3.1:
Molecules and Compounds
In nature, all matter is comprised of basic chemical units called elements having specific physical and chemical properties.
The periodic table lists a total of 118 elements identified to date. The constituent unit of each element is the atom. Some elements like helium, neon, silver, or platinum exist as units of independent individual atoms.
Other elements like hydrogen, oxygen, or sulfur exist as units of two or more identical atoms. These multiple atoms are held together by strong electrostatic forces referred to as chemical bonds, forming stable units called molecules.
Additionally, two or more different elements can also combine to form molecules like water or fructose. Such molecules that are formed by the combination of atoms of two or more elements, united chemically in a fixed proportion, are also called compounds. Hence, water and fructose molecules are also compounds.
Water is a molecule formed of two atoms of hydrogen and one atom of oxygen. In contrast, fructose is a molecule that contains six carbon atoms, twelve hydrogen atoms, and six oxygen atoms.
The individual atoms within a compound are held together by chemical bonds, dictating the type of compound formed such as a molecular or ionic compound.
Covalent or molecular compounds, like water, form when nonmetal atoms come together and share their valence electrons via a covalent bond. The shared electrons between two atoms lower the potential energy and create more stable molecules compared to the individual atoms.
Ionic compounds, like sodium chloride, form when metal and nonmetal atoms come together. Metals easily lose electrons becoming cations, while nonmetals readily gain electrons becoming anions. Thus metal atoms transfer electrons to nonmetal atoms and the electrostatic interactions between the oppositely charged ions form ionic bonds.
The ability of elements to form compounds contributes to the great diversity of matter that exists in nature.
3.1:
Molecules and Compounds
Atoms and Molecules
Everything in the universe is made up of matter, and matter is composed of a combination of elements. An atom is the smallest unit of an element that retains all properties of the element. For example, a silver coin is made up of silver atoms, and each silver atom retains the unique properties of the element — silver.
Elements are pure substances consisting of identical atoms that cannot be broken down into simpler substances by chemical changes. Atomic elements contain single atoms as their basic unit. Carbon is composed of carbon atoms, and sodium is composed of sodium atoms. Most elements are atomic elements. There are, however, a few elements that exist as a combination of atoms, and are never found in nature as single atoms. These are called molecular elements. Molecular elements exist as molecules with two or more atoms bonded together. They can be diatomic, like hydrogen (H2), oxygen (O2), or chlorine (Cl2) gas, or polyatomic, like sulfur (S8) and phosphorous (P4).
A molecule consists of two or more atoms joined by strong forces called chemical bonds. A molecule may consist of two or more identical atoms, or it may consist of two or more different atoms. When the molecule consists of atoms from two or more different elements that are combined in fixed whole number ratios, the molecule is also a compound. For example, water is a compound, and each water molecule (H2O) is a unit that contains two hydrogen atoms and one oxygen atom. The molecule glucose (C6H12O6) is also a compound that contains six carbon atoms, twelve hydrogen atoms, and six oxygen atoms.
Compounds are pure substances composed of two of more elements in fixed, definite proportions. Compounds are classified as molecular or ionic based on the bonds present.
Compounds
The nature of the attractive forces that hold atoms together within a compound decides the type of chemical bond formed. The complete transfer of electrons between a metal atom and a nonmetal atom results in the formation of oppositely charged ions. The electrostatic attraction between the ions is called an ionic bond, and the compounds formed through ionic bonds are ionic compounds. For example, sodium chloride NaCl is an ionic compound.
The sharing of electrons between two or more nonmetal atoms results in covalent bonds, and the compounds formed through covalent bonds are called covalent or molecular compounds. Covalent bonds are the attractive forces between the positively charged nuclei of the bonded atoms and negatively charged electrons shared between the atoms.
This text is adapted from OpenStax Chemistry 2e, Section 2.6: Molecular and Ionic Compounds.