Chemical element

A chemical element is a species of atom defined by its number of protons. The number of protons is called the atomic number of that element. For example, oxygen has an atomic number of 8: each oxygen atom has 8 protons in its nucleus. Atoms of the same element can have different numbers of neutrons in their nuclei, known as isotopes of the element.

Almost all baryonic matter in the universe is made of elements, with rare exceptions like neutron stars. By November 2016, the International Union of Pure and Applied Chemistry recognized 118 elements. The first 94 occur naturally on Earth, while the remaining 24 are synthetic elements created in nuclear reactions.

The history of discovering elements began with early human societies finding materials like carbon, sulfur, copper, and gold. The modern understanding of elements grew from the work of Dmitri Mendeleev, who published the first recognizable periodic table in 1869. This table organizes elements by their atomic numbers and shows patterns in their physical and chemical properties.

Description

A chemical element is a type of atom that has a specific number of protons, called its atomic number. For example, oxygen has an atomic number of 8, meaning every oxygen atom has 8 protons. Elements can be thought of in two ways: as single kinds of atoms, like pure gold, or as parts of other substances, like hydrogen and oxygen in water.

Chemical elements can be organized by their properties, such as atomic number and how they react with other elements. The periodic table is a common way to show elements, grouping those with similar properties together. Some elements exist naturally, like hydrogen and helium, which were created shortly after the universe began. Others are made by scientists in laboratories.

Atomic nucleus properties

The center of an atom, called the nucleus, is made up of protons and neutrons. Protons have a positive charge, while neutrons have no charge. They are held together by a strong force, even though the protons push apart from each other because of their like charges.

An atom’s identity is determined by the number of protons in its nucleus, known as the atomic number. For example, all carbon atoms have 6 protons, so carbon’s atomic number is 6. Atoms of the same element can have different numbers of neutrons, which are called isotopes. These isotopes have almost identical chemical properties but may behave differently in nuclear reactions.

Chemical and physical properties

Quantum mechanics helps explain how electrons are arranged in layers around the nucleus of an atom. These layers, called shells, can only hold a certain number of electrons. The way electrons are arranged in these shells determines many of an element's chemical properties and its place in the periodic table.

Elements are grouped based on whether they conduct electricity — metals do, nonmetals don’t, and metalloids are in between. Metals tend to lose electrons to form positive ions, while nonmetals gain electrons to form negative ions. The periodic table organizes elements into rows and columns that show patterns in their properties, making it a useful tool in many areas of science and technology.

Nomenclature and symbols

Chemical elements are identified by their unique atomic numbers, names, and symbols. The atomic number tells us how many protons are in an atom’s nucleus. For example, oxygen has an atomic number of 8, meaning each oxygen atom has 8 protons.

Elements are given names that may come from many sources, like their properties, locations, or scientists. These names are decided by groups of scientists to make sure everyone uses the same terms worldwide. Each element also has a special symbol, like “O” for oxygen, which helps scientists write and share their ideas clearly across different languages.

Origin of the elements

Main article: Nucleosynthesis

Most of the universe is made of something we can't see, called dark matter and dark energy. Only about 4.6% of the universe is ordinary matter, like the atoms that make up everything around us.

The naturally occurring elements we find on Earth and in space were created in different ways. The simplest elements, hydrogen and helium, were formed in the first few minutes after the Big Bang. Heavier elements, like carbon and iron, are made inside stars through a process called nuclear fusion. Even heavier elements, such as uranium, are created when stars explode or collide. Some lighter elements, like lithium and beryllium, are made when cosmic rays hit other atoms.

Abundance

Main article: Abundance of the chemical elements

The amount of each element in the universe depends on how it was created. Hydrogen and helium, the two most common elements, were made during the Big Bang. Heavier elements are created inside stars, where they are formed from smaller particles. This process makes elements with even numbers of protons more common because they are more stable.

On Earth, the mix of elements is different from the rest of the solar system. Lighter elements like hydrogen and helium are much less common because they were lost early in Earth’s formation. Oxygen is the most common element on Earth because it bonds with other elements like silicon and iron. Living things are made mostly of six key elements: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Some organisms also need extra elements, like magnesium for chlorophyll in plants or calcium for shells in molluscs.

History

The idea of an "element" has changed over time. Ancient thinkers like the Greeks thought of basic building blocks of nature as things like earth, water, air, and fire. Later, scientists began to think of elements as basic, unchanging substances that make up all matter.

In the 1600s, Robert Boyle suggested that matter is made of tiny particles that can combine in many ways. By the late 1700s, Antoine Lavoisier made one of the first lists of elements. In the 1800s, Dmitri Mendeleev created the periodic table to organize these elements, showing patterns in their properties. Today, we define an element by the number of protons in its atoms, a concept clarified in the early 1900s.

Many elements were known to ancient cultures, like gold and copper. Others, such as phosphorus, were discovered much later. Scientists continue to find and study new elements, with the most recent ones being very heavy and created in laboratories.