Chemical substance
A chemical substance is defined as a material composed of a single type of atom or molecule, possessing a specific chemical composition. These substances, also known as pure substances, cannot be separated into individual components without breaking chemical bonds. Chemical substances can be classified as elements, such as hydrogen or oxygen, or compounds like water and table salt, where atoms are bonded together. In addition to elements and compounds, chemical substances also include ions and alloys, which are formed from metals.
Chemical elements are listed on the periodic table, which, as of 2017, contains 118 elements, with 91 found naturally on Earth. Elements are categorized as metals, nonmetals, or metalloids, each exhibiting distinct properties. For example, metals tend to be good conductors of heat and electricity, whereas nonmetals may form negatively charged ions. Chemical compounds, such as water (H₂O) and table salt (NaCl), result from the bonding of different elements. In contrast, mixtures consist of substances combined without undergoing chemical reactions, retaining their individual properties, such as in the case of air or soil. Understanding these distinctions is crucial for exploring the vast field of chemistry and the interactions between different substances.
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Chemical substance
A chemical substance is a material consisting of one type of atom or molecule that exhibits a definite chemical composition. Also called pure substances, chemical substances cannot be separated into their individual components without breaking chemical bonds. A chemical substance can be a chemical element, such as hydrogen, oxygen, or iron. It can also be a chemical compound—a group of atoms held together by chemical bonds—such as water, sugar, or table salt. Other types of chemical substances include ions and alloys. To be classified as chemical substance, the individual elements must bond at the molecular level. Materials that combine without a chemical reaction are called mixtures. For example, water and salt are chemical substances, but saltwater is a mixture.
Background
Atoms are the smallest units of matter that make up the chemical elements and still maintain the characteristics of that element. Atoms are composed of three fundamental particles: protons, neutrons, and electrons. Protons and neutrons together make up the nucleus, or center, of an atom. Protons have a positive charge, while neutrons have no charge at all. The number of protons in an atom is referred to as its atomic number. It determines the chemical properties of an atom and defines it as a chemical element. Carbon, for example, is a chemical element with six protons. Oxygen atoms have eight protons, gold has seventy-nine, and uranium has ninety-two.
Electrons are negatively charged particles that orbit the nucleus of an atom like planets circling the sun. In stable atoms, the number of electrons matches the number of protons. Electrons orbit the nucleus in shells; the more electrons an atom has, the more shells it has as well. The outer shell of an atom determines how well it bonds with other elements. Stable atoms tend to have full outer shells. Elements with less-than-full outer shells have a tendency to react, or bond, with other atoms so that both can have the maximum number of electrons in their outer shells.
Hydrogen and helium atoms, for example, both have an outer shell that can hold up to two electrons. Hydrogen has one electron and helium has two. As a result, hydrogen atoms are more reactive and tend to bond with other elements. Helium is more stable and less likely to chemically bond with another atom.
Overview
All chemical elements are considered chemical substances. The elements are materials that cannot be broken down further or changed into another element by chemical means. The elements are listed according to their atomic number on the periodic table. As of 2017, the periodic table contained 118 chemical elements. Ninety-one of those elements occur naturally on Earth. About seven elements can be found in very small amounts naturally, but are usually created in a laboratory; the remaining twenty elements can only be made in a laboratory.
Most of the chemical elements are classified as metals. Metals are typically good conductors of heat and electricity and can be shaped into other forms. Common examples of metals are gold, iron, and copper. Mercury is a metal that exists in liquid form at room temperature. Seventeen elements are considered nonmetals. These include hydrogen, oxygen, and sulphur. Nonmetals also include the noble gases. These gases have full outer electron shells, and they are considered very stable. Helium and neon are noble gases. A group of six elements called metalloids display properties of both metals and nonmetals. Silicon is a metalloid.
An atom or molecule that gains or loses an electron is called an ion. Because electrons are negatively charged, an atom or molecule that gains an electron takes on a negative electrical charge. Ions with a negative charge are called anions. Nonmetals tend to form anions. Ions that lose an electron take on a positive charge and are called cations. Metals tend to form cations.
When the electrons of two or more atoms bond together, a molecule is formed. Molecules can form from atoms of the same element or of different elements. Chemical compounds are formed from the chemical bonding of atoms or molecules of two or more elements.
Water is an example of a simple chemical compound. Water is made from the chemical bonding of two hydrogen atoms with one oxygen atom. Its chemical symbol is written as H2O. Common table salt is formed by the bonding of a sodium atom (Na)—designated by its Latin name, natrium—and a chlorine atom (Cl). Also known as sodium chloride, salt's chemical formula is written as NaCl. Chalk (CaCO3) is a chemical compound made from a calcium atom, carbon atom, and three oxygen atoms. The sugar used to sweeten food is called sucrose, and it is a more complex chemical compound. It is made of twelve carbon atoms, twenty-two hydrogen atoms, and eleven oxygen atoms. Its formula is written as C12H22O11.
Alloys are formed by melting together two chemical substances, one of which must be a metal. Solid substances are heated until they become liquid and then combined. Upon cooling, the material crystallizes into an alloy. Alloys can become a chemical substance if their individual components undergo chemical bonding. Brass, for example, is an alloy made from the chemical bonding of copper and zinc. Most alloys remain mixtures.
A mixture occurs when two or more chemical substances are combined but do not undergo chemical bonding. The substances may join together, but each substance retains its chemical identity. Some mixtures are obvious, such as soil, which is a mixture of broken rocks, mineral, organic matter, and decaying matter. Others seem to form a more coherent substance. Air is an example of this type of mixture. Air is made up of nitrogen, oxygen, argon, carbon dioxide, and other gases, but its components do not chemically bond. The individual elements of air can be removed through a process of extreme cooling.
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