The periodic table organizes elements by atomic structure and common properties

By organizing elements by atomic weight, Dmitri Mendeleev created the periodic table

The organization included groupings based on similar chemical properties and patterns in atomic weights. This left gaps that were used to predict the properties of elements yet to be discovered—including germanium and gallium—with high accuracy.

The history of the periodic table and how it hinted at atoms possessing internal structure

In 1869, more than two decades before the discovery of electrons, Russian chemist Dmitrii Mendeleev organized known elements by atomic weight and chemical properties using index cards. His organization revealed previously undiscovered elements and identified patterns in the reactivity of elements, which would later be linked to each element's outermost electron shell. (Some readers may experience a paywall.)

Understanding how atomic structure contributes to each element's properties

The outermost layer of electrons in an atom dictates how it will bond to other atoms. These bonds determine whether elements exist as solids, liquids, or gases in the environment and other characteristics. For example, electrons are freely shared between atoms in metallic bonds, allowing them to immediately re-emit light that strikes them, thus making most metals shiny.

How the number of subatomic particles helps identify different types of atoms

Different elements, such as oxygen and nitrogen, can be distinguished by the number of protons in each, since all atoms of the same element have the same number of protons. Isotopes of a given element have the same number of protons but different numbers of neutrons, while ions are atoms possessing different numbers of electrons than protons.

Similarities in reactivity complicate separating rare earths from ores

The proximity of these elements in the periodic table mean they have similar electron structure and require complex separation processes during processing. However, their uncommon magnetic, luminescent, and electrical properties make them invaluable for critical industrial applications.

Build atoms to learn about atomic number, mass number, and charge notations

In this simulation, users can add protons, neutrons, and electrons to create atoms, view their stability and atomic configurations, and identify how the amounts of these subatomic particles are detailed around the abbreviation for the resulting element using AZE notation.

View the evolution of atomic models, with four successively overturned between 1900 and 1926

Originally thought to be indivisible spheres unique to each element, the discovery of the electron by J.J. Thomson revealed them to be made of smaller particles. Ernest Rutherford's gold foil experiment uncovered the existence of nuclei, which Niels Bohr suggested were surrounded by electrons in fixed orbits. By applying quantum mechanics, Erwin Schrödinger replaced orbits with probability clouds.

Distinguishing polar and nonpolar molecules and why charges get distributed unevenly in molecules

Because each element has a unique number of protons and configuration where electrons exist, they attract electrons that are shared with other elements in chemical bonds to varying degrees. When this attraction—electronegativity—varies between two bonded elements, electrons are more likely to be near the more electronegative element and away from the less electronegative element. This can create regions of opposite charge, or poles, and thus polar molecules.

Unusually stable, undiscovered elements may exist near atomic number 126

While heavier elements like uranium and plutonium are unstable and decay quickly, simulations suggest a pocket of stability exists for elements with approximately 126 protons and 184 neutrons. Such elements may have practical applications, such as americium, a synthetic element used in smoke detectors.

Time crystals are systems of atoms organized into periodic patterns of motion

Unlike traditional crystals—whose atomic structures cause them to look identical no matter how you look at them in space—time crystals exhibit this behavior through time, where the motion of atoms will bring them to the same positions relative to one another regularly.

The Standard Model reveals an underlying organization to the universe

Using the framework of quantum field theory, it structures all known particles into 12 matter particles, alongside five particles that carry three of the four fundamental forces. Although remarkably precise and predictive, it cannot yet explain gravity, dark matter, or dark energy.

An overview of helium, the nonrenewable resource that's lighter than air

The second element in the periodic table naturally forms deep underground over the course of billions of years through the decay of radioactive elements, but it is used and lost to space at rates faster than it is generated. Because of this and its extensive applications in scientific technologies, scientists continue to seek avenues for its exploration, storage, and recycling.

Create anything from the elements in Little Alchemy 2

Various ancient cultures believed the most abundant substances in the world were its basic elements: earth, water, air, and fire. We may know better now, but you can make almost anything from those four things. Little Alchemy 2 is a fun, cartoonish game that allows users to create more complex items from simple substances.

While atoms are normally made of protons, neutrons, and electrons, exotic ones have been built with antimatter—alternate versions of normal matter that live for fractions of a second and are about a billion times as rare.

Fireworks' chemical composition and internal layout produce their unique displays

The burning of different metal compounds, facilitated by combining fuel with oxidizers, results in explosions of varying colors. The timing and pattern are engineered through the arrangement of pellets and fuses.

Psychedelic compounds: a visual library

However psychedelic experiences manifest, they all begin with the presence of psychoactive compounds. Learn about their various chemical compositions in this structural library of psychedelic compounds. Clicking on an entry will take readers to a new page where they can learn about the chemistry and pharmacology of each compound, as well as the drug's possible applications.

Cells are categorized by the presence of contained genetic material and other organelles

While prokaryotes contain one or more cells with free-floating genetic material in a region called the nucleoid, eukaryotes possess a nucleus specialized for DNA storage. Plant cells possess cell walls—a reinforced structure surrounding their membrane—made of cellulose, while fungal cell walls contain chitin. Animal cells have lysosomes housing enzymes that break down biomolecules. (Some readers may experience a paywall.)

See how fireworks are made and learn how metals like magnesium and copper are responsible for different colors.

