the periodic table.

A diagram of an atom based on the Rutherford model

Standard form of the periodic table of chemical elements. The colors represent different blocks of elements.

Compounds and Molecules

When atoms bond together, they form chemical compounds—substances composed of two or more different elements. A molecule is the smallest particle of a compound that retains its chemical properties. For example, carbon dioxide (CO2) is a common compound, while more complex structures like caffeine (C8H10N4O2) demonstrate the intricate ways atoms can arrange themselves.

Carbon dioxide (CO2), an example of a chemical compound

A ball-and-stick representation of the caffeine molecule (C8H10N4O2)

A 2-D structural formula of a benzene molecule (C6H6)

Substances, Mixtures, and the Mole

Chemistry distinguishes between pure substances and mixtures. To quantify these substances, scientists use the mole, a unit of measurement denoting a chemical amount. One mole contains exactly 6.022 140 76 × 1023 particles (atoms, molecules, ions, or electrons), a value known as the Avogadro constant. Molar concentration is typically reported as the amount of substance per volume of solution (mol/dm3).

Chemical Bonding and Reactions

Types of Bonding

Chemical bonds are the forces that hold atoms together. Ionic bonding occurs when one atom takes valence electrons from another, creating ions that attract each other, such as in sodium chloride (table salt). In contrast, covalent bonding involves the sharing of electron pairs, as seen in methane (CH4), where carbon shares electrons with four hydrogen atoms to satisfy the octet rule.

An animation of the process of ionic bonding between sodium (Na) and chlorine (Cl) to form sodium chloride, or common table salt. Ionic bonding involves one atom taking valence electrons from another (as opposed to sharing, which occurs in covalent bonding).

In the methane molecule (CH4), the carbon atom shares a pair of valence electrons with each of the four hydrogen atoms. Thus, the octet rule is satisfied for C-atom (it has eight electrons in its valence shell) and the duet rule is satisfied for the H-atoms (they have two electrons in their valence shells).

The crystal lattice structure of potassium chloride (KCl), a salt which is formed due to the attraction of K+ cations and Cl− anions. The overall charge of the ionic compound is zero.

Phases and Reactions

Matter exists in different phases, and changes between these phases are governed by energy. During a chemical reaction, existing bonds break and new ones form, resulting in substances with entirely different properties. A practical example is the blast furnace, where iron oxide reacts with carbon monoxide to produce pure iron and carbon dioxide.

Diagram showing relationships among the phases and the terms used to describe phase changes

During chemical reactions, bonds between atoms break and form, resulting in different substances with different properties. In a blast furnace, iron oxide, a compound, reacts with carbon monoxide to form iron, one of the chemical elements, and carbon dioxide.

Hydrogen bromide exists in the gas phase as a diatomic molecule.

The Evolution of Chemical Science

From Alchemy to Modern Chemistry

Chemistry has evolved from ancient philosophies and alchemy. Early thinkers like Democritus proposed atomist philosophies, which were later adopted by Epicurus. Pioneers such as Jābir ibn Hayyān (Geber) laid the groundwork for organic chemistry. The modern name "chemistry" is attributed to Georgius Agricola, who dropped the Arabic prefix "al-".

Democritus' atomist philosophy was later adopted by Epicurus (341–270 BCE).

15th-century artistic impression of Jābir ibn Hayyān (Geber), a Perso-Arab alchemist and pioneer in organic chemistry

Georgius Agricola, author of De re metallica, was the first to drop the Arabic definite article al-, exclusively writing chymia and chymista, giving chemistry its modern name.[63][64][65]

Defining the Science

The definition of chemistry has shifted over centuries. In 1730, Georg Ernst Stahl viewed it as the art of resolving and composing bodies. By 1837, Jean-Baptiste Dumas focused on molecular forces. In 1947, Linus Pauling defined it as the science of substances, their structure, and their reactions. Today, as noted by Professor Raymond Chang, it is broadly the study of matter and the changes it undergoes.

Antoine-Laurent de Lavoisier is considered the "Father of Modern Chemistry".[68]

Key Historical Milestones

The 19th century saw rapid advancement, including Friedrich Wöhler's synthesis of urea, which helped develop organic chemistry. Edward Frankland introduced valence bonding in 1852, and J.W. Gibbs and Svante Arrhenius applied thermodynamics to the field in the 1870s. Dmitri Mendeleev's creation of the periodic table was a watershed moment, as he correctly placed 60 known elements and predicted seven new ones.

In his periodic table, Dmitri Mendeleev predicted the existence of 7 new elements,[71] and placed all 60 elements known at the time in their correct places.[72]

Top: Expected results: alpha particles passing through the plum pudding model of the atom undisturbed. Bottom: Observed results: a small portion of the particles were deflected, indicating a small, concentrated charge.

Chemical Laws and Analysis

Chemical reactions are governed by fundamental laws that allow scientists to predict behavior. These include Boyle's law (pressure and volume), Charles's law (volume and temperature), and the Ideal Gas Law. In the lab, spectroscopy—the study of interactions between light and matter—is used to analyze substances using tools like spectrophotometers.

In analytical chemistry, spectroscopy studies interactions between electromagnetic radiation (light) and matter.[82] A spectrophotometer is a machine used to measure the effect light has on matter. The model pictured is the Beckman DU-640

Uranium hexafluoride is the inorganic compound that allow the purification of 235U.

The Solvay Conferences have been devoted to resolving unsolved problems in chemistry and physics since their inception in 1911. The 1927 conference (participants pictured) discussed electrons and photons.

Law/Principle	Primary Relationship	Key Focus
Boyle's Law	Pressure & Volume	Gas behavior at constant temperature
Charles's Law	Volume & Temperature	Gas expansion/contraction
Gay-Lussac's Law	Pressure & Temperature	Gas pressure changes
Avogadro's Law	Volume & Amount	Molar volume of gases
Le Chatelier's Principle	Equilibrium & Stress	System response to change

Frequently Asked Questions

Why is chemistry called the central science?

It is called the central science because it provides a foundational understanding of matter that connects physics (the study of energy and forces) with biology (the study of living organisms), supporting numerous other scientific disciplines.

What is the difference between an element and a compound?

An element consists of only one type of atom and cannot be broken down into simpler substances by chemical means. A compound consists of two or more different elements chemically bonded together.

What is the significance of the Avogadro constant?

The Avogadro constant (6.022 140 76 × 1023) defines the number of particles in one mole of a substance, allowing chemists to bridge the gap between the microscopic world of atoms and the macroscopic world of grams and liters.

How does ionic bonding differ from covalent bonding?

Ionic bonding occurs when electrons are transferred from one atom to another, creating oppositely charged ions that attract. Covalent bonding occurs when atoms share pairs of valence electrons to achieve stability.

Who is considered the Father of Modern Chemistry?

Antoine-Laurent de Lavoisier is widely regarded as the "Father of Modern Chemistry" for his contributions to the science.