Take some Cs, Hs, Ns, Os, Ps, and Ss, combine them in many different combinations, and what do you get?
In just the right combinations, you get life. Carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur. Some of the most basic of elements, but some of the most important. Together they can form countless combinations of organic compounds. And in just the right combinations, anything can happen.
Living things are made of matter. In fact, matter is the “stuff” of which all things are made. Anything that occupies space and has mass is known as matter. Matter, in turn, consists of chemical substances. A chemical substance is a material that has a definite chemical composition. It is also homogeneous, so the same chemical composition is found uniformly throughout the substance. A chemical substance may be an element or a chemical compound.
An element is a pure substance that cannot be broken down into different types of substances. There are almost 120 known elements (Figure below), each with its own personality. The chemical and physical properties of one element differ from any other. Elements are arranged according to their properties in the Periodic Table.
Examples of elements include carbon, oxygen, hydrogen, gold, silver and iron. Each element is made up of just one type of atom. An atom is the smallest particle of an element that still characterizes the element. As shown in Figure below, at the center of an atom is a nucleus. The nucleus contains positively charged particles called protons and electrically neutral particles called neutrons. Surrounding the nucleus is a much larger electron cloud consisting of negatively charged electrons. Electrons are arranged into distinct energy levels, at various distances from the nucleus. An atom is electrically neutral if it has the same number of protons as electrons. Each element has atoms with a characteristic number of protons, which defines the atomic number of the element. For example, all carbon atoms have six protons, and all oxygen atoms have eight protons. A combination of the number of protons and neutrons in the nucleus gives the approximate atomic mass of the atom, measured in an amu, or atomic mass unit. For example, hydrogen has an atomic number of 1 and an atomic mass of 1.00794 amu; carbon has an atomic number of 6 and an atomic mass of 12.0107 amu; oxygen has an atomic number of 8 and an atomic mass of 15.9994 amu.
The majority of known elements are classified as metals. Metals are elements that are lustrous, or shiny. They are also good conductors of electricity and heat. Examples of metals include iron, gold, and copper. Fewer than 20 elements are classified as nonmetals. Nonmetals lack the properties of metals. Examples of nonmetals include oxygen, hydrogen, and sulfur. Certain other elements have properties of both metals and nonmetals. They are known as metalloids. Examples of metalloids include silicon and boron.
A chemical compound is a new substance that forms when atoms of two or more elements react with one another. A chemical reaction is a process that changes some chemical substances into other chemical substances. A compound that results from a chemical reaction always has a unique and fixed chemical composition. The substances in the compound can be separated from one another only by another chemical reaction. This is covered further in Concept Biochemistry (Advanced). Atoms bond with each other through the interactions of their electrons, specifically their outermost or valence electrons.
The atoms of a compound are held together by chemical bonds. Chemical bonds form when atoms share electrons. There are different types of chemical bonds, and they vary in how strongly they hold together the atoms of a compound. Two of the strongest types of bonds are covalent and ionic bonds. Covalent bonds form between atoms that have little if any difference in electronegativity, and result when atoms share electrons. Electronegativity is the power of an atom to attract electrons toward itself. Ionic bonds, in contrast, form between atoms that are significantly different in electronegativity. An ion is an atom that has gained or lost at least one electron. Ionic bonds form between ions of opposite charges.
An example of a chemical compound is water. A water molecule forms when oxygen (O) and hydrogen (H) atoms react and are held together by covalent bonds. Like other compounds, water always has the same chemical composition: a 2:1 ratio of hydrogen atoms to oxygen atoms. This is expressed in the chemical formula H2O. A model of a water molecule is shown in Figure below.
Compounds that contain mainly the elements carbon and hydrogen are called organic compounds. This is because they are found mainly in living organisms. Most organic compounds are held together by covalent bonds. An example of an organic compound is glucose (C6H12O6), which is shown in Figure below. Glucose is a simple sugar that living cells use for energy. All other compounds are called inorganic compounds. Water is an example of an inorganic compound.
Mixtures vs. Compounds
Like a chemical compound, a mixture consists of more than one chemical substance. Unlike a compound, a mixture does not have a fixed chemical composition. The substances in a mixture can be combined in any proportions. A mixture also does not involve a chemical reaction. Therefore, the substances in a mixture are not changed into unique new substances, and they can be separated from each other without a chemical reaction.
The following examples illustrate these differences between mixtures and compounds. Both examples involve the same two elements: the metal iron (Fe) and the nonmetal sulfur (S).
- When iron filings and sulfur powder are mixed together in any ratio, they form a mixture. No chemical reaction occurs, and both elements retain their individual properties. A magnet can be used to mechanically separate the two elements by attracting the iron filings out of the mixture and leaving the sulfur behind.
- When iron and sulfur are mixed together in a certain ratio and heated, a chemical reaction occurs. This results in the formation of a unique new compound, called iron sulfide (FeS). A magnet cannot be used to mechanically separate the iron from the iron sulfide because metallic iron does not exist in the compound. Instead, another chemical reaction is required to separate the iron and sulfur.
- Matter consists of elements and compounds.
- A compound forms when elements combine in fixed proportions and undergo a chemical reaction.
- A mixture forms when substances combine in any proportions without a chemical reaction.
- Define element, and give an example of an element.
- State how a compound differs from an element, and give an example of a compound.
- Compare and contrast mixtures and compounds.
- Describe the difference between an ionic bond and a covalent bond.