Visualize how data becomes organized through a variety of sorting algorithms

This interactive shows how different algorithms shift elements in a randomized set to order them from smallest to largest. By comparing the different procedures and their corresponding functions, users can explore how runtimes vary across algorithms that complete the same task.

Dow Theory laid the groundwork for modern market analysis

Developed by Charles Dow in the late 19th century, Dow Theory argued that markets move in trends. Its principles—confirmation, trends and volume—still influence technical analysis today.

Comets can be categorized based on their orbital period

Short-period comets orbit around the sun once every 200 years or less and are made up of Jupiter-family (less than 20 years) and Halley-type (between 20 and 200 years) comet subgroups. Long-period comets have orbits greater than 200 years, while exocomets originate in solar systems outside our own.

Stacking nucleotide molecules like toy blocks helps build DNA's structure

Each nucleotide is made up of a sugar—deoxyribose—a phosphate group, and one of four bases that contain nitrogen. Like building a two-tone, spiral staircase from the ground up, nucleotides are linked one on top of another so that sugars and phosphates alternate. The twisted strands are held together by hydrogen bonds between base pairs.

Paleontologists apply rules about rock layering, fossilization, and radioactivity to date geological samples

Concepts such as the understanding that older rock layers will form beneath newer ones allow scientists to compare the ages of samples from various layers. Although no geological site preserves evidence across all time, samples can be combined with fossils of known age, such as those dated by analyzing the decay of their radioactive isotopes, to reconstruct Earth's timeline.

Radioactive elements can introduce software errors because the ionizing radiation they produce can detach electrons from atoms, and computer code relies on the distribution of electrons.

During manufacturing, lithium-ion and alkaline batteries are tested for short-circuiting

Lithium-ion batteries are made by manufacturing anodes, cathodes, and electrolytes via chemical processes, assembling these components into cells, and enclosing multiple interconnected cells within a protective casing. Disposable alkaline battery production involves filling steel containers with preforms—rings of powdered cathode material—paper-based separators, electrolyte solution, and an anode gel.

A timeline of key events during each period of Egyptian history

This timeline shows the dates, periods, dynasties, and significant events in the history and art of ancient Egypt from the Predynastic Period (before 3100 BCE) to the end of the Roman Period (395 CE).

The Köppen-Geiger system divides Earth’s climate into five main categories

This classification system, developed by a German-Russian botanist and a German climatologist, results from differences in the intensity of solar radiation and rainfall in different parts of the world. Each category is further divided into multiple coded zones.

The five most common chemicals found in modern food

Most modern processed foods rely on a number of preservatives to stay fresh from production to our plates. The most common include citric acid (preservative and flavor), sodium benzoate (preservative), xanthan gum (thickening and stabilizing), carrageenan (thickening), and red dyes 3 and 4 (for color).

As of 2025, the most cited papers in science include a review of graphene, methods to improve image recognition, streamlining electron density calculations, and optimizing protein studies.

The hormonal disorder PCOS was renamed PMOS in 2026, a move designed to improve care and research by shifting the focus to the hormonal causes of this condition that affects 1 in 8 women.

See how each of the more than two million known species relates to each other—and to common ancestors—in this zoomable tree of life.

Pure water does not conduct electricity because this flow of energy requires the movement of charged particles, and water is a neutral molecule.

Sony set the standard for cylinder-shaped batteries when it repurposed manufacturing equipment used to make rolls of magnetic recording tape.

While the sun's gravity keeps the planets in orbit, their gravity also pulls on the sun, causing it and everything else in the solar system to orbit around a mutual balance point called the barycenter—the true center of the solar system.

At the beginning of the 20th century, a group of young men ate standard American food—replete with food additives like borax and formaldehyde—as part of government research to better understand food safety.

The US Constitution is widely regarded as the longest-standing written national constitution still in force—see how it has influenced rights worldwide.

The term 'algorithm' originated from the Latinized name of al-Khwārizmī, a 9th-century mathematician whose works introduced Arabic numerals and the decimal-based numeral system to Europe and whose treatise 'Al-Jabr' gave algebra its name.

A century of Billboard issues documenting the evolution of popular music.

Chaos theory—informally known as the butterfly effect—is the reason why weather forecasts are presented with probabilities rather than certainties.

Iran's Revolutionary Guard Corps is independent of Iran's regular army and is one of the most powerful and influential military-political organizations in the Middle East.

Animal phobias are likely the most common type of phobias, followed by an extreme fear of heights.

Graffiti dates back to at least ancient Pompeii, where citizens scrawled messages to one another on public walls

Warren Buffett's former professor Benjamin Graham is known as the 'father of value investing,' a strategy in which investors seek to buy stocks trading below their intrinsic worth.

Poison ivy's characteristic rash is caused by an allergic response to an oily compound in the plant called urushiol.

While El Niño moves warm Pacific Ocean waters east along the equator, La Niña moves them west, shifting where rainfall will become more common for months.

For centuries, it was common to sleep in two shifts: a few hours at the beginning of the night, and a longer period beginning around midnight

The syncytiotrophoblast is the outer layer of the placenta and the largest human cell, spanning an area about twice that of a king-sized bedsheet.

Alzheimer's can lead to brain shrinkage, with an average of around 2% of volume reduction per year

SpongeBob exemplifies Aristotle's philosophy for living well because he understands that happiness doesn't come from external factors like money, recognition, or power.

Some coin denominations have ridges etched on their sides, partly to prevent counterfeiting, in a practice called 'reeding.'